Data Link Layer

CS 3516 ndash Computer Networks

Chapter 5 The Data Link LayerGoals bull Understand principles behind data link layer

servicesndash Error detection correctionndash Sharing a broadcast channel multiple accessndash Link layer addressingndash Reliable data transfer flow control (done in

Ch3)bull Instantiation and implementation of various

link layer technologies

Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Link Layer IntroductionSome terminologybull Hosts and routers are nodesbull Communication channels

that connect adjacent nodes along communication path are linksndash Wired linksndash Wireless linksndash LANs

bull Layer-2 packet is a frame encapsulates datagram

data-link layer has responsibility of transferring datagram from one node to adjacent node over link

Link Layer Contextbull Datagram transferred

by different link protocols over different linksndash eg Ethernet on first

link frame relay on intermediate links 80211 on last link

bull Each link protocol provides different servicesndash eg may or may not

provide rdt over link

Transportation analogybull Trip from Princeton to

Lausannendash limo Princeton to JFKndash plane JFK to Genevandash train Geneva to

Lausannebull Tourist = datagrambull Transport hop =

communication linkbull Transportation mode =

link layer protocolbull Travel agent = routing

algorithm

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailer

ndash Channel access if shared mediumndash Medium Access Control (MAC) addresses used

in frame headers to identify source and dest bull Different from IP address

bull Reliable delivery between adjacent nodesndash We learned how to do this already (in ch3)ndash Seldom used on low bit-error link (fiber some

twisted pair)ndash Used for wireless links with high error rates

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noisendash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame bull Error correction

ndash Receiver identifies and corrects bit error(s) without resorting to retransmission

bull Half-duplex and full-duplexndash With half duplex nodes at both ends of link can

transmit but not at same time

Where is Link Layer Implementedbull In each and every hostbull Link layer implemented

in ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI card

80211 cardndash Implements link physical

layerbull Attaches into hostrsquos

system busesbull Combination of

hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

Adaptors Communicating

bull Sending sidendash Encapsulates datagram

in framendash Adds error checking

bits rdt flow control etc

bull Receiving sidendash Looks for errors rdt flow

control etcndash Extracts datagram passes

to upper layer

controller controller

sending host receiving host

datagram datagram

datagram

frame

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliable― Protocol may miss some errors but rarely― Larger EDC field yields better detection and correction

otherwise

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Chapter 5 The Data Link LayerGoals bull Understand principles behind data link layer

servicesndash Error detection correctionndash Sharing a broadcast channel multiple accessndash Link layer addressingndash Reliable data transfer flow control (done in

Ch3)bull Instantiation and implementation of various

link layer technologies

Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Link Layer IntroductionSome terminologybull Hosts and routers are nodesbull Communication channels

that connect adjacent nodes along communication path are linksndash Wired linksndash Wireless linksndash LANs

bull Layer-2 packet is a frame encapsulates datagram

data-link layer has responsibility of transferring datagram from one node to adjacent node over link

Link Layer Contextbull Datagram transferred

by different link protocols over different linksndash eg Ethernet on first

link frame relay on intermediate links 80211 on last link

bull Each link protocol provides different servicesndash eg may or may not

provide rdt over link

Transportation analogybull Trip from Princeton to

Lausannendash limo Princeton to JFKndash plane JFK to Genevandash train Geneva to

Lausannebull Tourist = datagrambull Transport hop =

communication linkbull Transportation mode =

link layer protocolbull Travel agent = routing

algorithm

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailer

ndash Channel access if shared mediumndash Medium Access Control (MAC) addresses used

in frame headers to identify source and dest bull Different from IP address

bull Reliable delivery between adjacent nodesndash We learned how to do this already (in ch3)ndash Seldom used on low bit-error link (fiber some

twisted pair)ndash Used for wireless links with high error rates

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noisendash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame bull Error correction

ndash Receiver identifies and corrects bit error(s) without resorting to retransmission

bull Half-duplex and full-duplexndash With half duplex nodes at both ends of link can

transmit but not at same time

Where is Link Layer Implementedbull In each and every hostbull Link layer implemented

in ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI card

80211 cardndash Implements link physical

layerbull Attaches into hostrsquos

system busesbull Combination of

hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

Adaptors Communicating

bull Sending sidendash Encapsulates datagram

in framendash Adds error checking

bits rdt flow control etc

bull Receiving sidendash Looks for errors rdt flow

control etcndash Extracts datagram passes

to upper layer

controller controller

sending host receiving host

datagram datagram

datagram

frame

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliable― Protocol may miss some errors but rarely― Larger EDC field yields better detection and correction

otherwise

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Link Layer IntroductionSome terminologybull Hosts and routers are nodesbull Communication channels

that connect adjacent nodes along communication path are linksndash Wired linksndash Wireless linksndash LANs

bull Layer-2 packet is a frame encapsulates datagram

data-link layer has responsibility of transferring datagram from one node to adjacent node over link

Link Layer Contextbull Datagram transferred

by different link protocols over different linksndash eg Ethernet on first

link frame relay on intermediate links 80211 on last link

bull Each link protocol provides different servicesndash eg may or may not

provide rdt over link

Transportation analogybull Trip from Princeton to

Lausannendash limo Princeton to JFKndash plane JFK to Genevandash train Geneva to

Lausannebull Tourist = datagrambull Transport hop =

communication linkbull Transportation mode =

link layer protocolbull Travel agent = routing

algorithm

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailer

ndash Channel access if shared mediumndash Medium Access Control (MAC) addresses used

in frame headers to identify source and dest bull Different from IP address

bull Reliable delivery between adjacent nodesndash We learned how to do this already (in ch3)ndash Seldom used on low bit-error link (fiber some

twisted pair)ndash Used for wireless links with high error rates

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noisendash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame bull Error correction

ndash Receiver identifies and corrects bit error(s) without resorting to retransmission

bull Half-duplex and full-duplexndash With half duplex nodes at both ends of link can

transmit but not at same time

Where is Link Layer Implementedbull In each and every hostbull Link layer implemented

in ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI card

80211 cardndash Implements link physical

layerbull Attaches into hostrsquos

system busesbull Combination of

hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

Adaptors Communicating

bull Sending sidendash Encapsulates datagram

in framendash Adds error checking

bits rdt flow control etc

bull Receiving sidendash Looks for errors rdt flow

control etcndash Extracts datagram passes

to upper layer

controller controller

sending host receiving host

datagram datagram

datagram

frame

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliable― Protocol may miss some errors but rarely― Larger EDC field yields better detection and correction

otherwise

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Link Layer IntroductionSome terminologybull Hosts and routers are nodesbull Communication channels

that connect adjacent nodes along communication path are linksndash Wired linksndash Wireless linksndash LANs

bull Layer-2 packet is a frame encapsulates datagram

data-link layer has responsibility of transferring datagram from one node to adjacent node over link

Link Layer Contextbull Datagram transferred

by different link protocols over different linksndash eg Ethernet on first

link frame relay on intermediate links 80211 on last link

bull Each link protocol provides different servicesndash eg may or may not

provide rdt over link

Transportation analogybull Trip from Princeton to

Lausannendash limo Princeton to JFKndash plane JFK to Genevandash train Geneva to

Lausannebull Tourist = datagrambull Transport hop =

communication linkbull Transportation mode =

link layer protocolbull Travel agent = routing

algorithm

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailer

ndash Channel access if shared mediumndash Medium Access Control (MAC) addresses used

in frame headers to identify source and dest bull Different from IP address

bull Reliable delivery between adjacent nodesndash We learned how to do this already (in ch3)ndash Seldom used on low bit-error link (fiber some

twisted pair)ndash Used for wireless links with high error rates

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noisendash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame bull Error correction

ndash Receiver identifies and corrects bit error(s) without resorting to retransmission

bull Half-duplex and full-duplexndash With half duplex nodes at both ends of link can

transmit but not at same time

Where is Link Layer Implementedbull In each and every hostbull Link layer implemented

in ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI card

80211 cardndash Implements link physical

layerbull Attaches into hostrsquos

system busesbull Combination of

hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

Adaptors Communicating

bull Sending sidendash Encapsulates datagram

in framendash Adds error checking

bits rdt flow control etc

bull Receiving sidendash Looks for errors rdt flow

control etcndash Extracts datagram passes

to upper layer

controller controller

sending host receiving host

datagram datagram

datagram

frame

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliable― Protocol may miss some errors but rarely― Larger EDC field yields better detection and correction

otherwise

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Link Layer Contextbull Datagram transferred

by different link protocols over different linksndash eg Ethernet on first

link frame relay on intermediate links 80211 on last link

bull Each link protocol provides different servicesndash eg may or may not

provide rdt over link

Transportation analogybull Trip from Princeton to

Lausannendash limo Princeton to JFKndash plane JFK to Genevandash train Geneva to

Lausannebull Tourist = datagrambull Transport hop =

communication linkbull Transportation mode =

link layer protocolbull Travel agent = routing

algorithm

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailer

ndash Channel access if shared mediumndash Medium Access Control (MAC) addresses used

in frame headers to identify source and dest bull Different from IP address

bull Reliable delivery between adjacent nodesndash We learned how to do this already (in ch3)ndash Seldom used on low bit-error link (fiber some

twisted pair)ndash Used for wireless links with high error rates

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noisendash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame bull Error correction

ndash Receiver identifies and corrects bit error(s) without resorting to retransmission

bull Half-duplex and full-duplexndash With half duplex nodes at both ends of link can

transmit but not at same time

Where is Link Layer Implementedbull In each and every hostbull Link layer implemented

in ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI card

80211 cardndash Implements link physical

layerbull Attaches into hostrsquos

system busesbull Combination of

hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

Adaptors Communicating

bull Sending sidendash Encapsulates datagram

in framendash Adds error checking

bits rdt flow control etc

bull Receiving sidendash Looks for errors rdt flow

control etcndash Extracts datagram passes

to upper layer

controller controller

sending host receiving host

datagram datagram

datagram

frame

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliable― Protocol may miss some errors but rarely― Larger EDC field yields better detection and correction

otherwise

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailer

ndash Channel access if shared mediumndash Medium Access Control (MAC) addresses used

in frame headers to identify source and dest bull Different from IP address

bull Reliable delivery between adjacent nodesndash We learned how to do this already (in ch3)ndash Seldom used on low bit-error link (fiber some

twisted pair)ndash Used for wireless links with high error rates

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noisendash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame bull Error correction

ndash Receiver identifies and corrects bit error(s) without resorting to retransmission

bull Half-duplex and full-duplexndash With half duplex nodes at both ends of link can

transmit but not at same time

Where is Link Layer Implementedbull In each and every hostbull Link layer implemented

in ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI card

80211 cardndash Implements link physical

layerbull Attaches into hostrsquos

system busesbull Combination of

hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

Adaptors Communicating

bull Sending sidendash Encapsulates datagram

in framendash Adds error checking

bits rdt flow control etc

bull Receiving sidendash Looks for errors rdt flow

control etcndash Extracts datagram passes

to upper layer

controller controller

sending host receiving host

datagram datagram

datagram

frame

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliable― Protocol may miss some errors but rarely― Larger EDC field yields better detection and correction

otherwise

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noisendash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame bull Error correction

ndash Receiver identifies and corrects bit error(s) without resorting to retransmission

bull Half-duplex and full-duplexndash With half duplex nodes at both ends of link can

transmit but not at same time

Where is Link Layer Implementedbull In each and every hostbull Link layer implemented

in ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI card

80211 cardndash Implements link physical

layerbull Attaches into hostrsquos

system busesbull Combination of

hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

Adaptors Communicating

bull Sending sidendash Encapsulates datagram

in framendash Adds error checking

bits rdt flow control etc

bull Receiving sidendash Looks for errors rdt flow

control etcndash Extracts datagram passes

to upper layer

controller controller

sending host receiving host

datagram datagram

datagram

frame

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliable― Protocol may miss some errors but rarely― Larger EDC field yields better detection and correction

otherwise

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Where is Link Layer Implementedbull In each and every hostbull Link layer implemented

in ldquoadaptorrdquo (aka network interface card NIC)ndash Ethernet card PCMCI card

80211 cardndash Implements link physical

layerbull Attaches into hostrsquos

system busesbull Combination of

hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

Adaptors Communicating

bull Sending sidendash Encapsulates datagram

in framendash Adds error checking

bits rdt flow control etc

bull Receiving sidendash Looks for errors rdt flow

control etcndash Extracts datagram passes

to upper layer

controller controller

sending host receiving host

datagram datagram

datagram

frame

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliable― Protocol may miss some errors but rarely― Larger EDC field yields better detection and correction

otherwise

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Adaptors Communicating

bull Sending sidendash Encapsulates datagram

in framendash Adds error checking

bits rdt flow control etc

bull Receiving sidendash Looks for errors rdt flow

control etcndash Extracts datagram passes

to upper layer

controller controller

sending host receiving host

datagram datagram

datagram

frame

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliable― Protocol may miss some errors but rarely― Larger EDC field yields better detection and correction

otherwise

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliable― Protocol may miss some errors but rarely― Larger EDC field yields better detection and correction

otherwise

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields

bull Error detection not 100 reliable― Protocol may miss some errors but rarely― Larger EDC field yields better detection and correction

otherwise

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Simple - Parity CheckingSingle Bit ParityDetect single bit errors

Two Dimensional Bit ParityDetect and correct single bit errors

0 0

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Internet Checksum (review)

Senderbull Treat segment contents

as sequence of 16-bit integers

bull Checksum addition (1rsquos complement sum) of segment contents

bull Sender puts checksum value into UDP checksum field

Receiverbull Compute checksum of

received segmentbull Check if computed

checksum equals checksum field valuendash NO - error detectedndash YES - no error detected

But maybe errors nonetheless

Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Checksumming Cyclic Redundancy Check (CRC)

bull View data bits D as a binary numberbull Choose r+1 bit pattern (generator) G bull Goal choose r CRC bits R such that

ndash ltDRgt exactly divisible by G (modulo 2) ndash Receiver knows G divides ltDRgt by G

bull If non-zero remainder error detectedndash Can detect all burst errors less than r+1 bits

bull Widely used in practice (Ethernet 80211 WiFi)

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

CRC Example ndash Choosing RWant

D2r XOR R = nGEquivalently

D2r = nG XOR R Equivalently If we divide D2r by

G want remainder R

R = remainder[ ]D2r

G

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

CRC Standards

bull Defined for 8 12 16 and 32 bit genrators (G)

bull CRC-32 adopted by many IEEE link-layer protocols uses generatorndash Gcrc-32 =

100000100110000010001110110110111bull Detects all errors burst less than 33 bitsbull Detects all odd number bit errorsbull Burst errors greater than 33 bits with

probability 1-05r

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull point-to-point (not shared)

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and host

bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Multiple Access Protocolsbull Single shared broadcast channel bull Two or more simultaneous transmissions by nodes

interference ndash collision if node receives two or more signals at the same

timeMultiple access protocolbull Distributed algorithm determines how nodes share

channel (ie determine whenwho node can transmit)

bull Communication about channel sharing must use channel itself ndash no ldquoout-of-bandrdquo channel for coordination

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Ideal Multiple Access ProtocolBroadcast channel of rate R bps1 When one node wants to transmit it can send

at rate R2 When M nodes want to transmit each can

send at average rate RM (no overhead)3 Fully decentralized

ndash No special node to coordinate transmissionsndash No synchronization of clocks slots

4 Simple

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

MAC Protocols a TaxonomyThree broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency)

ndash Allocate piece to node for exclusive usebull Random Access

ndash Channel not divided allow collisionsndash ldquoRecoverrdquo from collisions

bull Taking turnsndash Nodes take turns but nodes with more to send can

perhaps take longer turns

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length =

pkt trans time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots

256 idle

1 3 4 1 3 4

6-slotframe

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access bull Channel spectrum divided into frequency bandsbull Each station assigned fixed frequency bandbull Unused transmission time in frequency bands go

idle bull Example 6-station LAN 134 have pkt

frequency bands 256 idle fr

eque

ncy

band

s time

FDM cable

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Random Access Protocolsbull When node has packet to send

ndash Transmit at full channel data rate Rndash No a priori coordination among nodes

bull Two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash How to detect collisionsndash How to recover from collisions (eg via delayed

retransmissions)bull Examples of random access MAC protocols

ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Slotted ALOHAAssumptionsbull All frames same sizebull Time divided into

equal size slots (time to transmit 1 frame)

bull Nodes start to transmit only slot beginning

bull Nodes are synchronized

bull If 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains

fresh frame transmits in next slotndash If no collision node can

send new frame in next slot

ndash If collision node retransmits frame in each subsequent slot with prob p until success

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Slotted ALOHA

Prosbull Single active node

can continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting

slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Slotted Aloha Efficiency

bull Suppose N nodes with many frames to send each transmits in slot with probability p

bull Prob that given node has success in a slot = p(1-p)N-1

bull Prob that any node has a success = Np(1-p)N-1

bull Max efficiency find prsquo that maximizes Np(1-p)N-1

bull For many nodes take limit of Nprsquo(1-prsquo)N-1 as N goes to infinity gives

Max efficiency = 1e ~ 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Pure (Unslotted) ALOHAbull Unslotted Aloha simpler no synchronizationbull When frame first arrives

ndash Transmit immediately bull Collision probability increases

ndash Frame sent at t0 collides with other frames sent in [t0-1t0+1]

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Pure Aloha EfficiencyP(success by given node) = P(node transmits) P(no other node transmits in [p0-1p0]

P(no other node transmits in [p0-1p0] = p (1-p)N-1 (1-p)N-1

= p (1-p)2(N-1)

hellip choosing optimum p and then letting n -gt infty

= 1(2e) = 18

Even worse than slotted Aloha

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogy someone else talking Donrsquot interrupt

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

CSMA Collisions

Collisions can still occurPropagation delay means two nodes may not heareach otherrsquos transmission

CollisionEntire packet transmission time wasted

spatial layout of nodes

NoteRole of distance amp propagation delay in determining collision probability

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in

CSMAndash Collisions detected within short timendash Colliding transmissions aborted reducing

channel wastage bull Collision detection

ndash Easy in wired LANsbull Measure signal strengths compare transmitted

received signalsndash Difficult in wireless LANs

bull Received signal strength overwhelmed by local transmission strength

bull Human analogy the polite conversationalist

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

CSMACD (Collision Detection)

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash Share channel efficiently and fairly at high load

ndash Inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully

utilize channelndash High load collision overhead

ldquoTaking turnsrdquo protocolsndash Look for best of both worlds

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

ldquoTaking Turnsrdquo MAC protocolsPolling bull Master node

ldquoinvitesrdquo slave nodes to transmit in turn

bull Typically used with ldquodumbrdquo slave devices

bull Concernsndash Polling overhead ndash Latencyndash Single point of

failure (master)

master

slaves

poll

data

data

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

ldquoTaking Turnsrdquo MAC protocolsToken passingbull Control token

passed from one node to next sequentially

bull Token messagebull Concerns

- token overhead - latency- single point of failure

(token)

T

data

(nothingto send)

T

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Summary of MAC protocolsbull Channel partitioning

ndash Time Division Frequency Divisionbull Random access (dynamic)

ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull Taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

MAC Addresses

bull 32-bit IP address ndash Network-layer addressndash Used to get datagram to destination IP subnet

bull MAC (or LAN or physical or Ethernet) address ndash Function get frame from one interface to

another physically-connected interface (same network)

ndash 48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software

settable

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

LAN AddressesEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

LAN Address (more)bull MACLAN address allocation administered by

IEEEbull Manufacturer buys portion of MAC address

space (to assure uniqueness)bull Analogy (a) MAC address like Social Security Number (b) IP address like postal addressbull MAC flat address portability

ndash Can move LAN card from one LAN to anotherbull IP hierarchical address NOT portable

ndash Address depends on IP subnet to which node is attached

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

ARP Address Resolution Protocolbull Each IP node (host

router) on LAN has ARP table

bull ARP table IPMAC address mappings for some LAN nodes

IP address MAC address TTLndash TTL (Time To Live) time

after which address mapping will be forgotten (typically 20 min)

Question how to determineMAC address of Bknowing Brsquos IP address

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53 LAN

137196723

137196778

137196714

137196788

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

ARP Protocol Same LANbull A wants to send

datagram to B and Brsquos MAC address not in Arsquos ARP table

bull A broadcasts ARP query packet containing Bs IP address ndash dest MAC address =

FF-FF-FF-FF-FF-FFndash all machines on LAN

receive ARP query bull B receives ARP packet

replies to A with its (Bs) MAC addressndash frame sent to Arsquos MAC

address (unicast)

bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) ndash soft state information that times out (goes away)

unless refreshedbull ARP is ldquoplug-and-playrdquo

ndash nodes create their ARP tables without intervention from net administrator

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Addressing Routing to Another LAN

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

Walkthrough send datagram from A to B via R assume A knows Brsquos IP address

bull Two ARP tables in router R one for each IP network (LAN)

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

bull A creates IP datagram with source A destination B bull A uses ARP to get Rrsquos MAC address for 111111111110bull A creates link-layer frame with Rs MAC address as dest

frame contains A-to-B IP datagrambull Arsquos NIC sends frame bull Rrsquos NIC receives frame bull R removes IP datagram from Ethernet frame sees its

destined to Bbull R uses ARP to get Brsquos MAC address bull R creates frame containing A-to-B IP datagram sends to

B

R

1A-23-F9-CD-06-9B

222222222220111111111110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111111111112

111111111111

A74-29-9C-E8-FF-55

222222222221

88-B2-2F-54-1A-0F

B222222222222

49-BD-D2-C7-56-2A

This is a really importantexample ndash make sure youunderstand

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

EthernetDominant wired LAN technology bull Cheap ($20) for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Topology (Bus and Star)bull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in center (contrast with hub)ndash Each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

switch

bus coaxial cable star

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Ethernet Frame StructureSending adapter encapsulates IP datagram (or

other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011bull Used to synchronize receiver sender clock

rates

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash If adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP

but others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected

frame is dropped

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Ethernet Unreliable Connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can have

gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps

bull Ethernetrsquos MAC protocol unslotted CSMACD

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random

wait will be longerbull First collision choose K

from 01 delay is K 512 bit transmission times

bull After second collision choose K from 0123hellip

bull After ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

CSMACD Efficiencybull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame

bull Efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity

bull Better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

511

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bpsndash Different physical layer media fiber

cableapplicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twistedpair) physical layer

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Manchester Encoding

bull Used in 10BaseTbull Each bit has a transitionbull Allows clocks in sending and receiving nodes

to synchronize to each otherndash No need for a centralized global clock among nodes

bull Hey this is physical-layer stuff

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-layer

Addressingbull 55 Ethernet

bull 56 Link-layer switches LANs VLANs

bull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Hubshellip physical-layer (ldquodumbrdquo) repeaters

ndash bits coming in one link go out all other links at same rate

ndash all nodes connected to hub can collide with one another

ndash no frame bufferingndash no CSMACD at hub host NICs detect collisions

twisted pair

hub

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Switchbull Link-layer device smarter than hubs

take active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment

bull Transparentndash hosts are unaware of presence of switches

bull Plug-and-play self-learningndash switches do not need to be configured

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Switch Allows Multiple Simultaneous Transmissions

bull Hosts have dedicated direct connection to switch

bull Switches buffer packetsbull Ethernet protocol used on

each incoming link but no collisions full duplexndash Each link is its own collision

domainbull Switching A-to-Arsquo and B-

to-Brsquo simultaneously without collisions ndash Not possible with dumb hub

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Switch Tablebull Q how does switch know

that Arsquo reachable via interface 4 Brsquo reachable via interface 5

bull A each switch has a switch table each entryndash (MAC address of host

interface to reach host time stamp)

bull looks like a routing tablebull Q how are entries created

maintained in switch table ndash something like a routing

protocol

A

Arsquo

B

Brsquo

C

Crsquo

switch with six interfaces(123456)

1 2 345

6

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Switch Self-learningbull Switch learns which

hosts can be reached through which interfacesndash When frame received

switch ldquolearnsrdquo location of sender incoming LAN segment

ndash Records senderlocation pair in switch table

A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Switch Frame Filtering Forwarding

When frame received

1 Record link associated with sending host2 Index switch table using MAC dest address3 if entry found for destination

then if dest on segment from which frame arrived

then drop the frame else forward the frame on interface indicated else flood

forward on all but the interface on which the frame arrived

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Self-learning Forwarding

example A

Arsquo

B

Brsquo

C

Crsquo

1 2 345

6

A Arsquo

Source ADest Arsquo

MAC addr interface TTLSwitch table

(initially empty)A 1 60

A ArsquoA ArsquoA ArsquoA ArsquoA Arsquobull frame destination

unknownflood

Arsquo A

bull Destination A location known

Arsquo 4 60

selective send

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Interconnecting Switchesbull Switches can be connected together

AB

bull Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3

bull A self learning (works exactly the same as in single-switch case)

S1

C DE

FS2

S4

S3

HI

G

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Self-learning multi-switch example

Suppose C sends frame to I I responds to C

bull Q show switch tables and packet forwarding in S1 S2 S3 S4

AB

S1

C DE

FS2

S4

S3

HI

G

12

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Institutional Network

to externalnetwork

router

IP subnet

mail server

web server

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Switches vs Routersbull both store-and-forward devices

ndash routers network layer devices (examine network layer headers)

ndash switches are link layer devicesbull routers maintain routing tables implement

routing algorithmsbull switches maintain switch tables implement

filtering learning algorithms

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Data Link Layerbull 51 Introduction and

servicesbull 52 Error detection

and correction bull 53 Multiple access

protocolsbull 54 Link-Layer

Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

But First Elements of Wireless (WiFi)

bull Note some key characteristics of Wireless that differ from wired

bull 80211 (WiFi) as contrast to 8023 (Ethernet)

(Bits of Ch 61 ndash 63)

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Characteristics of Selected Wireless Link Standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor200m ndash 4 Km

Long-rangeoutdoor5Km ndash 20 Km

056

384

14

5-1154

IS-95 CDMA GSM 2G

UMTSWCDMA CDMA2000 3G

80215

80211b

80211ag

UMTSWCDMA-HSPDA CDMA2000-1xEVDO 3G cellularenhanced

80216 (WiMAX)

200 80211n

Dat

a ra

te (M

bps) data

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Wireless Link Characteristics (1)Differences from wired link hellip

ndash decreased signal strength radio signal attenuates as it propagates through matter (path loss)

ndash interference from other sources standardized wireless network frequencies (eg 24 GHz) shared by other devices (eg phone) devices (motors) interfere as well

ndash multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times

hellip make communication across (even a point to point) wireless link much more ldquodifficultrdquo

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Wireless Link Characteristics (2)bull SNR signal-to-noise ratio

ndash larger SNR ndash easier to extract signal from noise (a ldquogood thingrdquo)

bull SNR versus BER tradeoffsndash Given physical layer

increase power increase SNR decrease BER

ndash Given SNR choose physical layer that meets BER requirement giving highest thruputbull SNR may change with

mobility dynamically adapt physical layer (modulation technique rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)B

ER

10-1

10-2

10-3

10-5

10-6

10-7

10-4

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Wireless Network CharacteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access)

AB

C

Hidden terminal problembull B A hear each otherbull B C hear each otherbull A C can not hear each

othermeans A C unaware of their

interference at B

A B C

Arsquos signalstrength

space

Crsquos signalstrength

Signal attenuationbull B A hear each otherbull B C hear each otherbull A C can not hear each

other interfering at B

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

IEEE 80211 Wireless LANbull 80211b

ndash 24-5 GHz unlicensed spectrum

ndash up to 11 Mbpsndash direct sequence spread

spectrum (DSSS) in physical layerbull all hosts use same

chipping code

bull 80211a ndash 5-6 GHz rangendash up to 54 Mbps

bull 80211g ndash 24-5 GHz rangendash up to 54 Mbps

bull 80211n multiple antennaendash 24-5 GHz rangendash up to 200 Mbps

bull All use CSMACA for multiple accessbull All have base-station and ad-hoc network

versions

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

IEEE 80211 multiple accessbull Avoid collisions 2+ nodes transmitting at same

timebull 80211 CSMA - sense before transmitting

ndash donrsquot collide with ongoing transmission by other nodebull 80211 no collision detection

ndash difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

ndash canrsquot sense all collisions in any case hidden terminal fading

ndash goal avoid collisions CSMAC(ollision)A(voidance)

A B

CA B C

Arsquos signalstrength

space

Crsquos signalstrength

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff

interval repeat 280211 receiver- if frame received OK return ACK after SIFS (ACK needed due

to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

80211 Advanced CapabilitiesRate Adaptationbull Base station mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1 SNR decreases BER increase as node moves away from base station2 When BER becomes too high switch to lower transmission rate but with lower BER

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

More Wireless

bull Power managementbull Other protocols Zigbee 3G WiMax hellipbull Mobilitybull Security

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Link Layerbull 51 Introduction and servicesbull 52 Error detection and correction bull 53Multiple access protocolsbull 54 Link-Layer Addressingbull 55 Ethernet

bull 56 Link-layer switchesbull 57 PPPbull 58 Link virtualization

MPLSbull 59 A day in the life of

a web request

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Synthesis a day in the life of a Web request

bull Journey down protocol stack completendash Application Transport Network Data

Linkbull Putting-it-all-together synthesis

ndash goal identify review understand protocols (at all layers) involved in seemingly simple scenario requesting www page

ndash scenario student attaches laptop to campus network requestsreceives wwwgooglecom

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

A day in the life Scenario

Comcast network 68800013

Googlersquos network 64233160019 64233169105

web server

DNS server

school network 68802024

browser

web page

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

A day in the lifehellip connecting to the Internet

bull connecting laptop needs to get its own IP address addr of first-hop router addr of DNS server use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulated in UDP encapsulated in IP encapsulated in 8021 Ethernet Ethernet frame broadcast (dest FFFFFFFFFFFF) on LAN received at router running DHCP server

Ethernet demuxrsquoed to IP demuxrsquoed UDP demuxrsquoed to DHCP

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

A day in the lifehellip connecting to the Internet

bull DHCP server formulates DHCP ACK containing clientrsquos IP address IP address of first-hop router for client name amp IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

bull Encapsulation at DHCP server frame forwarded (switch learning) through LAN demultiplexing at client

Client now has IP address knows name amp addr of DNS server IP address of its first-hop router

bull DHCP client receives DHCP ACK reply

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

A day in the lifehellip ARP (before DNS before HTTP)

bull Before sending HTTP request need IP address of wwwgooglecom DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

bull DNS query created encapsulated in UDP encapsulated in IP encasulated in Eth In order to send frame to router need MAC address of router interface ARP

bull ARP query broadcast received by router which replies with ARP reply giving MAC address of router interfacebull Client now knows MAC address of first hop router so can now send frame containing DNS query

ARP query

EthPhy

ARP

ARP

ARP reply

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

A day in the lifehellip using DNS

DNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

DNS

bull IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

bull IP datagram forwarded from campus network into comcast network routed (tables created by RIP OSPF IS-IS andor BGP routing protocols) to DNS server

bull demuxrsquoed to DNS serverbull DNS server replies to

client with IP address of wwwgooglecom

Comcast network 68800013

DNS serverDNSUDP

IPEthPhy

DNS

DNS

DNS

DNS

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

A day in the lifehellip TCP connection carrying HTTP

HTTPTCPIP

EthPhy

HTTP

bull To send HTTP request client first opens TCP socket to web server

bull TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

bull TCP connection established

64233169105web server

SYN

SYN

SYN

SYN

TCPIPEthPhy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

bull Web server responds with TCP SYNACK (step 2 in 3-way handshake)

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

A day in the lifehellip HTTP requestreply HTTPTCPIP

EthPhy

HTTP

bull HTTP request sent into TCP socket

bull IP datagram containing HTTP request routed to wwwgooglecom

bull IP datgram containing HTTP reply routed back to client

64233169105web server

HTTPTCPIPEthPhy

bull Web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTPHTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

bull Web page finally () displayed

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Chapter 5 Summarybull Principles behind data link layer services

ndash error detection correctionndash sharing a broadcast channel multiple accessndash link layer addressing

bull Instantiation and implementation of various link layer technologiesndash Addressingndash Ethernetndash Switched LANS

bull Synthesis a day in the life of a web request

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath

Chapter 5 Letrsquos take a breathbull Journey down protocol stack complete

(except PHY)bull Solid understanding of networking

principles practicebull hellip could stop here hellip but lots of

interesting topicsndash Wirelessndash Multimediandash Security ndash Network management

• Data Link Layer CS 3516 ndash Computer Networks
• Chapter 5 The Data Link Layer
• Link Layer
• Link Layer Introduction
• Link Layer Context
• Link Layer Services
• Link Layer Services (more)
• Where is Link Layer Implemented
• Adaptors Communicating
• Data Link Layer
• Error Detection
• Simple - Parity Checking
• Internet Checksum (review)
• Checksumming Cyclic Redundancy Check (CRC)
• CRC Example ndash Choosing R
• CRC Standards
• Data Link Layer (2)
• Multiple Access Links and Protocols
• Multiple Access Protocols
• Ideal Multiple Access Protocol
• MAC Protocols a Taxonomy
• Channel Partitioning MAC protocols TDMA
• Channel Partitioning MAC protocols FDMA
• Random Access Protocols
• Slotted ALOHA
• Slotted ALOHA (2)
• Slotted Aloha Efficiency
• Pure (Unslotted) ALOHA
• Pure Aloha Efficiency
• CSMA (Carrier Sense Multiple Access)
• CSMA Collisions
• CSMACD (Collision Detection)
• CSMACD (Collision Detection) (2)
• ldquoTaking Turnsrdquo MAC protocols
• ldquoTaking Turnsrdquo MAC protocols (2)
• ldquoTaking Turnsrdquo MAC protocols (3)
• Summary of MAC protocols
• Data Link Layer (3)
• MAC Addresses
• LAN Addresses
• LAN Address (more)
• ARP Address Resolution Protocol
• ARP Protocol Same LAN
• Addressing Routing to Another LAN
• Slide 45
• Data Link Layer (4)
• Ethernet
• Topology (Bus and Star)
• Ethernet Frame Structure
• Ethernet Frame Structure (more)
• Ethernet Unreliable Connectionless
• Ethernet CSMACD algorithm
• Ethernetrsquos CSMACD (more)
• CSMACD Efficiency
• 8023 Ethernet Standards Link amp Physical Layers
• Manchester Encoding
• Data Link Layer (5)
• Hubs
• Switch
• Switch Allows Multiple Simultaneous Transmissions
• Switch Table
• Switch Self-learning
• Switch Frame Filtering Forwarding
• Self-learning Forwarding example
• Interconnecting Switches
• Self-learning multi-switch example
• Institutional Network
• Switches vs Routers
• Data Link Layer (6)
• But First Elements of Wireless (WiFi)
• Characteristics of Selected Wireless Link Standards
• Wireless Link Characteristics (1)
• Wireless Link Characteristics (2)
• Wireless Network Characteristics
• IEEE 80211 Wireless LAN
• IEEE 80211 multiple access
• IEEE 80211 MAC Protocol CSMACA
• Slide 78
• More Wireless
• Link Layer (2)
• Synthesis a day in the life of a Web request
• A day in the life Scenario
• A day in the lifehellip connecting to the Internet
• A day in the lifehellip connecting to the Internet (2)
• A day in the lifehellip ARP (before DNS before HTTP)
• A day in the lifehellip using DNS
• A day in the lifehellip TCP connection carrying HTTP
• A day in the lifehellip HTTP requestreply
• Chapter 5 Summary
• Chapter 5 Letrsquos take a breath