ATM - Asynchronous transfer mode

来源： 作者： 出处：Vlan9.com 2007-10-14 进入论坛

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　　Asynchronoustransfermode(ATM)consistsofasetofhostsconnectedbyATMlinkstoanATMswitch.AnATMswitchgetsitsdatafromthehostsconnectedtoitandpassesthisdatatothedestination.Thedestinationhostcaneitherbeanend-point,oritcanbeanintermediateATMswitchonthewayfromthedata'ssourcetoit'sdestination.

DataistransmittedoveranATMnetworkinATMcells.Acellisafixedsizeat53bytedatastructurethatcontains48bytesofdataand5bytesofcontrolinformation.Eachcell'scontrolinformationincludesavirtualcircuitnumber.ThisnumberisusedbyATMswitchestodeterminewherethecellshouldbesent,andisusedbyreceivinghoststodeterminewhichbuffersshouldreceivethedata(thusallowinginboundnetworkdatatobedemultiplexedinhardware).

Thevirtualcircuitnumberiscomposedoftwonumbers:thevirtualchannelidentifier(VCI)andthevirtualpathidentifier(VPI).AlldatasentoveranATMnetworkisassociatedwithavirtualcircuit.Therearetwotypesofvirtualcircuits:permanentvirtualcircuits(PVC)andswitchedvirtualcircuits(SVC).PermanentvirtualcircuitsareusuallysetupinanATMswitchbyanetworkadministrator.Switchedvirtualcircuitsareconnectionsthatareestablishedasneededthroughtheuseofsignallingprotocols

The48bytedataareaofanATMcellisquitesmallwhencomparedtothedataareaofanEthernetorFDDIpacket.ATMincludesanumberofATMadaptationlayers(orAAL).AAL5allowsahosttosendandreceiveframesofupto64Kinsize.WhenahostsendsalargeAAL5frame,theATMhost-networkinterfacebreaksitupintoATMcells.Whenthecellsarriveatthereceivinghost,thesecellsarereassembledbythereceivingmachine'sATMhost-networkinterface.AAL5allowshoststosendandreceiveframesandnotworryaboutthesmallATMcells.

OneadvantageofATMisitsuseofvirtualcircuitsmakesiteasiertoprovidenetworkperformanceguaranteestoapplications.EachactivevirtualcircuitonanATMnetworkcanbeallocatedafixedportionofthenetwork'sbandwidth. 更多请看Cisco与华为技术网（Vlan9.com）ATM网络技术教程、ATM协议专题，或进入论坛讨论。