1、国家示范性软件学院 http:/国家示范性软件学院1Chapter 2 Link Layer and LANsA note on the use of these ppt slides:Were making these slides freely available to all (faculty, students, readers). Theyre in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. T
2、hey obviously represent a lot of work on our part. In return for use, we only ask the following:q If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, wed like people to use our book!)q If you post any slides in substantially unaltered
3、 form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.Thanks and enjoy! JFK/KWRAll material copyright 1996-2005J.F Kurose and K.W. Ross, All Rights Reserved国家示范性软件学院 http:/国家示范性软件学院2Our goals: lunderstand principle
4、s behind data link layer services:lerror detection, correctionlsharing a broadcast channel: multiple accessllink layer addressinglreliable data transfer, flow control: done!linstantiation and implementation of various link layer technologies国家示范性软件学院 http:/国家示范性软件学院3l2.1 Introduction and services of
5、 Link Layer国家示范性软件学院 http:/国家示范性软件学院4Some terminology:l hosts and routers are nodesl communication channels that connect adjacent nodes along communication path are linksl wired linksl wireless linksl LANsl layer-2 packet is a frame, encapsulates datagram“link”data-link layer has responsibility of t
6、ransferring datagram from one node to adjacent node over a link国家示范性软件学院 http:/国家示范性软件学院5lDatagram transferred by different link protocols over different links:le.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last linklEach link protocol provides different servicesle.g., m
7、ay or may not provide RDT over linktransportation analogyl trip from Princeton to Lausannellimo: Princeton to JFKlplane: JFK to Genevaltrain: Geneva to Lausanneltourist = datagramltransport segment = communication linkltransportation mode = link layer protocolltravel agent = routing algorithm国家示范性软件
8、学院 http:/国家示范性软件学院6Link Layer ServiceslFraming, link access: lencapsulate datagram into frame, adding header, trailerlchannel access if shared mediuml“MAC” addresses used in frame headers to identify source, dest ldifferent from IP address!lReliable delivery between adjacent nodeslwe learned how to
9、do this already (chapter 3)!lseldom used on low bit error link (fiber, some twisted pair)lwireless links: high error rateslQ: why both link-level and end-end reliability?国家示范性软件学院 http:/国家示范性软件学院7lFlow Control: lpacing between adjacent sending and receiving nodeslError Detection: lerrors caused by s
10、ignal attenuation, noise. lreceiver detects presence of errors: lsignals sender for retransmission or drops frame lError Correction: lreceiver identifies and corrects bit error(s) without resorting to retransmissionlHalf-duplex and full-duplexlwith half duplex, nodes at both ends of link can transmi
11、t, but not at same time国家示范性软件学院 http:/国家示范性软件学院8Adaptors Communicatingllink layer implemented in “adaptor” (aka NIC)lEthernet card, PCMCI card, 802.11 cardlsending side:lencapsulates datagram in a frameladds error checking bits, rdt, flow control, etc.lreceiving sidellooks for errors, rdt, flow con
12、trol, etclextracts datagram, passes to rcving nodeladapter is semi-autonomousllink & physical layerssendingnodeframercvingnodedatagramframeadapter adapterlink layer protocol国家示范性软件学院 http:/国家示范性软件学院92.2 Error detection and correction 国家示范性软件学院 http:/国家示范性软件学院10EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields Error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction
