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