计算机网络英文版课件：5-1 数据链路层服务

数据链路层服务LinkLayer:IntroductionSometerminology:hostsandroutersarenodescommunicationchannelsthatconnectadjacentnodesalongcommunicationpatharelinkswiredlinkswirelesslinksLANslayer-2packetisaframe,

encapsulatesdatagramdata-linklayerhasresponsibilityoftransferringdatagramfromonenodetoadjacentnodeoveralinkLinklayer:contextdatagramtransferredbydifferentlinkprotocolsoverdifferentlinks:e.g.,Ethernetonfirstlink,framerelayonintermediatelinks,802.11onlastlinkeachlinkprotocolprovidesdifferentservicese.g.,mayormaynotproviderdtoverlinktransportationanalogytripfromPrincetontoLausannelimo:PrincetontoJFKplane:JFKtoGenevatrain:GenevatoLausannetourist=datagramtransportsegment=communicationlinktransportationmode=linklayerprotocoltravelagent=routingalgorithmLinkLayerServicesframing,linkaccess:

encapsulatedatagramintoframe,addingheader,trailerchannelaccessifsharedmedium“MAC”addressesusedinframeheaderstoidentifysource,destdifferentfromIPaddress!reliabledeliverybetweenadjacentnodeswelearnedhowtodothisalready(chapter3)!seldomusedonlowbit-errorlink(fiber,sometwistedpair)wirelesslinks:higherrorratesQ:whybothlink-levelandend-endreliability?LinkLayerServices(more)flowcontrol:

pacingbetweenadjacentsendingandreceivingnodeserrordetection:

errorscausedbysignalattenuation,noise.receiverdetectspresenceoferrors:signalssenderforretransmissionordropsframeerrorcorrection:

receiveridentifiesandcorrectsbiterror(s)withoutresortingtoretransmissionhalf-duplexandfull-duplexwithhalfduplex,nodesatbothendsoflinkcantransmit,butnotatsametimeWhereisthelinklayerimplemented?ineachandeveryhostlinklayerimplementedin“adaptor”(akanetworkinterfacecardNIC)Ethernetcard,PCMCIcard,802.11cardimplementslink,physicallayerattachesintohost’ssystembusescombinationofhardware,software,firmwarecontrollerphysicaltransmissioncpumemoryhostbus(e.g.,PCI)networkadaptercardhostschematicapplicationtransportnetworklinklinkphysicalAdaptorsCommunicatingsendingside:encapsulatesdatagraminframeaddserrorcheckingbits,rdt,flowcontrol,etc.receivingsidelook