1.- lan basics
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Local Area Networks/School of Engineering in Computer Science/2009-2010
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/1.- LAN basics
Networking basics The Internet TCP/IP
LANs topologiesMedia Access Control (MAC) techniques
Local Area Networks/School of Engineering in Computer Science/2009-2010
http
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rali
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/1.- LAN basics
Networking basics
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Goals of computer networks
to provide ubiquitous access to shared resources (e.g., printers, databases, file systems...),
to allow remote users to communicate (e.g., email, IP telephony),
to do transactions (banking, e-commerce, stock trading), and…
… save money: downsizing
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A “nuts and bolts” view of a network
Millions of connected computing devices: hosts, end-systems pc’s workstations, servers PDA’s phones, toastersrunning network apps
communication links fiber, copper, radio, satellite
routers: forward packets (chunks) of data thru network
protocols: control sending, receiving of msgs TCP, IP, and HTTP, FTP, PPP,
…
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local ISP
companynetwork
regional ISP
router workstationserver mobile
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A closer look at the network structure
1. The network edge: applications and hosts
2. The network core: routers network of networks
3. The access networks and physical media: communication links
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The network edge
End systems (hosts): run application programs
at the “edge of network” client/server model
client host requests, receives service from server
e.g., WWW client (browser)/ server; email client/server
peer-peer model: host interaction symmetric e.g.: Gnutella, KaZaA
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The network core
Mesh of interconnected routers The fundamental question: how
is data transferred through net? Circuit switching:
dedicated circuit per call: telephone net
Packet switching: data sent through the network in discrete “chunks”
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The network core: Circuit switching
End-end resources reserved for “call”
Characterizing parameters: link bandwidth, switch capacity
dedicated resources: no sharing
circuit-like (guaranteed) performance
call setup required
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The network core: Packet switching
Data traffic divided into packets Each packet contains a header (with address)
Packets travel separately through network Packet forwarding based on the header Network nodes may store packets temporarily
Destination reconstructs the message
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The network core: Packet switching (routing)
Goal: move packets among routers from source to destination
datagram network: destination address determines next hop routes may change during session analogy: driving, asking directions
virtual circuit network: each packet carries tag (virtual circuit ID), tag determines next
hop fixed path determined at call setup time, remains fixed thru call routers maintain per-call state
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The access networks and physical media
How to connect end systems to edge router? Residential access networks Institutional access
networks (school, company) Wireless access networks
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Residential access networks: point to point access
Dialup via modem up to 56Kbps direct access
to router (conceptually) ISDN: integrated services digital
network: 128Kbps all-digital connect to router
ADSL: asymmetric digital subscriber line up to 1 Mbps home-to-
router up to 8 Mbps router-to-
home ADSL deployment:
happening HFC: hybrid fiber coax
asymmetric: up to 10Mbps upstream, 1 Mbps downstream
network of cable and fiber attaches homes to ISP router
shared access to router among home
issues: congestion, dimensioning
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Residential access networks: cable modems
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Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
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Institutional access networks: local area networks
company/univ local area network (LAN) connects end system to edge router
Ethernet: shared or dedicated cable
connects end system and router
10 Mbs, 100Mbps, Gigabit Ethernet
deployment: institutions, home LANs happening now
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Wireless access networks
Shared wireless access network connects end system to router
Wireless LANs: radio spectrum replaces wire e.g., WiFi, Bluetooth, WiMAX
Wireless WANs: GPRS/EDGE over GSM High-Speed Downlink Packet
Access (HSDPA) a 3G (third generation) mobile telephony communications based on Universal Mobile Telecommunications System (UMTS) networks.
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basestation
mobilehosts
router
Local Area Networks/School of Engineering in Computer Science/2009-2010
http
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rali
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/1.- LAN basics
Networking basics The Internet
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Internet structure: network of networks
Roughly hierarchical National/international
backbone providers (NBPs) e.g. BBN/GTE, Sprint, AT&T,
IBM, UUNet interconnect (peer) with each
other privately, or at public Network Access Point (NAPs)
A point of presence (POP) is a machine that is connected to the Internet.
Internet Service Providers (ISPs) provide dial-up or direct access to POPs. regional ISPs
connect into NBPs local ISP, company
connect into regional ISPs
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NBP A
NBP BNAP NAP
regional ISP
regional ISP
localISP
localISP
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Network Access Points (NAPs)
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Source: Boardwatch.com
Note: Peers in this context are commercial backbones.
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MCI/WorldCom/UUNET Global Backbone
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Source: Boardwatch.com
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The situation in Europe
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See: http://www.redes.upv.es/ralir/en/MforS/GEANT2.WMVAlso: http://video.google.com/googleplayer.swf?docId=-4949195951027294198&hl=en-GBMore about technolgies: http://video.google.com/googleplayer.swf?docId=-4634094763983277329&hl=en-GB
Local Area Networks/School of Engineering in Computer Science/2009-2010
http
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/1.- LAN basics
Networking basics TCP/IP
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A simple TCP/IP Example
A user on host argon.tcpip-lab.edu (“Argon”) makes a web access to URL
http://neon.tcpip-lab.edu/index.html.
What actually happens in the network?
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argon.tcpip-lab.edu("Argon")
neon.tcpip-lab.edu("Neon")
Web request
Web page
Web client Web server
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HTTP Request and HTTP response
Web browser runs an HTTP client program Web server runs an HTTP server program HTTP client sends an HTTP request to HTTP server HTTP server responds with HTTP response
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HTTP client
Argon
HTTP server
Neon
HTTP requestHTTP response
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HTTP Request
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GET /index.html HTTP/1.1
Accept: image/gif, */*
Accept-Language: en-us
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0
Host: neon.tcpip-lab.edu
Connection: Keep-Alive
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HTTP Response
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HTTP/1.1 200 OKDate: Sat, 25 May 2002 21:10:32 GMTServer: Apache/1.3.19 (Unix) Last-Modified: Sat, 25 May 2002 20:51:33 GMTETag: "56497-51-3ceff955"Accept-Ranges: bytesContent-Length: 81Keep-Alive: timeout=15, max=100Connection: Keep-AliveContent-Type: text/html <HTML><BODY><H1>Internet Lab</H1>Click <a href="http://www.tcpip-lab.net/index.html">here</a> for the Internet Lab webpage.</BODY></HTML>
• How does the HTTP request get from Argon to Neon ?
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From HTTP to TCP
To send a request, the HTTP client program establishes an TCP connection to the HTTP server at Neon.
The HTTP server at Neon has a TCP server running
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HTTP client
TCP client
Argon
HTTP server
TCP server
Neon
HTTP request / HTTP response
TCP connection
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Resolving hostnames and port numbers
Since TCP does not work with hostnames and also does not know how to find the HTTP server program at Neon, two things must happen:
1. The name “neon.tcpip-lab.edu” must be translated into a 32-bit IP address.
2. The HTTP server at Neon must be identified by a 16-bit port number.
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Translating a hostname into an IP address
The translation of the hostname neon.tcpip-lab.edu into an IP address is done via a database lookup
The distributed database used is called the Domain Name System (DNS)
All machines on the Internet have an IP address:argon.tcpip-lab.edu
128.143.137.144neon.tcpip-lab.edu 128.143.71.21
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HTTP client DNS Server
argon.tcpip-lab.edu 128.143.136.15
neon.tcpip-lab.edu
128.143.71.21
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Finding the port number
Note: Most services on the Internet are reachable via well-known ports. E.g. All HTTP servers on the Internet can be reached at port number “80”.
So: Argon simply knows the port number of the HTTP server at a remote machine.
On most Unix systems, the well-known ports are listed in a file with name /etc/services. The well-known port numbers of some of the most popular services are:
ftp 21 finger 79telnet 23 http 80smtp 25 nntp 119
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Requesting a TCP Connection
The HTTP client at argon.tcpip-lab.edu requests the TCP client to establish a connection to port 80 of the machine with address 128.141.71.21
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HTTP client
TCP client
argon.tcpip-lab.edu
Establish a TCP connectionto port 80 of 128.143.71.21
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Invoking the IP Protocol
The TCP client at Argon sends a request to establish a connection to port 80 at Neon
This is done by asking its local IP module to send an IP datagram to 128.143.71.21
(The data portion of the IP datagram contains the request to open a connection)
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TCP client
argon.tcpip-lab.edu
IP
Send an IP datagram to128.143.71.21
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Sending the IP datagram to an IP router
Argon (128.143.137.144) can deliver the IP datagram directly to Neon (128.143.71.21), only if it is on the same IP network (sometimes called “subnet”).
But Argon and Neon are not on the same IP network (Q: How does Argon know this?)
So, Argon sends the IP datagram to its default gateway The default gateway is an IP router The default gateway for Argon is Router137.tcpip-
lab.edu (128.143.137.1).
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The route from Argon to Neon
Note that the gateway has a different name for each of its interfaces.
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neon.tcpip-lab.edu"Neon"
128.143.71.21
argon.tcpip-lab.edu"Argon"128.143.137.144
router137.tcpip-lab.edu"Router137"
128.143.137.1
router71.tcpip-lab.edu"Router71"128.143.71.1
Ethernet NetworkEthernet Network
Router
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Finding the MAC address of the gateway
To send an IP datagram to Router137, Argon puts the IP datagram in an Ethernet frame, and transmits the frame.
However, Ethernet uses different addresses, so-called Media Access Control (MAC) addresses (also called: physical address, hardware address)
Therefore, Argon must first translate the IP address 128.143.137.1 into a MAC address.
The translation of addressed is performed via the Address Resolution Protocol (ARP)
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Address resolution with ARP
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argon.tcpip-lab.edu128.143.137.14400:a0:24:71:e4:44
ARP message: What is the MACaddress of 128.143.137.1?
ARP message: IP address 128.143.137.1belongs to MAC address 00:e0:f9:23:a8:20
router137.tcpip-lab.edu128.143.137.100:e0:f9:23:a8:20
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Invoking the device driver
The IP module at Argon, tells its Ethernet device driver to send an Ethernet frame to address 00:e0:f9:23:a8:20
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argon.tcpip-lab.edu
IP module
Ethernet
Send an Ethernet frameto 00:e0:f9:23:a8:20
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Sending an Ethernet frame
The Ethernet device driver of Argon sends the Ethernet frame to the Ethernet network interface card (NIC)
The NIC sends the frame onto the wire
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argon.tcpip-lab.edu128.143.137.14400:a0:24:71:e4:44
IP Datagram for Neon
router137.tcpip-lab.edu128.143.137.100:e0:f9:23:a8:20
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Forwarding the IP datagram
The IP router receives the Ethernet frame at interface 128.143.137.1, recovers the IP datagram and determines that the IP datagram should be forwarded to the interface with name 128.143.71.1
The IP router determines that it can deliver the IP datagram directly
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neon.tcpip-lab.edu"Neon"
128.143.71.21
argon.tcpip-lab.edu"Argon"128.143.137.144
router137.tcpip-lab.edu"Router137"
128.143.137.1
router71.tcpip-lab.edu"Router71"128.143.71.1
Ethernet NetworkEthernet Network
Router
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Another lookup of a MAC address
The router needs to find the MAC address of Neon. Again, ARP is invoked, to translate the IP address of
Neon (128.143.71.21) into the MAC address of neon (00:20:af:03:98:28).
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ARP message: What is the MACaddress of 128.143.71.21?
ARP message: IP address 128.143.71.21belongs to MAC address 00:20:af:03:98:28
neon.tcpip-lab.edu128.143.71.21
00:20:af:03:98:28
router71.tcpip-lab.edu128.143.71.1
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Invoking the device driver at the router
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The IP protocol at Router71, tells its Ethernet device driver to send an Ethernet frame to address 00:20:af:03:98:28
router71.tcpip-lab.edu
IP module
Ethernet
Send a frame to00:20:af:03:98:28
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Sending another Ethernet frame
The Ethernet device driver of Router71 sends the Ethernet frame to the Ethernet adapter, which transmits the frame onto the wire.
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IP Datagram for Neon
neon.tcpip-lab.edu128.143.71.21
00:20:af:03:98:28
router71.tcpip-lab.edu128.143.71.1
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Data has arrived at Neon
Neon receives the Ethernet frame The payload of the Ethernet frame is
an IP datagram which is passed to the IP protocol.
The payload of the IP datagram is a TCP segment, which is passed to the TCP server
Note: Since the TCP segment is a connection request (SYN), the TCP protocol does not pass data to the HTTP program for this packet. Instead, the TCP protocol at neon will respond with a SYN segment to Argon.
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HTTP server
Neon.cerf.edu
TCP server
IP module
Ethernet
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Wrapping-up the example
So far, Neon has only obtained a single packet Much more work is required to establish an actual TCP
connection and the transfer of the HTTP Request
The example was simplified in several ways: No transmission errors The route between Argon and Neon is short (only one IP router) Argon knew how to contact the DNS server (without routing or address resolution) ….
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Local Area Networks/School of Engineering in Computer Science/2009-2010
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/1.- LAN basics
LANs topologies
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LAN basics
A local area network is a communication network that interconnects a variety of data devices within a small geographic area and broadcasts data at high data transfer rates with very low error rates.
They are typically private Since the local area network first appeared in the 1970s, its
use has become widespread in commercial and academic environments.
Functions of a LAN: a few examples File server - A large storage disk drive that acts as a central storage
repository. Print server - Provides the authorization to access a particular printer,
accept and queue print jobs, and provides a user access to the print queue to perform administrative duties.
Interconnection - A LAN can provide an interconnection to other LANs and to wide area networks
Manufacturing support - LANs can support manufacturing and industrial environments.
Distributed processing - LANs can support network operating systems which perform the operations of distributed processing.
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LAN Selection Criteria
Cost For most of us, cost is an overriding constraint, and you must
choose the best solution within your budget. Usually, cost is the most inflexible constraint under which you must operate, and in the final analysis the LAN must be a cost-effective solution to your problem.
Number of Workstations Each LAN is physically capable of supporting some maximum
number of workstations. If you exceed that maximum number, you must make some provision for extending the maximum number.
Number of Concurrent Users / type of use As the number of concurrent users goes up, so does the LAN
workload. As the LAN workload increases, you have two basic choices: You can allow system responsiveness to decrease, or you can increase the work potential of the system.
Many concurrent users may increase the LAN workload.
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LAN Selection Criteria (cont.)
Distance and Medium Attaining high speed over long distances can be very expensive.
Thus, each LAN has a maximum distance it can cover. Speed
It is important to you select a LAN capable of meeting your performance goals. Available LAN speeds are 10, 100, and 1,000 Mbps, and the trend is for increasing speeds.
Device connectivity Some organizations need to attach special devices to the LAN, for
example, a plotter or scanner. LAN interfaces for such devices may not be available on some LANs or on some LAN file servers.
Connectivity to Other Networks A variety of connection capabilities exist, but a given LAN may not
support all of them. Adherence to Established Standards
There are several standards for LAN implementation. Some LANs conform to these standards whereas others do not.
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Simple LAN Topologies
Physical topology: Physical layout of a network Bus topology consists of a single cable—called a bus—
connecting all nodes on a network without intervening connectivity devices
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Simple LAN Topologies
Ring topology Each node is connected to the two nearest nodes so the entire
network forms a circle Active topology
Each workstation transmits data Each workstation functions as a repeater
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Simple LAN Topologies
Star topology Every node on the network is connected through a central device
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Hybrid LAN Topologies
Hybrid topology Complex combination of the simple physical topologies
Star-wired ring Star-wired topologies use physical layout of a star in conjunction
with token ring-passing data transmission method
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Hybrid LAN Topologies
Star-wired bus In a star-wired bus topology, groups of workstations are star-
connected to hubs and then networked via a single bus
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Hybrid LAN Topologies
Daisy-Chained Daisy chain is linked series of devices
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Hybrid LAN Topologies
Hierarchical Uses layers to separate devices by their priority or function
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The UPV extended LAN
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Local Area Networks/School of Engineering in Computer Science/2009-2010
http
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/1.- LAN basics
Media Access Control (MAC) techniques
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Media Access Control (MAC)
single shared communication channel two or more simultaneous transmissions by nodes:
interference only one node can send successfully at a time
Media Access Control: distributed algorithm that determines how stations share channel,
i.e., determine when a station can transmit communication about channel sharing must use channel itself! Takes also care of:
Assembly of data into frame with address and error detection fields Disassembly of frame
Address recognition Error detection
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Media Access Control (MAC)
For the same LLC, several MAC options may be available
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MAC Protocols: a taxonomy
Three broad classes: Channel Partitioning
divide channel into smaller “pieces” (time slots, frequency) allocate piece to node for exclusive use
Random Access allow collisions “recover” from collisions
“Taking turns” tightly coordinate shared access to avoid collisions
Goal: efficient, fair, simple, decentralized
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Channel Partitioning MAC protocols TDMA
TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans
time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
inefficient with low duty cycle users and at light load.
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Channel Partitioning MAC protocolsFDMA
FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands
2,5,6 idle
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frequ
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Random Access MAC protocols
When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes
two or more transmitting nodes -> “collision”, random access MAC protocol specifies:
how to detect collisions how to recover from collisions (e.g., via delayed retransmissions)
Examples of random access MAC protocols: pure ALOHA slotted ALOHA CSMA and CSMA/CD
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Random Access MAC protocols Pure (unslotted) ALOHA
unslotted Aloha: simpler, no synchronization pkt needs transmission:
send without awaiting for beginning of slot collision probability increases:
pkt sent at t0 collide with other pkts sent in [t0 -1, t0 +1]
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Random Access MAC protocols Slotted Aloha
time is divided into equal size slots (= pkt trans. time) node with new arriving pkt: transmit at beginning of next
slot if collision: retransmit pkt in future slots with probability
p, until successful.
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Success (S), Collision (C), Empty (E) slots
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Random Access MAC protocols CSMA: Carrier Sense Multiple Access
CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission
Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability)
Non-persistent CSMA: retry after random interval human analogy: don’t interrupt others!
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Random Access MAC protocols CSMA collisions
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collisions can occur:propagation delay means two nodes may not hear each other’s transmissioncollision:entire packet transmission time wasted
spatial layout of nodes along ethernet
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“Taking Turns” MAC protocols
“taking turns” protocols look for best of both worlds, because: Channel partitioning MAC protocols:
share channel efficiently at high load inefficient at low load: delay in channel access, 1/N bandwidth
allocated even if only 1 active node! Random access MAC protocols
efficient at low load: single node can fully utilize channelhigh load: collision overhead
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“Taking Turns” MAC protocols
Polling: master node “invites” slave
nodes to transmit in turn Request to Send, Clear to Send
msgs concerns:
polling overhead latency single point of failure (master)
Token passing: control token passed from one
node to next sequentially. token message concerns:
token overhead latency single point of failure (token)
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