计算机网络 自顶向下方法 第五章(英文版课件).ppt

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1、Chapter5:TheDataLinkLayerOurgoals:understandprinciplesbehinddatalinklayerservices:errordetection,correctionsharingabroadcastchannel:multipleaccesslinklayeraddressingreliabledatatransfer,flowcontrol:done!instantiationandimplementationofvariouslinklayertechnologies15:DataLinkLayerLinkLayer5.1Introd

2、uctionandservices5.2Errordetectionandcorrection5.3Multipleaccessprotocols5.4Link-LayerAddressing5.5Ethernet5.6Hubsandswitches5.7PPP5.8LinkVirtualization:ATMandMPLS25:DataLinkLayerLinkLayer:IntroductionSometerminology:hostsandroutersarenodescommunicationchannelsthatconnectadjacentnodesalongcommuni

3、cationpatharelinkswiredlinkswirelesslinksLANslayer-2packetisaframe,encapsulatesdatagram“link”data-linklayerhasresponsibilityoftransferringdatagramfromonenodetoadjacentnodeoveralink35:DataLinkLayerLinklayer:contextDatagramtransferredbydifferentlinkprotocolsoverdifferentlinks:e.g.,Ethernetonfirstli

4、nk,framerelayonintermediatelinks,802.11onlastlinkEachlinkprotocolprovidesdifferentservicese.g.,mayormaynotproviderdtoverlinktransportationanalogytripfromPrincetontoLausannelimo:PrincetontoJFKplane:JFKtoGenevatrain:GenevatoLausannetourist=datagramtransportsegment=communicationlinktransportationmod

5、e=linklayerprotocoltravelagent=routingalgorithm45:DataLinkLayerLinkLayerServicesFraming,linkaccess:encapsulatedatagramintoframe,addingheader,trailerchannelaccessifsharedmedium“MAC”addressesusedinframeheaderstoidentifysource,destdifferentfromIPaddress!Reliabledeliverybetweenadjacentnodeswelearnedh

6、owtodothisalready(chapter3)!seldomusedonlowbiterrorlink(fiber,sometwistedpair)wirelesslinks:higherrorratesQ:whybothlink-levelandend-endreliability?55:DataLinkLayerLinkLayerServices(more)FlowControl:pacingbetweenadjacentsendingandreceivingnodesErrorDetection:errorscausedbysignalattenuation,noise.r

7、eceiverdetectspresenceoferrors:signalssenderforretransmissionordropsframeErrorCorrection:receiveridentifiesandcorrectsbiterror(s)withoutresortingtoretransmissionHalf-duplexandfull-duplexwithhalfduplex,nodesatbothendsof