lect11b_linklayerIr1_Ross

lect11b_linklayerIr1_Ross - Link Layer Lecture 11b ELEC300U...

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Unformatted text preview: Link Layer Lecture 11b ELEC300U * The slides are adapted from ppt slides (in substantially unaltered form) available from “Computer Networking: A Top-Down Approach,” 4 th edition, by Jim Kurose and Keith Ross, Addison-Wesley, July 2007. Part of the materials are also adapted from MIT 6.02 course notes. • Link layer basics • Framing and checksum • ALOHA channel access protocol • Analysis of ALOHA throughput 1 2 Internet protocol stack • application: supporting network applications – HTTP, SMTP, FTP, DNS • transport: process-process data transfer – TCP, UDP • network: routing of datagrams from source to destination – IP, routing protocols • l i n k : data transfer between neighboring network elements – 802.11, Ethernet • p h y s i c a l : bits “on the wire” A pplication Transport N etwork Link P hysical Link Layer: Introduction Some terminology: • hosts and routers are nodes • communication channels that connect adjacent nodes along communication path are links – w i r e d l i n k s – wireless links – LANs • layer-2 packet is a frame , encapsulates datagram data-link layer has responsibility of transferring datagram from one node to adjacent node over a link 3 Frame – Link-layer packet message datagram frame message message Trans.-layer header message Trans.-layer header IP-layer header message Trans.-layer header IP-layer header Link-layer header segment 4 Link layer: Introduction • datagram transferred by different link protocols over different links: – e.g., Ethernet on first link, 802.11 on last link • each link protocol provides different services – e.g., may or may not provide reliable data transfer over link transportation analogy • trip from UST to Beida – taxi: UST to HK airport – plane: HK to Beijing – bus: Beijing airport to Beida • tourist = datagram • UST-to-airport, HK-to- Beijing, airport-to-Beida = communication links • transportation mode = link layer protocol • travel agent = routing lgorithm 5 Link Layer Services • framing, link access: – encapsulate datagram into frame, adding header, trailer – channel access if shared medium – “MAC” addresses used in frame headers to identify source, dest • different from IP address! • reliable delivery between adjacent nodes – seldom used on low bit-error link (fiber, some twisted pair) – wireless links: high error rates 6 Where is the link layer implemented? • in each and every host • link layer implemented in “adaptor” (aka network interface card NIC) – E t h e r n e t c a r d , P C M C I card, 802.11 card – implements link, physical layer • attaches into host’s system buses • combination of hardware, software, firmware controller physical transmission cpu memory host bus (e.g., PCI) network adapter card host schematic application transport network link link physical 7 Adaptors Communicating • sending side: – encapsul...
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lect11b_linklayerIr1_Ross - Link Layer Lecture 11b ELEC300U...

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