计算机网络英文课件Chapter5

5:DataLinkLayer5-1Chapter5LinkLayerandLANsComputerNetworking:ATopDownApproachFeaturingtheInternet,3rdedition.JimKurose,KeithRossAddison-Wesley,July2004.5:DataLinkLayer5-2Chapter5:TheDataLinkLayerOurgoals:understandprinciplesbehinddatalinklayerservices:errordetection,correctionsharingabroadcastchannel:multipleaccesslinklayeraddressingreliabledatatransfer,flowcontrol:done!instantiationandimplementationofvariouslinklayertechnologies5:DataLinkLayer5-3LinkLayer5.1Introductionandservices5.2Errordetectionandcorrection5.3Multipleaccessprotocols5.4Link-LayerAddressing5.5Ethernet5.6Hubsandswitches5.7PPP5:DataLinkLayer5-4LinkLayer:IntroductionSometerminology:hostsandroutersarenodescommunicationchannelsthatconnectadjacentnodesalongcommunicationpatharelinkswiredlinkswirelesslinksLANslayer-2packetisaframe,encapsulatesdatagram“link”data-linklayerhasresponsibilityoftransferringdatagramfromonenodetoadjacentnodeoveralink5:DataLinkLayer5-5Linklayer:contextDatagramtransferredbydifferentlinkprotocolsoverdifferentlinks:e.g.,Ethernetonfirstlink,framerelayonintermediatelinks,802.11onlastlinkEachlinkprotocolprovidesdifferentservicese.g.,mayormaynotproviderdtoverlinktransportationanalogytripfromPrincetontoLausannelimo:PrincetontoJFKplane:JFKtoGenevatrain:GenevatoLausannetourist=datagramtransportsegment=communicationlinktransportationmode=linklayerprotocoltravelagent=routingalgorithm5:DataLinkLayer5-6LinkLayerServicesFraming,linkaccess:encapsulatedatagramintoframe,addingheader,trailerchannelaccessifsharedmedium“MAC”addressesusedinframeheaderstoidentifysource,dest•differentfromIPaddress!Reliabledeliverybetweenadjacentnodeswelearnedhowtodothisalready(chapter3)!seldomusedonlowbiterrorlink(fiber,sometwistedpair)wirelesslinks:higherrorrates•Q:whybothlink-levelandend-endreliability?5:DataLinkLayer5-7LinkLayerServices(more)FlowControl:pacingbetweenadjacentsendingandreceivingnodesErrorDetection:errorscausedbysignalattenuation,noise.receiverdetectspresenceoferrors:•signalssenderforretransmissionordropsframeErrorCorrection:receiveridentifiesandcorrectsbiterror(s)withoutresortingtoretransmissionHalf-duplexandfull-duplexwithhalfduplex,nodesatbothendsoflinkcantransmit,butnotatsametime5:DataLinkLayer5-8Adaptors(NIC)Communicatinglinklayerimplementedin“adaptor”(akaNIC)Ethernetcard,PCMCIcard,802.11cardsendingside:encapsulatesdatagraminaframeaddserrorcheckingbits,rdt,flowcontrol,etc.receivingsidelooksforerrors,rdt,flowcontrol,etcextractsdatagram,passestorcvingnodeadapterissemi-autonomouslink&physicallayerssendingnodeframercvingnodedatagramframeadapteradapterlinklayerprotocol5:DataLinkLayer5-9LinkLayer5.1Introductionandservices5.2Errordetectionandcorrection5.3Multipleaccessprotocols5.4Link-LayerAddressing5.5Ethernet5.6Hubsandswitches5.7PPP5:DataLinkLayer5-10ErrorDetectionEDC=ErrorDetectionandCorrectionbits(redundancy)D=Dataprotectedbyerrorchecking,mayincludeheaderfields•Errordetectionnot100%reliable!•protocolmaymisssomeerrors,butrarely•largerEDCfieldyieldsbetterdetectionandcorrection5:DataLinkLayer5-11ParityCheckingSingleBitParity:DetectsinglebiterrorsTwoDimensionalBitParity:Detectandcorrectsinglebiterrors005:DataLinkLayer5-12InternetchecksumSender:treatsegmentcontentsassequenceof16-bitintegerschecksum:addition(1’scomplementsum)ofsegmentcontentssenderputschecksumvalueintoUDPchecksumfieldReceiver:computechecksumofreceivedsegmentcheckifcomputedchecksumequalschecksumfieldvalue:NO-errordetectedYES-noerrordetected.Butmaybeerrorsnonetheless?Morelater….Goal:detect“errors”(e.g.,flippedbits)intransmittedsegment(note:usedattransportlayeronly)5:DataLinkLayer5-13Checksumming:CyclicRedundancyCheckviewdatabits,D,asabinarynumberchooser+1bitpattern(generator),Ggoal:chooserCRCbits,R,suchthatD,RexactlydivisiblebyG(modulo2)receiverknowsG,dividesD,RbyG.Ifnon-zeroremainder:errordetected!candetectallbursterrorslessthanr+1bitswidelyusedinpractice(ATM,HDCL)5:DataLinkLayer5-14CRCExampleWant:D.2rXORR=nGequivalently:D.2r=nGXORRequivalently:ifwedivideD.2rbyG,wantremainderRR=remainder[]D.2rG5:DataLinkLayer5-15LinkLayer5.1Introductionandservices5.2Errordetectionandcorrection5.3Multipleaccessprotocols5.4Link-LayerAddressing5.5Ethernet5.6Hubsandswitches5.7PPP5:DataLinkLayer5-16MultipleAccessLinksandProtocolsTwotypesof“links”:point-to-pointPPPfordial-upaccesspoint-to-pointlinkbetweenEthernetswitchandhostbroadcast(sharedwireormedium)traditionalEthernetupstreamHFC802.11wirelessLAN5:DataLinkLayer5-17MultipleAccessprotocolssinglesharedbroadcastchanneltwoormoresimultaneoustransmissionsbynodes:interferencecollisionifnodereceivestwoormoresignalsatthesametimemultipleaccessprotocoldistributedalgorithmthatdetermineshownodessharechannel,i.e.,determinewhennodecantransmitcommunicationaboutchannelsharingmustusechannelitself!noout-of-bandchannelforcoordination5:DataLinkLayer5-18IdealMulitpleAccessProtocolBroadcastchannelofrateRbps1.Whenonenodewantstotransmit,itcansendatrateR.2.WhenMnodeswanttotrans