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1、Chapter5数据链路层和局域网宇缨ComputerNetworking:ATopDownApproach5thedition. JimKurose,KeithRoss Addison-Wesley,April2009.15:DataLinkLayerChapter5:数据链路层（LinkLayer）要点:掌握数据链路层服务的原理:差错检验、纠正errordetection,correction广播链路的共享:数据链路层地址可靠的数据传输,流量控制各种数据链路层技术的实现以太网PPP25:DataLi
2、nkLayerLinkLayer5.1数据链路层：引言和服务5.2差错检验和纠正5.3多路访问协议5.4链路层地址5.5以太网5.6集线器、网桥和交换机5.7PPP35:DataLinkLayer数据链路层：引言和服务部分术语:hostsandroutersarenodescommunicationchannelsthatconnectadjacentnodesalongcommunicationpatharelinkswiredlinkswirelesslinksLANslayer-2packetis
3、aframe,encapsulatesdatagramdata-linklayerhasresponsibilityoftransferringdatagramfromonenodetoadjacentnodeoveralink45:DataLinkLayer数据链路层:续数据报在不同的链路段可能使用不同的链路层协议传输:e.g.,Ethernetonfirstlink,framerelayonintermediatelinks,802.11onlastlink每一链路层协议提供不同的服务：e.g.,m
4、ayormaynotproviderdt(可靠数据传输)overlinktransportationanalogytripfromPrincetontoLausannelimo:PrincetontoJFKplane:JFKtoGenevatrain:GenevatoLausannetourist=datagramtransportsegment=communicationlinktransportationmode=linklayerprotocoltravelagent=routingalgorit
5、hm55:DataLinkLayer数据链路层服务封装成帧encapsulatedatagramintoframe,addingheader,trailer“MAC”addressesusedinframeheaderstoidentifysource,destdifferentfromIPaddress!链路访问:channelaccessifsharedmedium相邻节点间的可靠交付welearnedhowtodothisalready(chapter3)!seldomusedonlowbit-e
6、rrorlink(fiber,sometwistedpair)wirelesslinks:higherrorratesQ:whybothlink-levelandend-endreliability?65:DataLinkLayer数据链路层服务(续)流量控制:pacingbetweenadjacentsendingandreceivingnodes差错检测:errorscausedbysignalattenuation,noise.receiverdetectspresenceoferrors:sig
7、nalssenderforretransmissionordropsframe差错纠正:receiveridentifiesandcorrectsbiterror(s)withoutresortingtoretransmission半双工和全双工withhalfduplex,nodesatbothendsoflinkcantransmit,butnotatsametime75:DataLinkLayer数据链路层协议的实现？ineachandeveryhostlinklayerimplementedin
8、“adaptor”(akanetworkinterfacecardNIC)Ethernetcard,PCMCIcard,802.11cardimplementslink,physicallayerattachesintohost’ssystembusescombinationofhardware,software,firmwarecontrollerphysicaltransmissioncpumemoryhostbus(e.g.,PCI)
