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1、5:DataLinkLayer5-1Chapter5LinkLayerandLANsComputerNetworking:ATopDownApproach4thedition.JimKurose,KeithRossAddison-Wesley,July2007.5:DataLinkLayer5-2Chapter5:TheDataLinkLayerOurgoals:understandprinciplesbehinddatalinklayerservices:errordetection,correction
2、sharingabroadcastchannel:multipleaccesslinklayeraddressingreliabledatatransfer,flowcontrol:done!instantiationandimplementationofvariouslinklayertechnologies5:DataLinkLayer5-3LinkLayer5.1Introductionandservices5.2Errordetectionandcorrection5.3Multipleaccess
3、protocols5.4Link-layerAddressing5.5Ethernet5.6Link-layerswitches5.7PPP5.8Linkvirtualization:ATM,MPLS5:DataLinkLayer5-4LinkLayer:IntroductionSometerminology:hostsandroutersarenodescommunicationchannelsthatconnectadjacentnodesalongcommunicationpatharelinkswi
4、redlinkswirelesslinksLANslayer-2packetisaframe,encapsulatesdatagramdata-linklayerhasresponsibilityoftransferringdatagramfromonenodetoadjacentnodeoveralink5:DataLinkLayer5-5Linklayer:contextdatagramtransferredbydifferentlinkprotocolsoverdifferentlinks:e.g.,
5、Ethernetonfirstlink,framerelayonintermediatelinks,802.11onlastlinkeachlinkprotocolprovidesdifferentservicese.g.,mayormaynotproviderdtoverlinktransportationanalogytripfromPrincetontoLausannelimo:PrincetontoJFKplane:JFKtoGenevatrain:GenevatoLausannetourist=d
6、atagramtransportsegment=communicationlinktransportationmode=linklayerprotocoltravelagent=routingalgorithm5:DataLinkLayer5-6LinkLayerServicesframing,linkaccess:encapsulatedatagramintoframe,addingheader,trailerchannelaccessifsharedmedium“MAC”addressesusedinf
7、rameheaderstoidentifysource,destdifferentfromIPaddress!reliabledeliverybetweenadjacentnodeswelearnedhowtodothisalready(chapter3)!seldomusedonlowbit-errorlink(fiber,sometwistedpair)wirelesslinks:higherrorratesQ:whybothlink-levelandend-endreliability?5:DataL
8、inkLayer5-7LinkLayerServices(more)flowcontrol:pacingbetweenadjacentsendingandreceivingnodeserrordetection:errorscausedbysignalattenuation,noise.receiverdetectspresenceoferrors:signalssenderforretransmissionor
