《数据与计算机通信》ppt课件

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1、第16单元高速局域网Chapter16HighSpeedLANsWilliamStallingsDataandComputerCommunications7thEdition1IntroductionRangeoftechnologiesFastandGigabitEthernetFibreChannelHighSpeedWirelessLANs(seeChapter17)216.1WhyHighSpeedLANs?OfficeLANsusedtoprovidebasicconnectivityConnectingPCsandter

2、minalstomainframesandmidrangesystemsthatrancorporateapplicationsProvidingworkgroupconnectivityatdepartmentallevelTrafficpatternslightEmphasisonfiletransferandelectronicmailSpeedandpowerofPCshasrisenGraphics-intensiveapplicationsandGUIsMISorganizationsrecognizeLANsasesse

3、ntialBeganwithclient/servercomputingNowdominantarchitectureinbusinessenvironmentIntranetworksFrequenttransferoflargevolumesofdata3ApplicationsRequiringHighSpeedLANsCentralizedserverfarms中央服务器集群UserneedstodrawhugeamountsofdatafrommultiplecentralizedserversE.g.Colorpublis

4、hingServerscontaintensofgigabytesofimagedataDownloadedtoimagingworkstationsPowerworkgroups超级工作组SmallnumberofcooperatingusersDrawmassivedatafilesacrossnetworkE.g.Softwaredevelopmentgrouptestingnewsoftwareversionorcomputer-aideddesign(CAD)runningsimulationsHigh-speedlocal

5、backbone高速本地干线ProcessingdemandgrowsLANsproliferateatsiteHigh-speedinterconnectionisnecessary416.2Ethernet(CSMA/CD)以太网CarriersSenseMultipleAccesswithCollisionDetectionXerox–EthernetXerox/DEC/Intel3comIEEE802.3516.2.1IEEE802.3MediumAccessControlRandomAccessStationsaccessm

6、ediumrandomlyContentionStationscontentfortimeonmedium6ALOHAPacketRadioWhenstationhasframe,itsendsStationlistens(formaxroundtriptime)plussmallincrementIfACK,fine.Ifnot,retransmitIfnoACKafterrepeatedtransmissions,giveupFramechecksequence(asinHDLC)IfframeOKandaddressmatche

7、sreceiver,sendACKFramemaybedamagedbynoiseorbyanotherstationtransmittingatthesametime(collision)AnyoverlapofframescausescollisionMaxutilization18%7SlottedALOHATimeinuniformslotsequaltoframetransmissiontime时隙长度等于发送时间Needcentralclock(orothersyncmechanism)Transmissionbegins

8、atslotboundaryFrameseithermissoroverlaptotallyMaxutilization37%8CSMAPropagationtimeismuchlessthantransmissiont