ATM - Asynchronous transfer mode

2019-11-04 11:46:59

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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.

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