外文翻译校园网络多媒体的演变.docx

外文翻译校园网络多媒体的演变.docx

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外文翻译校园网络多媒体的演变

一、英文原文：

TheEvolutionofCampusNetworksTowardsMultimedia

CiroA.NoronhaJr.FouadA.Tobagi

4TheBackbone

Thecampusbackbonecarriestheaggregatetrafficfromthesubnetworks.Fromatheoreticalpoint,ofview,thedesignofthebackboneisnodifferent,fromthedesignofthesubnetworksthemselves;thestepstakeninsection3canberepeatedhere,withthedifferencethatthenetworkbeingdesignedisthebackbone,andits"users"arethesubnetworks.Thedifferenceisoneofscale.Inthissection,wewillbrieflycommentontheeffectofthebackbonetrafficontheaggregatethroughputofthesubnetwork（whichwasignoredinsection3）,andthencomputethebackbonetrafficunderavarietyofscenarios.

4.1ConnectingtheSubnetworktotheBackbone

Themaindifferencebetweenthetrafficto/fromthebackboneandtheinternalsubnetworktrafficisthefactthatformerisconcentratedinasinglepoint（thebackboneconnection）;inotherwords,allthetrafficfromthethesubnetworktothebackboneisdirectedtothebackboneconnection,andallthetrafficfromthebackbonetothesubnetworkoriginatesinthesamelocation.Intheotherhand,thesourcesanddestinationsoftheintra-subnetworktrafficarespreadoverthesubnetwork.

Theeffectofaddingthebackboneconnectionintheaggregatesubnetworkbandwidth'dependsonitstopology.Insubnetworksorganizedastrees,thecongestioninthebranchesnearthelocationofthebackboneconnectionmightlimittheaggregatethroughputtoavaluelowerthanwhatisattainedwhennobackbonetrafficispresent.Inthestarconfiguration,however,thehigh-speedchannelistheideallocationforattachingthebackboneconnection,andtheaggregatethroughputinthiscaseisalwayshigherthanitsvaluewhenthereisnotrafficfromthebackbone.Thereasonforthisissimple:

theinternalsubnetworktrafficusesbandwidthbothatthesourceandatthedestinationsegments,butthetrafficto/fromthebackboneusesbandwidthonlyatthesource/destinationsegments,leadingtoahigheraggregatethroughput.

Theactualincreaseordecreaseintheaggregatebandwidthwillbeafunctionofa,thefractionofthetrafficthat,isintra-subnetwork,andA,theratiobetweenthetrafficfromthebackbonetothetrafficintothebackbone（definedinsection2.）.Theincreaseishigherforsmallvaluesofa（mostofthetrafficisintersubnetwork）.

4.2StructureoftheBackbone

Inthissection,wewillgiveanumberofscenariosthatillustratehowthestructureofthebackbonecanchangeasafunctionofthesizeofthecampusnetwork;wewillusefigure3todeterminetherequiredcapacity.Theresultsareshownintable1;notethatthesolutionsforthestructureofthebackbonearenotunique,andwehavelistedoneofsuchsolutionsforeachcase.

Infigure5weshowonepossiblebackbonetopologywhichhasallthemainelementswehavebeendiscussing.Inthefigureweshowthesubnetworksbeingservedbyswitchinghubs.TheswitchinghubsservingsubnetworkswithlittleexternaltrafficcanbegroupedbyanATMmultiplexerbeforereachingtheATMswitch;thismultiplexercanperformsomelocalswitchingorrelycompletelyinthecentralswitch.HubsgeneratinglargertrafficcanbeconnecteddirectlytotheATMswitch.Theswitchisalsothebestlocationtoconnectcentralizedsharedresources,suchasvideoservers.TheATMswitchesthemselvescanbearrangedinahierarchicalfashion,dependingonthetrafficrequirements.

5AddingmultimediatotheotherOSIlayers

InthissectionwediscusstheissuesraisedbytheadditionofmultimediaservicesontheotherlayersoftheOSImodel.

5.1TheMediumAccessControl（MAC）Layer

Thesynchronousnatureofaudioandvideotrafficdictatesthatdatabedeliveredtoitsdestinationwithinstricttimingconstraints.Failuretodeliverdataontimeresultsinadiscontinuityofthevideooradegradationinthequality.Thebustynatureofdatatraffic,ontheotherhand,meansthatastationtransmitsdatainanunpredictablefashion,andtheamountofdatathatthestationtransmitsinaburstisalsorandom.

Bothtypesoftrafficaretobesupportedbythesamelocalnetwork.Indeed,multimediaapplicationsnaturallyinvolvebothtypesoftrafficsimultaneously;existinglocalareanetworksareexpectedtocarryaudioandvideotrafficalongsideexistingdataapplications.Mixingthetwotypesoftrafficonthesamenetworkrequiresspecialattention,particularlywheresharedresourcesarecontendedtobybothtypes.Thebandwidthavailableonlocalareanetworksegmentsisonesuchsharedresource,andisthesubjectofthissection.Itissharedbyalltrafficoriginatingatallstations,andaccessedbymeansoftheMediaAccessControl（MAC）protocol.Ideally,onewouldliketheMACprotocoltoincludetechniqueswhichreservethebandwidthnecessaryforsynchronoustraffic,soastoremoveanyeffectofbustytraffic.

TheCSMA/CDprotocol（IEEE802.3）doesnotdifferentiateamongthedifferenttraffictypes.Inessence,itoperatesasfollows.Astationwithapacketreadyfortransmissiontransmitsthepacketassoonasthechannelissensedidle.Ifitcollideswithanotherpacket,thenitattemptsagainbutafterincurringarandomreschedulingdelay.Themeanreschedulingdelayisdoubledwitheachcollisionincurred.Thisisclearlyinadequateforreal-timetraffic.Notonlyisthedelayincurredbypacketsnondeterministic,butitsvarianceisquitelarge,owingtotheexponentialbackoffalgorithm.However,sinceatlowloadsthisprotocolworkswell,itcanstillbeusedifonesegmentsthenetworktoinsurethatthisconditionismet.

IEEE802.5isatokenprotocoldevisedforaringnetwork.Itusesasingletoken,inthesensethatastationthathascompletedtransmissionwillnotissueanewtokenuntilthebusytokenreturnstoit.Sinceinaringnetworktheconnectiontothemediumisactive,apriorityschemecanbeimplementedbyassigningdynamicallyapriorityvaluetothetoken,andbyrestrictingaccesstotheringtopacketsofpriorityequalorhigherthanthecurrentlyassignedvalue.Thisschemecanbeappropriatelyusedtointegratethetwotypesoftrafficonthesamechannelsimplybygivingsynchronoustraffichigherprioritythanthebustydatatraffic.

InFDDI（ANSIX3T9.5）,anotherschemewithmultipleprioritylevelsisemployed,bytheuseofasetoftimers,thatregulateforhowlongthestationcantransmittrafficofeachpriority.

Insummary:

theassignmentofdifferenttypesofserviceforreal-timeanddatatrafficattheMAClayerisveryimportantwhencommunicatingmultimediainformation.ThisisnotofferedinthewidelydeployedIEEE802.3Othernetworks,suchastheIEEE802.5tokenringandFDDIcanprovidethisfunctionality,althoughmanyvendorshavechosennottoimplementit.Giventhelargebaseofdeployednetworks,itisnotreasonabletoexpectthatchangesintheMAClayertosupportmultimediawouldbeimplemented.Therefore,onemustseekotheroptions（suchasmakingsurethatthenetworkoperateswithlowenoughloadthattheproblemsdescribedinthissectionwillnotbeseen）.

5.2TheNetworkLayer

Asthechangesdescribedbeforetakeplaceinthenetwork,theroutetousewhenmovinginformationfromsegmenttosegmentbecomesincreasinglyimportant.Insimpletopologies,suchasstarwiththeswitchingelementinthecenter,routingistrivialbecausethereisonlyoneroutefromthesourcetothedestination.However,inpracticemultipleroutesmightbeprovidedbetweenagivenpairofstations,forreliabilityand/orincreased.

Currently,thepathtakenbyapacketfromitssourcetoitsdestinationthroughthenetworkisdeterminedbythebridgesandroutersthatdefinethenetworktopology.Transparentbridgesusethespanningtreealgorithm,i.e.,thebridges,workingtogether,identifyasubsetofthenetworktopologythatconstitutesaspanningtree,anddirectalltrafficthroughthisspanningtree.Linksnotonthespanningtreearenotusedandkeptinreserve,tobeactivatedincaseoffailureofalinkinthespanningtree.Moreover,thespanningtreeidentifiedforatopologyisessentiallychosenatrandom,anditisnotoptimuminanysense.Thishastwoconsequences:

first,extracapacityontheredundantlinksisneverused;andsecond,bottlenecksmaydevelopduetotrafficconcentrationinsomesegments,asaresultoftheparticularspanningtreechosen.Wehavediscussedthistrafficconcentrationinsection3.

Routers,ontheotherhand,havetheabilitytousemultiplespanningtrees;asamatteroffact,eachrouterforwardsthepacketsoverthespanningtreehavingitselfastheroot.Manyalgorithmsexisttocomputethoseroutesinadistributedfashion（RIP,OSPF,IS-IS,etc）,buttheyallemploythecostofthelinkasthemetricusedwhencomputingthespanningtree（thecostofalinkisanarbitraryfunction,inverselyproportionaltothelink’sbandwidth）.Routerscanrecoverfromnetworkfailures,andcanmakeuseofthebandwidthofalllinksavailable.However,inmostcases,allthetrafficbetweenasourceandadestinationwillbesentoverthesameroute.Moreover,current,routershavenoprovisionfortakingintoaccountreal-timerequirements（suchasdelay）whencomputingroutes,andthesystemofprioritiesimplementedisclearlyinadequateformultimediacommunications.

Ofspecialimportanceistheroutingofmulticasttraffic（video-conferenceisanexampleofasituationwherethiskindoftrafficisgenerated）.Formulticasttraffic,twokindsofroutescanbeidentified:

minimumdelayroutesandminimumcostroutes.Currentmulticastroutersusetheminimumdelaycriteriontocomputetheroutes,butthismightleadtou