ATM - Asynchronous transfer mode

　　 

　　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.

原文转自：http://www.ltesting.net