tcp/ip protocol suite 1 copyright © the mcgraw-hill companies, inc. permission required for...

Chapter 3

UnderlyingTechnology

TCP/IP Protocol Suite 2

OBJECTIVES:OBJECTIVES: To briefly discuss the technology of dominant wired LANs,

Ethernet, including traditional, fast, gigabit, and ten-gigabit Ethernet.

To briefly discuss the technology of wireless WANs, including IEEE 802.11 LANs, and Bluetooth.

To briefly discuss the technology of point-to-point WANs including 56K modems, DSL, cable modem, T-lines, and SONET.

To briefly discuss the technology of switched WANs including X.25, Frame Relay, and ATM.

To discuss the need and use of connecting devices such as repeaters (hubs), bridges (two-layer switches), and routers (three-layer switches).

Chapter Chapter OutlineOutline

3.1 Wired Local Area Network3.1 Wired Local Area Network

3.2 Wireless LANs3.2 Wireless LANs

3.3 Point-to-Point WANs3.3 Point-to-Point WANs

3.4 Switched WANs3.4 Switched WANs

3.5 Connecting Devices3.5 Connecting Devices

3-1 WIRED LOCAL AREA NETWORKS

A local area network (LAN) is a computer network that is designed for a limited geographic area such as a building or a campus. Although a LAN can be used as an isolated network to connect computers in an organization for the sole purpose of sharing resources, most LANs today are also linked to a wide area network (WAN) or the Internet. The LAN market has seen several technologies such as Ethernet, token ring, token bus, FDDI, and ATM LAN, but Ethernet is by far the dominant technology.

Topics Discussed in the SectionTopics Discussed in the Section

IEEE StandardsFrame FormatAddressingEthernet EvolutionStandard EthernetFast EthernetGigabit EthernetTen-Gigabit Ethernet

Figure 3.1 IEEE standard for LANs

Figure 3.2 Ethernet Frame

Figure 3.3 Maximum and minimum lengths

Minimum length: 64 bytes (512 bits)

Maximum length: 1518 bytes (12,144 bits)

Figure 3.4 Ethernet address in hexadecimal notation

Figure 3.5 Unicast and multicast addresses

multicast: 1unicast: 0

The broadcast destination address is a special case of the multicast address

in which all bits are 1s.

The least significant bit of the first byte defines the type of address.

If the bit is 0, the address is unicast; otherwise, it is multicast.

Define the type of the following destination addresses: a. 4A:30:10:21:10:1A b. 47:20:1B:2E:08:EE c. FF:FF:FF:FF:FF:FF

SolutionTo find the type of the address, we need to look at the secondhexadecimal digit from the left. If it is even, the address is unicast. If it is odd, the address is multicast. If all digits are F’s, the address is broadcast. Therefore, we have the following:a. This is a unicast address because A in binary is 1010 (even).b. This is a multicast address because 7 in binary is 0111 (odd).c. This is a broadcast address because all digits are F ’s.

ExampleExample 3.1

Show how the address 47:20:1B:2E:08:EE is sent out on line.

SolutionThe address is sent left-to-right, byte by byte; for each byte, it is sent right-to-left, bit by bit, as shown below:

ExampleExample 3.2

← 11100010 00000100 11011000 01110100 00010000 01110111

Figure 3.6 Ethernet evolution through four generations

Figure 3.7 Space/time model of a collision in CSMA

Time Time

BA C D

B startsat time t1

Area whereA’s signal exists

C startsat time t2

Area whereB’s signal exists

Area whereboth signals exist

Figure 3.8 Collision of the first bit in CSMA/CD

In the standard Ethernet, if the maximum

propagation time is 25.6 μs, what is the minimum

size of the frame?

Solution

The frame transmission time is Tfr = 2 × Tp = 51.2

μs. This means, in the worst case, a station needs

to transmit for a period of 51.2 μs to detect the

collision. The minimum size of the frame is 10

Mbps × 51.2 μs = 512 bits or 64 bytes. This is

actually the minimum size of the frame for

Standard Ethernet, as we discussed before.

ExampleExample 3.3

Figure 3.9 CSMA/CD flow diagram

Figure 3.10 Standard Ethernet implementation

Figure 3.11 Fast Ethernet implementation

In the full-duplex mode of Gigabit Ethernet, there is no collision;

the maximum length of the cable is determined by the signal attenuation

in the cable.

Figure 3.12 Gigabit Ethernet implementation

3-2 WIRELESS LANS

Wireless communication is one of the fastest growing technologies. The demand for connecting devices without the use of cables is increasing everywhere. Wireless LANs can be found on college campuses, in office buildings, and in many public areas. In this section, we concentrate on two wireless technologies for LANs: IEEE 802.11 wireless LANs, sometimes called wireless Ethernet, and Bluetooth, a technology for small wireless LANs.

IEEE 802.1MAC SublayerAddressing MechanismBluetooth

Figure 3.13 Basic service sets (BSSs)

Figure 3.14 Extended service sets (ESSs)

Figure 3.15 CSMA/CA flow diagram

All other stations

• • •

Source Destination

TimeTime Time Time

Figure 3.16 CSMA/CA and NAV

NAV(No carrier sensing)

Figure 3.17 Frame format

Figure 3.18 Control frames

Figure 3.19 Hidden station problem

The CTS frame in CSMA/CA handshake can prevent collision from a hidden

station.

Time Time Time

Figure 3.20 Use of handshaking to prevent hidden station problem

CTS CTS

Figure 3.21 Exposed station problem

Figure 3.22 Use of handshaking in exposed station problem

RTS RTSRTS

DataData

RTSRTS

Collisionhere

3-5 CONNECTING DEVICES

LANs or WANs do not normally operate in isolation. They are connected to one another or to the Internet. To connect LANs and WANs together we use connecting devices. Connecting devices can operate in different layers of the Internet model. We discuss three kinds of connecting devices: repeaters (or hubs), bridges (or two-layer switches), and routers (or three-layer switches).

RepeatersBridgesRouters

Figure 3.40 Connecting devices

Figure 3.41 Repeater or hub

SentDiscarded

Maintained DiscardedDiscarded

A repeater forwards every bit; it has no filtering capability.

A bridge has a table used in filtering decisions.

A bridge does not change the physical (MAC) addresses in a frame.

Figure 3.42 Bridge

71:2B:13:45:61:41 1

43271:2B:13:45:61:42

64:2B:13:45:61:1264:2B:13:45:61:13

Address Port

Bridge table

Figure 3.43 Learning bridge

Gradual building of Table

a. Original

Address Port

c. After D sends a frame to B

71:2B:13:45:61:41 1464:2B:13:45:61:13

Address Port

d. After B sends a frame to A

71:2B:13:45:61:41 14

271:2B:13:45:61:42

64:2B:13:45:61:13

Address Port

e. After C sends a frame to D

71:2B:13:45:61:41 14

3271:2B:13:45:61:42

64:2B:13:45:61:12

64:2B:13:45:61:13

Address Port

M MM M

A router is a three-layer (physical, data link, and network) device.

A repeater or a bridge connects segments of a LAN.

A router connects independent LANs or WANs to create an internetwork

(internet).

Figure 3.44 Routing example

A router changes the physical addresses in a packet.

Ethernet Supplement

Ethernet

First practical local area network, built at Xerox PARC in 70’s

“Dominant” LAN technology: Cheap Kept up with speed race: 10, 100, 1000 Mbps

Metcalfe’s Ethernetsketch

Ethernet MAC – Carrier Sense Basic idea:

Listen to wire before transmission

Avoid collision with active transmission

Why didn’t ALOHA have this? In wireless, relevant

contention at the receiver, not sender

Hidden terminal Exposed terminal

Chicago

St.Louis

Chicago

Hidden Exposed

Ethernet MAC – Collision Detection But: ALOHA has collision detection also?

That was very slow and inefficient Basic idea:

Listen while transmitting If you notice interference assume collision

Why didn’t ALOHA have this? Very difficult for radios to listen and transmit Signal strength is reduced by distance for radio

Much easier to hear “local, powerful” radio station than one in NY

You may not notice any “interference”

Ethernet MAC (CSMA/CD)

Packet?

Sense Carrier

Discard Packet

Send Detect Collision

Jam channel b=CalcBackoff()

; wait(b);attempts++;

attempts < 16

attempts == 16

Carrier Sense Multiple Access/Collision Detection

Ethernet CSMA/CD: Making it word

Jam Signal: make sure all other transmitters are aware of collision; 48 bits;

Exponential Backoff: If deterministic delay after collision, collision

will occur again in lockstep Why not random delay with fixed mean?

Few senders needless waiting Too many senders too many collisions

Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer

Ethernet Backoff Calculation

Exponentially increasing random delay Infer senders from # of collisions More senders increase wait time

First collision: choose K from {0,1}; delay is K x 512 bit transmission times

After second collision: choose K from {0,1,2,3}…

After ten or more collisions, choose K from {0,1,2,3,4,…,1023}

Outline

Ethernet MAC

Collisions

Ethernet Frames

CollisionsT

Minimum Packet Size

What if two people sent really small packets How do you find

collision?

Ethernet Collision Detect

Min packet length > 2x max prop delay If A, B are at opposite sides of link, and B

starts one link prop delay after A Jam network for 32-48 bits after collision,

then stop sending Ensures that everyone notices collision

End to End Delay

c in cable = 60% * c in vacuum = 1.8 x 10^8 m/s Modern 10Mb Ethernet

2.5km, 10Mbps ~= 12.5us delay +Introduced repeaters (max 5 segments) Worst case – 51.2us round trip time!

Slot time = 51.2us = 512bits in flight After this amount, sender is guaranteed sole access to

link 51.2us = slot time for backoff

Packet Size

What about scaling? 3Mbit, 100Mbit, 1Gbit... Original 3Mbit Ethernet did not have minimum

packet size Max length = 1Km and No repeaters

For higher speeds must make network smaller, minimum packet size larger or both

What about a maximum packet size? Needed to prevent node from hogging the network 1500 bytes in Ethernet

10BaseT and 100BaseT

10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair (wiring) Minimum packet size requirement

Make network smaller solution for 100BaseT

Gbit Ethernet

Minimum packet size requirement Make network smaller?

512bits @ 1Gbps = 512ns 512ns * 1.8 * 10^8 = 92meters = too small !!

Make min pkt size larger! Gigabit Ethernet uses collision extension for small pkts and

backward compatibility

Maximum packet size requirement 1500 bytes is not really “hogging” the network Defines “jumbo frames” (9000 bytes) for higher

efficiency

Outline

Ethernet MAC

Collisions

Ethernet Frames

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Ethernet Frame Structure (cont.) Preamble: 8 bytes

101010…1011 Used to synchronize receiver, sender clock

rates CRC: 4 bytes

Checked at receiver, if error is detected, the frame is simply dropped

Ethernet Frame Structure (cont.)

Each protocol layer needs to provide some hooks to upper layer protocols Demultiplexing: identify which upper layer

protocol packet belongs to E.g., port numbers allow TCP/UDP to identify

target application Ethernet uses Type field

Type: 2 bytes Indicates the higher layer protocol, mostly IP but

others may be supported such as Novell IPX and AppleTalk)

Addressing Alternatives

Broadcast all nodes receive all packets Addressing determines which packets are kept and which

are packets are thrown away Packets can be sent to:

Unicast – one destination Multicast – group of nodes (e.g. “everyone playing Quake”) Broadcast – everybody on wire

Dynamic addresses (e.g. Appletalk) Pick an address at random Broadcast “is anyone using address XX?” If yes, repeat

Static address (e.g. Ethernet)

Ethernet Frame Structure (cont.)

Addresses: 6 bytes Each adapter is given a globally unique address at

manufacturing time Address space is allocated to manufacturers

24 bits identify manufacturer E.g., 0:0:15:* 3com adapter

Frame is received by all adapters on a LAN and dropped if address does not match

Special addresses Broadcast – FF:FF:FF:FF:FF:FF is “everybody” Range of addresses allocated to multicast

Adapter maintains list of multicast groups node is interested in

Why Did Ethernet Win?

Failure modes Token rings – network unusable Ethernet – node detached

Good performance in common case Deals well with bursty traffic Usually used at low load

Volume lower cost higher volume …. Adaptable

To higher bandwidths (vs. FDDI) To switching (vs. ATM)

Easy incremental deployment Cheap cabling, etc

And .. It is Easy to Manage

You plug in the host and it basically works No configuration at the datalink layer Today: may need to deal with security

Protocol is fully distributed Broadcast-based.

In part explains the easy management Some of the LAN protocols (e.g. ARP) rely on

broadcast Networking would be harder without ARP

Not having natural broadcast capabilities adds complexity to a LAN

Example: ATM

Ethernet Problems: Unstable at High Load Peak throughput worst with

More hosts – more collisions to identify single sender Smaller packet sizes – more frequent arbitration Longer links – collisions take longer to observe, more wasted

bandwidth But works well in

practice Can improve

efficiency by avoiding

above conditions

ut ”

G = offered load = N X p0.5 1.0 1.5 2.0

Pure Aloha

Slotted Aloha

1/e = 37%

Summary

CSMA/CD carrier sense multiple access with collision detection Why do we need exponential backoff? Why does collision happen? Why do we need a minimum packet size?

How does this scale with speed?

Ethernet What is the purpose of different header fields? What do Ethernet addresses look like?

What are some alternatives to Ethernet design?

tcp/ip protocol suite 1 copyright © the mcgraw-hill companies, inc. permission required for...

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