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1、ComputerNetworksandInternets《计算机网络与因特网》课件林坤辉2019-02-6PARTIIPacketTransmissionChapter11ExtendingLANs:FiberModems,Repeaters,Bridges,andSwitches局域网扩展：光纤调制解调器、中继器、网桥和交换机11.1IntroductionEachLANtechnologyisdesignedforaspecificcombinationofspeed,distance,andcost.Thedesign
2、erspecifiesamaximumdistancethattheLANcanspan,withtypicalLANsdesignedtospanafewhundredmeters.网络互联设备11.2DistanceLimitationandLANDesignDistancelimitationisafundamentalpartofLANdesigns.Engineerschooseacombinationofcapacity,maximumdelay,anddistancethatcanbeachievedatagi
3、vencost.Tohelpsaveexpense,LANtechnologiesusuallyuseasharedcommunicationmedium.ALANdesignmustincludeamechanismthatguaranteeseachstationfairaccesstothesharedmedium.Hardwareisengineeredtoemitafixedamountofelectricalpower,thesignalcannotreacharbitrarilyfar.LANhardwarei
4、sengineeredforafixedmaximumlengthcable,Thehardwarewillnotworkcorrectlyoverwirethatexceedthebound.11.3FiberOpticExtensionsApairoffibermodemsandopticalfiberscanbeusedtoprovideaconnectionacomputerandaremoteLAN.Becausedelaysacrossfiberarelowandbandwidthishigh,themechan
5、ismwilloperatecorrectlyacrossdistancesofseveralkilometers.11.4Repeaters中继器Arepeaterisusuallyananalogelectronicdevicethatcontinuouslymonitorselectricalsignalsoneachcable.Whenitsensesasignalononecable,therepeatertransmitsanamplifiedcopyontheothercable.中继器（1）中继器（2）Are
6、peaterconnectstwoEthernetcablescalledsegments,eachofwhichhastheusualtermination.Repeatersdonotunderstandtheframeformat,nordotheyhavephysicaladdresses.AnypairofcomputersontheextendedLANcancommunication,thecomputersdonotknowwhetherarepeaterseparatesthem.Therepeaterat
7、tachesdirectlytotheEthernetcablesandsendscopiesofelectricalsignalsfromonetootherwithoutwaitingforacompleteframe.Arepeaterdoesnotdistinguishbetweenthesignalsthatcorrespondtoavalidframeandotherelectricalsignals.Ifacollisionorelectricalinterferenceoccursononesegment,r
8、epeaterscausethesameproblemtooccuronallothersegments.Ifmorethanfourrepeatersseparateanypairofstations,thenetworkwillnotoperatecorrectly.11.5Bridg
