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LAN technologies

LAN technologies 21
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Dr.ShivJindal,India,Teacher
Published Date:19-07-2017
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Computer Communication Networks (CCN) Chapter 5: Data Link Layer: Part II 1Summary of MAC protocols • What do you do with a shared media? • Channel Partitioning: time, frequency or code • Time Division,Code Division, Frequency Division • Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD • carrier sensing: easy in some technoligies (wire), hard in others (wireless) • CSMA/CD used in Ethernet • Taking Turns • polling from a central cite, token passing Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 2LAN technologies Data link layer so far: • services, error detection/correction, multiple access Next: LAN technologies • addressing • Ethernet • hubs, bridges, switches • 802.11 • PPP • ATM Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 3LAN Addresses and ARP - 1 32-bit IP address: • network-layer address • used to get datagram to destination network (recall IP network definition) LAN (or MAC or physical) address: • used to get datagram from one interface to another physically-connected interface (same network) • 48 bit MAC address (for most LANs) burned in the adapter ROM Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 4LAN Addresses and ARP - 2 Each adapter on LAN has unique LAN address Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 5LAN Address (more) • MAC address allocation administered by IEEE • manufacturer buys portion of MAC address space (to assure uniqueness) • Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address • MAC flat address = portability • can move LAN card from one LAN to another • IP hierarchical address NOT portable • depends on network to which one attaches Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 6Recall earlier routing discussion A Starting at A, given IP 223.1.1.1 223.1.2.1 datagram addressed to B: 223.1.1.2 223.1.2.9 223.1.1.4 • look up net. address of B, B 223.1.2.2 E 223.1.1.3 223.1.3.27 find B on same net. as A 223.1.3.2 223.1.3.1 • link layer send datagram to B inside link-layer frame frame source, datagram source, dest address dest address A’s IP B’s IP B’s MAC A’s MAC IP payload addr addr addr addr datagram frame Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 7ARP: Address Resolution Protocol Question: how to • Each IP node (Host, Router) determine on LAN has ARP module, MAC address of B table given B’s IP address? • ARP Table: IP/MAC address mappings for some LAN nodes IP address; MAC address; TTL ………………………….. • TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 8ARP protocol • A knows B's IP address, wants to learn physical address of B • A broadcasts ARP query pkt, containing B's IP address • all machines on LAN receive ARP query • B receives ARP packet, replies to A with its (B's) physical layer address • A caches (saves) IP-to-physical address pairs until information becomes old (times out) • soft state: information that times out (goes away) unless refreshed Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 9Routing to another LAN - 1 Walkthrough: routing from A to B via R Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 10Routing to another LAN - 2 • A creates IP packet with source A, destination B • A uses ARP to get R’s physical layer address for 111.111.111.110 • A creates Ethernet frame with R's physical address as dest, Ethernet frame contains A-to-B IP datagram • A’s data link layer sends Ethernet frame Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 11Routing to another LAN - 3 • R’s data link layer receives Ethernet frame • R removes IP datagram from Ethernet frame, sees its destined to B • R uses ARP to get B’s physical layer address • R creates frame containing A-to-B IP datagram sends to B Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 12Ethernet “Dominant” LAN technology: • Cheap 20 for 100Mbs • First widely used LAN technology • Simpler, cheaper than token LANs and ATM • Kept up with speed race: 10, 100, 1000 Mbps Metcalfe’s Ethernet sketch Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 13Ethernet Frame Structure - 1 Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: • 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 • Used to synchronize receiver, sender clock rates Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 14Ethernet Frame Structure - 2 • Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match • Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) • CRC: checked at receiver, if error is detected, the frame is simply dropped Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 15Ethernet • Ethernet uses 1-persistent CSMA/CD on coaxial cable at 10 Mbps (802.3 allows other speeds & media) • The maximum cable length allowed: 500m • Longer distances covered using repeaters to connect multiple “segments” of cable • No two stations can be separated by more than 2500 meters and 4 repeaters • Including the propagation delay for 2500m and the store and forward delay in 4 repeaters, the maximum time for a bit to travel between any two stations is  =25.6se (one way) max Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 16Ethernet: uses CSMA/CD A: sense channel, if idle then transmit and monitor the channel; If detect another transmission then abort and send jam signal; update collisions; delay as required by exponential backoff algorithm; goto A else done with the frame; set collisions to zero else wait until ongoing transmission is over and goto A Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 17Ethernet’s CSMA/CD • In order to ensure that every collision i s “heard" by all stations, when a station detects a collision , it jams the channel for • Example • Two stations, A and B, are close together • A third station, C, is far away • A and B will detect each other’s transmission very quickly and shut off • This will only cause a short blip which may not be detected by C but will still cause enough errors to destroy C’s packet Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 18Ethernet’s CSMA/CD • When collisions occur, Ethernet uses a random retransmission scheme called exponential backoff: 1.If your packet is in a collision, set K=2 2.Pick a number k at random from 0, 1,..,K-1 3.After  seconds, sense channel, transmit if max idle 4.If collision occurs, let K=2 x K, go to step 2 • After 10 repeats, stop doubling K • After 16, give up and tell layer above “I give up” • “Fixes” random access stability problem by passing it to the layer above Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 19Ethernet Technologies: 10Base2 • 10: 10Mbps; 2: under 200 meters max cable length • Thin coaxial cable in a bus topology • Repeaters used to connect up to multiple segments • Repeater repeats bits it hears on one interface to its other interfaces: physical layer device Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 20