电子信息专业英语（第二版）课件：Modern Switching Technique

ModernSwitchingTechniquePassageAThePrinciplesofDigitalSwitchingPassageBCircuitSwitchingandPacketSwitchingPassageCATMPassageAThePrinciplesofDigitalSwitchi

Digitalswitchingsystemhasplayedaveryimportantroleintoday’stelecommunicationnetwork.

1.SystemConfiguration

Theelectronicswitchingsystem(ESS)withstoredprogramcontrol,asshownin

Figure21.1,isdividedintothefollowing3subsystems:

·SpeechPathSubsystem.Thissubsystemcontrolsthespeechpathstoinformtheconnectionbetweensubscribers,betweensubscribersandtrunksorbetweentrunks.

Figure21.1ESSSystemConfigurations

·InputandOutputSubsystem.ThissubsystemperformstheMan-Machineinterfacebetweentheswitchingsystemandtheoperationandmaintenancepersonnel.Itiscomposedofinputandoutputequipment,suchasteletypewriters,magnetictapeunits,etc.

·CentralProcessorSubsystem.Thissubsystemperformsbystoredprogramcontrolthedigitanalysis,thecontrolofthespeechpathfortheestablishmentofacall,thecontroloftheMan-Machineinterface,etc.

2.CompositionoftheSpeechPath

Thespeechpathofthedigitalswitchingsystem,forexample,incaseofsubscribertosubscriberconnection,iscomposedbythefollowingcomponentsasshowninFigure21.2.

·LineCircuit(LC).

Providedtoeachsubscriberline,itsuppliestheringingandtalkingcurrentandperformsthe2to4wireconversion.Inthiscircuit,theA/DandD/Aconversionsarealsocompleted.Inoneword,ithastheBORSCHTfunction［1］.

AftertheCODECwithinthiscircuit,thePCMsignalsoffirstorderareobtainedandsenttothePMUX.

Figure21.2StructureoftheSpeechPath

·Multiplexer(MUX).MultiplexerperformsthemultiplexingofthefirstorderPCMsignal.InthecaseofNEAX61,fromtheprimarymultiplexer(PMUX)120channelsareobtainedandfromthesecondarymultiplexer(SMUX)480channelsareobtained.Theprimarydemultiplexer(PDMUX)andthesecondarydemultiplexer(SDMUX)performtheoppositeoperations.

·TimeDivisionSwitch(TSW).Switchesthemultiplexedsignalsaccordingtothestoredprogramcontrol,performinginterchangeoftimeslotsandbetweenHighwaysthatwillbeexplainedlater.［2］

TheTSWiscomposedofcombinedtimeandspaceswitches.

3.TimeSwitch

Thetimeswitchisimplementedwithrandomaccessmemories(RAM).AsshowninFigure21.3,thedigitalsignalismultiplexedintheMUXbeforeenteringthetimeswitch.Thishasmemoryforallthemultiplexedtimeslotscomingfromthehighwaywhicharewritteninthismemoryinsequenceaccordingtotheclockfrequency.

Figure21.3TimeSwitchControl

Thecentralprocessorcontrolstheoutputsequenceofthetimeslotsthatarereadoutfromthetimeswitchmemory.Inotherwords,thesequenceoftheaddressestobereadfromthetimeswitchiswrittenintheTSW-CTLM.［3］

ThereadingisperformedwithclockaccordingtothesequenceoftheaddressesindicatedintheTSW-CTLM.Thusintheoutputhighway,thetimeslotshavetheirpositionsinterchanged.Namely,theswitchingoftimeslotsisthefundamentalfunctionofthetimeswitch.

ThisswitchingiselementarilyshowninFigure21.4.ThetimeslotsthatcomefromhighwayarewritteninsequenceinmemoriesM1,M2,…,Mrofthetimeswitch.Thiswritingoperationiscalledsequentialwriting(SW)becauseitisexecutedinaccordancewiththesequenceestablishedbythehardwareattheclockfrequency.［4］

However,thereadingsequenceisdeterminedbysoftware,andasafixedreadingorderdoesnotexist,itiscalledasrandomreading(RR).Figure21.4SwitchingofTimeSlots

Forexample,taketheswitchingofthetimeslotTS1withTS3,thewritingisexecutedinthesameorderthattheyhaveinthehighway,thecontentsofTS1“a”arewritteninthememoryM1ofthetimeswitch,thecontentsofTS2arewritteninM2,thatofTS3inM3andthussuccessively.

AswehavetoswitchTS1andTS3,M3isreadatthetimethatcorrespondstoTS1,M2isreadatthetimeofTS2(Thereisnoswitching)andM1isreadatthetimeofTS3.Thus“a”isswitchedfromTS1toTS3andthecontentsofTS3“C”switchedtoTS1.

Throughthiscontrolofreading,anyTScanbeswitchedtoanyTS.

4.SpaceSwitch

Torealizealargesizeswitchingnetworkusingonlytimeswitches,highspeedwritingandreadingandahighgradeoftimemultiplexingisrequired.［5］

Therefore,inlargedigitalswitchingsystems,thedifferenthighwaysofthetime

switchesare“crossed”insuchawaythattheycanbeswitchedamongthemselves.［6］Withthiscomposition,itispossibletogethighcapacityswitchingmatrixes.

Theswitchthatenableslinkageofdifferenthighwaysisthespaceswitch.

Thefundamentalcomponentsofthisswitcharegates.AsindicatedinFigure21.5,totransferthetimeslotTS2fromhighwayH2tohighwayH1,itisenoughtoopengateG12atthetimeofTS2.Inthesameway,totransferTS2fromhighH1tohighwayH2gateG21isopened.

Figure21.5HighwayConversion

Wecanseethen,thatitispossibletotransferatimeslotofanyincominghighwaytoanyoutgoinghighway,throughthehighseedopeningandclosingofthegatesthatarelocatedateach“crosspoint”.Thisoperationiscalledhighwayconversion.

Thespeechpathisretainedtotransferatimeslotfromonehighwaytoanotherevery125μsbyopeningandclosingaparticulargate.

Forthispurpose,asshowninFigure21.6,inthesamewayasinthecaseofthetimeswitch,thesequenceforopeningandclosingthegatesiswritteninthememoryandthespaceswitchcontrolisperformedaccordingtothecontentsofthatmemory.

InFigure21.6,thismemory,whichiscalledSpaceSwitchControlMemory,isshown.Figure21.6SpaceSwitchControl

ThespaceswitchshowninFigure21.5isequivalenttoagroupofmatrixes(Figure.21.7)whosenumberisequaltothatofthemultiplexedchannels(inthiscasen).

Thenumberofinputsineachmatrixisequaltothatoftheincominghighways(inthiscase2)andthenumberofoutputsofeachmatrixisequalthatofoutgoinghighways(inthiscase2).

Intheprecedingparagraphstheelementsthatcomposethetimedivisionswitch,namelythetimeandspaceswitches,werementioned.Intheactualsystemsbothtypeofswitchesarecombined.

Figure21.7EquivalentSpaceDivisionSwitchStructureNotes

［1］Providedtoeachsubscriberline,itsuppliestheringingandtalkingcurrentandperformsthe2to4wireconversion.Inthiscircuit,theA/DandD/Aconversionsarealsocompleted.Inoneword,ithastheBORSCHTfunction.

它连接用户线时,供给振铃和通话电流并执行2线与4线转换。在这个电路中,还完成A/D和D/A变换。总之,该电路具有BORSCHT功能。

·providedtoeachsubscriberline意为“(用户电路)被提供给每一条用户线路”。这是一个(过去)分词短语,做伴随状语。

·BORSCHT是由下列单词或词组的首字母组成,表示用户电路的基本功能。馈电(Battery):向用户线进行直流馈电;

过压保护(OverloadProtection):防止用户线上的电压冲击;振铃(Ringing):传送呼叫铃流;

监视(Supervision):监视用户状态;

混合(Hybrid):进行用户线的2/4线转换;

编解码(Codec):完成编解码的功能;

测试(Test):进行用户线路的测试。［2］Switchesthemultiplexedsignalsaccordingtothestoredprogramcontrol,

performinginterchangeoftimeslotsandbetweenHighwaysthatwillbeexplainedlater.

(时分交换器)依据存储程序控制来复用信号。该电路完成时隙交换和总线交换,稍后我们将对此进行说明。

·performing...为现在分词短语,做主句的伴随情况状语,可译为“完成时隙间和总线间的交换”。［3］Inotherwords,thesequenceoftheaddressestobereadfromthetimeswitchiswrittenintheTSW-CTLM.

换句话说,地址序列从时间接线器中读出，随后写入控制存储器(TSW-CTLM)。

·thesequenceoftheaddressestothereadfromthetimeswitch该短语中的“toberead”作定语,修饰“thesequenceoftheaddresses”,意为“(被)读的地址序列”。［4］Thiswritingoperationiscalledsequentialwriting(SW)becauseitisexecutedinaccordancewiththesequenceestablishedbythehardwareattheclockfrequency.

这种写操作称为顺序写入(SW),因为它是按照硬件设定的在时钟频率上的次序执行的。

·inaccordancewith意为“按照或与……一致”。

·attheclockfrequency意为“以时钟频率,在时钟频率上”。［5］Torealizealargesizeswitchingnetworkusingonlytimeswitches,highspeedwritingandreadingandahighgradeoftimemultiplexingisrequired.

如果只用时间接线器来实现一个大容量的交换网络,则需要高速度的读写操作和高度的时分复用。

·usingonlytimeswitches意为“仅用时间接线器(时)”。这是一个现在分词短语,在句中作条件状语。［6］Therefore,inlargedigitalswitchingsystems,thedifferenthighwaysofthetimeswitchesare“crossed”insuchawaythattheycanbeswitchedamongthemselves.

因此,在大容量的数字交换系统中,不同的时间接线器总线是以一种可在它们之间交换的方式进行“交换”的。

·insuchawaythat…以这样一种方式以使……

Exercises

1.PleasetranslatethefollowingphrasesintoEnglish.

(1)数字交换 (2)中继线

(3)用户电路 (4)总线

(5)电信网 (6)人-机接口

(7)时分交换 (8)时钟频率

(9)存储程序控制 (10)数字分析

(11)时间接线器 (12)交换矩阵

(13)电话用户 (14)话务量

(15)空间接线器

2.True/False.

(1)Thedistalswitchingsystemiscomposedofthreemainparts.()

(2)Theconnectionbetweensubscriberswouldbeimpossiblewithoutthecentral

professorsubsystem.()

(3)Foreachsubscriberline,theremustbeaLineCircuittotieconnectedwithit.

()

(4)Sincesubscribersusetelephonefrequently,thesubscriberlinesareverybusy.

()

(5)FortheNEAX61,theprimarymultiplexeroffers30voicechannels.()

(6)Thetimeswitchcanperformtheinterchangeofdifferenttimeslotcomingfromthedifferenthighways.()

(7)Thereareatleasttwomemoriesintimeswitch,oneisforstoringtheincomingtimeslotsfromthehighway,anotherisusedtostorethesequenceofaddressestobereadfromthetimeswitch.()

(8)ThespaceswitchcantransferthetimeslotsfromhighwayH1tohighwayH2,butcan’ttransmitthemtothesamehighway.()

3.Answerthequestions.

(1)Listthe3subsystemsoftheelectronicswitchingsystem.

(2)Listthecompositionofthespeechpath.

(3)Whatisthefunctionoflinecircuit?

(4)Whatisthefunctionoftimeswitch?

PassageBCircuitSwitchingandPacket

Switching

Therearetwobasictypesofswitchingtechniques:circuitswitchingandmessageswitching.Incircuitswitching,atotalpathofconnectedlinessetupfromtheorigintothedestinationatthetimethecallismade,andthepathremainsallocatedtothesource-destinationpair(whetherusedornot)untilitreleasedbythecommunicatingparties.［1］Theswitches,calledcircuitswitches(orofficeexchangeintelephonejargon),havenocapabilityofstoringormanipulatinguser’sdataontheirwaytothedestination.［2］Thecircuitissetupbyaspecialsignalingmessagethatfindsitswaythroughthenetwork,seizingchannelsinthepathasitproceeds.Oncethepathisestablished,arereturnsignalinformsthesourcetobegintransmission．DirecttransmissionofdatafromsourcetodestinationcanthentakePlacewithoutanyinterventiononthepartofthesubnet.

Inmessageswitching,thetransmissionunitisawell-definedblockofdatacalledamessage．Inadditiontothetexttobetransmitted,amessagecomprisesaheaderandachecksum．Theheadercontainsinformationregardingthesourceanddestinationaddressesaswellasothercontrolinformation;thechecksumisusedforerrorcontrolpurposes.Theswitchingelementisacomputerreferredtoasamessageprocessor,withprocessingandstoragecapabilities.Messagestravelindependentlyandasynchronously,timingtheirownwayfromsourcetodestination.Firstthemessageistransmittedfromthehosttothemessageprocessortowhichitisattached.Oncethemessageisentirelyreceived,themessageprocessorexaminesitsheader,andaccordinglydecidesonthenextoutgoingchannelonwhichtotransmitit.［3］

Ifthisselectedchannelisbusy,themessagewaitsinaqueueuntilthechannelbecomesfree,atwhichtimetransmissionbegins.Atthenextmessageprocessor,themessageisagainreceived,stored,examined,andtransmittedonsomeoutgoingchannel,andthesameprocesscontinuesuntilthemessageisdeliveredtoitsdestination.Thistransmissiontechniqueisalsoreferredtoasthestore-and-forwardtransmissiontechnique.

Avariationofmessageswitchingispacketswitching.Herethemessageisbrokenupintoseveralpiecesofagivenmaximumlength,calledpackets.Aswithmessageswitching,eachpacketcontainsaheaderandachecksum.Packetsaretransmittedindependently

inastore-and-forwardmanner.

Withcircuitswitching,thereisalwaysaninitialconnectioncostincurredinsettingupthecircuit.Itiscost-effectiveonlyinthosesituationswhereoncethecircuitissetupthereisaguaranteedsteadyflowofinformationtransfertoamortizetheinitialcost.Thisiscertainlythecasewithvoicecommunicationinthetraditionalway,andindeedcircuitswitchingisthetechniqueusedinthetelephonesystem.Communicationamongcomputers,however,ischaracterizedasbursty.Burstinessisaresultofthehighdegreeofrandomnessencounteredinthemessage-generationprocessandthemessagesize,andofthelowdelayconstraintrequiredbytheuser.[4]Theusersanddevicesrequirethecommunicationresourcesrelativelyinfrequently;butwhentheydo,theyrequirearelativelyrapid

response.Ifafixeddedicatedend-to-endcircuitweretobesetupconnectingtheendusers,thenonemustassignenoughtransmissionbandwidthtothecircuitinordertomeetthe

delayconstraintwiththeconsequencethattheresultingchannelutilizationislow.［5］Ifthecircuitofhighbandwidthweresetupandreleasedateachmessagetransmissionrequest,thentheset-uptimewouldbelargecomparedtothetransmissiontimeofthemessage,

resultingagaininlowchannelutilization.Therefore,forburstyusers(whichearlalsobecharacterizedbyhighpeak-to-averagedataraterequirements),store-and-forwardtransmissiontechniquesofferamorecost-effectivesolution,sinceamessageoccupiesaparticularcommunicationslinkonlyforthedurationofitstransmissiononthatlink;therestofthetimeitisstoredatsomeintermediatemessageswitchandthelinkisavailableforothertransmissions.Thusthemainadvantageofstore-and-forwardtransmissionovercircuitswitchingisthatthecommunicationbandwidthisdynamicallyallocated,andtheallocationisdoneonthefinebasisofaparticularlinkinthenetworkandaparticularmessage(foraparticularsource-destinationpair).

Packetswitchingachievesthebenefitsdiscussedsofarandoffersaddedfeatures.Itprovidesthefulladvantageofthedynamicallocationofthebandwidth,evenwhenmessagesarelong.Indeed,withpacketswitching,manypacketsofthesamemessagemaybeintransmissionsimultaneouslyoverconsecutivelinksofapathfromsourcetodestination,thusachievinga“pipelining”effectandreducingconsiderablytheoveralltransmission

delayofthemessageascomparedtomessageswitching.Ittendstorequiresmallerstorageallocationattheintermediateswitches.Italsohasbettererrorcharacteristicsandleadstomoreefficienterrorrecoveryprocedures,asitdealswithsmallerentities.Needlesstosay,packetswitchingpresentsdesignproblemsofitsown,suchastheneedtoreorderpacketsofagivenmessagethatmayarriveatthedestinationnodeoutofsequence.［6］

Notes

［1］Incircuitswitching,atotalpathofconnectedlinessetupfromtheorigintothedestinationatthetimethecallismade,andthepathremainsallocatedtothesource-destinationpair(whetherusedornot)untilitreleasedbythecommunicatingparties.

在电路交换中,当呼叫发生时,由呼叫源点到终点之间要建立整个通路的连线,而且在通信双方释放该电路之前,此通路一直保持分配给这对源点-终点(不管通路是否使用)。

·thecallismade是一个定语从句,修饰thetime,可译成:呼叫(被做出)的时候。

·allocated是过去分词,在句中作形容词用。［2］Theswitches,calledcircuitswitches(orofficeexchangeintelephonejargon),havenocapabilityofstoringormanipulatinguser’sdataontheirwaytothedestination.

称为电路交换机(电话行业用语称之为局交换)的交换设备没有存储或控制用户数据送往终点的能力。

·storing和manipulating均为动名词形式,usersdata系它们的动词宾语。［3］Oncethemessageisentirelyreceived,themessageprocessorexaminesitsheader,andaccordinglydecidesonthenextoutgoingchannelonwhichtotransmitit.

一旦报文被完全收到,报文处理机就检查其报头,并依此决定该报文传送的输出通道。

·onwhich中的which代表前面的channel。［4］Burstinessisaresultofthehighdegreeofrandomnessencounteredinthemessage-generationprocessandthemessagesize,andofthelowdelayconstraintrequiredbytheuser.

突发性是报文产生过程和报文长度的高随机性所造成的,也是用户对时延要求很短造成的结果。

·encounteredin…size系过去分词短语,作定语,修饰thehighdegreeofrandomness。［5］Ifafixeddedicatedend-to-endcircuitweretobesetupconnectingtheendusers,thenonemustassignenoughtransmissionbandwidthtothecircuitinordertomeetthe

delayconstraintwiththeconsequencethattheresultingchannelutilizationislow.

如果要建立一个固定专用的端到端电路以连接终端用户,则必须对该电路分配足够的传输带宽以合乎对时延的要求,其结果是电路的利用率很低。

·connectingtheendusers系现在分词短语,该现在分词在句中作状语,表示目的。

·withtheconsequencethat…可译为:其结果为……。［6］Needlesstosay,packetswitchingpresentsdesignproblemsofitsown,suchastheneedtoreorderpacketsofagivenmessagethatmayarriveatthedestinationnodeoutofsequence.

当然,分组交换亦有其设计上的麻烦,例如当报文无序地到达目的节点时,需要重新对该报文进行分组排序。

·needlesstosay:不用说,当然。

·needtoreorder中的动词不定式toreorder作need的定语。Exercises

1.PleasetranslatethefollowingphrasesintoEnglish.

(1)电路交换 (2)信头

(3)突发性 (4)返回信号

(5)随机性 (6)分组交换

(7)目的地址 (8)传输时延

(9)报文处理机 (10)专用电路

(11)报文交换 (12)误差控制

(13)中间交换设备 (14)给定最大长度

(15)电路利用率 (16)子网

(17)存储转发方式 (18)交换技术

(19)信息转移

2.PleasetranslatethefollowingphrasesintoChinese.

(1)thecapabilityofstoringandmanipulatinguser’sdata

(2)thespecialsignalingmessage

(3)awelldefinedblockofdatacalledamessage

(4)theinformationregardingthesourceanddestinationaddresses

(5)thecomputerreferredtoasamessageprocessor

(6)thestore-and-forwardtransmissiontechnique

(7)thedynamicallocationofthebandwidth

(8)theoveralltransmissiondelayofthemessage

(9)switchingtechnique

(10)totalpathofconnectedlines

(11)source-destinationpair

(12)communicationparties

(13)transmissionunit

(14)initialconnectioncostincurredinsettingupthecircuit

(15)lowdelayconstraintrequiredbytheuser

(16)thefixeddedicatedend-to-endcircuit

(17)lowchannelutilization

3.Filltheblankswiththebestchoice.

(1)Theswitch,

circuitswitches,havenocapabilityof

user’s

dataontheirwaytothedestination. a.calling,stored b.called,storing

c.tocall,tostore d.becalled,bestored

(2)Thecircuitissetupbyaspecialsignalingmessagefindsitswaythroughthenetwork,

channelsinthepathasitproceeds.

a.that,seizing b.where,seizing

c.who,seized d.one,beseized

(3)Theswitchingelementisacomputer

amessageprocessor,with

processingandstoragecapabilities.Messagetravelsindependentlyandasynchronously,

theirownwayfromsourcetodestination.

a.tocall,find b.calling,found

c.referredtoas,finding d.becalled,sending

(4)Avariationofmessageswitchingispacketswitching.Herethemessageisbrokenupintoseveralpiecesofa

maximumlength,

packets.

a.given,called b.give,call

c.giving,calling d.togive,tocall

(5)

,atotalpathofconnectedlinesissetupfromtheorigintothedestinationatthetimethecallismade,andthepathremainsallocatedtothesource-destinationpairuntilitisreleasedbythecommunicationparties.

a.Inmessageswitching

b.Inpacketswitching

c.Incircuitswitching

d.Indigitalswitching

(6)Inmessageswitching,thetransmissionunitisawelldefinedblockofdatacalledamessage.Inadditiontothetexttobetransmitted,amessagecomprises

.

a.astartandendofdatab.aheaderandchecksum

c.astartandastopbit

d.atransmitterandreceiver

(7)Avariationofmessageswitchingispacketswitching.Herethemessageisbrokenupintoseveralpiecesofagivenmaximumlength,

.

a.calledinformation

b.calledtexts

c.calledpackets

d.calledmessages

(8)Theheaderofamessagecontainsinformationregarding

,thechecksumisusedforerrorcontrolpurposes.

a.thetexttobetransmitted

b.thesourceanddestinationaddresses

c.thevoicechannels

c.manypacketsbetweentwocomputers

4.Answerthequestions.

(1)Listthetwobasictypesofswitchingtechnique.

(2)Whatiscircuitswitching?

(3)Whatispacketswitching?

(4)Listtheadvantageofstore-and-forwardtransmission.

PassageCATM

ATM(AsynchronousTransferMode)isbothamultiplexingandswitchingtechnique.Itwasinitiallyintendedtohandlehighbitrates,butithasinfactprovedtobeauniversaltechniquefortransportingandswitchinganytypeofdigitizedinformationatawidevarietyofbitrates.

ATMtransfersinformationinshortpacketscalled“cells”withafixedlengthof48bytesplusfiveheaderbytes,irrespectiveoftheunderlyingtypeoftransmission.Cellroutingisbasedontheprincipleoflogicalchannelswithdualidentification:thecellheadercontainstheidentifierofthebasicconnectiontowhichthecellbelongs-calledavirtualcircuit(VC)andtheidentifierofthegroupofVCstowhichtheconnectionbelongs-calledavirtualpath(VP).

ATMisrelatedtobothcircuitandpacketmodes.Becauseofthesimplicityoftheprotocolused,thetransferofcellstothenetworknodescanbehandledentirelybyhardware,whichleadstoveryshorttransittimeandhighusageoftransmissionpaths,evenatbitratesofseveralhundredmegabitsasecond.Ontheotherhand,ATMretainsalltheflexibilityofpacketmode,enablingonlyrequiredinformationtobeconveyed,offeringasimple,uniquemultiplexingmethodirrespectiveofthebitratesofthedifferentinformationflows,andallowingthesebitratestobevaried.

ATMdatedfromthebeginningofthe1980s:atthetime,peopleweretryingtofindthemostsuitabletechniqueforswitchinghighbitratechannelatmorethan100Mbps.In1988,theITUapprovedrecommendationI.121whichratifiedthechoiceofATMasthetargettransfermodeforbroadbandnetworksforalltypesofinformation,includinglowbitrateinformationsuchasvoice.In1991severaloperatorsandmanufacturersfoundedtheATMForumtoexpeditestandardization.TheATMForumnowhasmorethan600membersandhasasignificantinfluenceonATMstandardsandspecifications.ThefirstATMproductsappearedonthemarketin1992:theywereforlocalareanetworksandwere

designedtosolvetheproblemsofsharingthesamebearercircuitbetweencomputerterminalsastheycontinuetoincreaseinnumbersandpower.

AnATMnetworkcanbeconsidered,inafirstapproximation,asbeingthreeoverlaidfunctionallevels:aservicesandapplicationslevel,anATMnetworklevelandatransmissionlevel.Theapplicationsprovideanend-to-endservice.TheyusethelogicalconnectionsoftheATMnetworklevelwhichinturnmultiplexesandlogicallyroutestheinformationflowasATMcellsthroughthetransmissionlinkssharedbylogicalconnectionscalled

virtualconnections.Thetransmissionlevelprovidesthesephysicallinksandhandlestheactualphysicaltransportofthecells.

AnATMnetworkcantransportandswitchvoice,dataandvideowhich,seenfromtheaccess,usetraditionaldigitalinterfaceswiththesamequalityofservice.Thismeansthataphysicalconnectionbetweenanytwoterminalscanbereplacedwithanequivalentlogicalconnectionwhichismultiplexedwithothersinacommontransmissionlink.Thereso