Features
La rete Framerelay ha le seguenti caratteristiche:
(1)Commonchannelsignalingisadopted.Thelogicalconnectionanduserdatathatcarrycallcontrolsignalingareseparated.Forexample,AnsiT1.603andITU-TAnnexAbothuseDLCI=0asthesignalingchannel.Themultiplexingandexchangeoflogicalconnectionsoccursinthesecondlayer,therebyreducingthelevelofprocessing.
(2)Simplifythemechanism.FrameRelayisstreamlinedx.25protocol,cancelthesecondlayerofflowcontrolanderrorcontrol,onlybytheend-to-endhigh-levelprotocol.Theuser-networkinterfaceandtheinternalprocessingfunctionsofthenetworkaregreatlysimplified,resultinginlowlatencyandhighthroughputperformance.Framerelaysimplyprocessestheframeandthenforwardsit.Theprocessingfunctionsinclude:checkingwhethertheDLCIintheframeheaderisvalid,transmittingtheframewhenitisvalid,anddeletingitwhenitisinvalid;checkingwhethertheframeiscorrect,transmittingtheframeifitiscorrect,anddeletingitifitiswrong.Therefore,itisatransmissionmechanismthatdetectsbutdoesnotcorrect.
(3)Usehigh-speedvirtualcircuits.Framerelaycancelsthethirdlayer,transfersmultiplexingtothesecondlayer,andestablishesmultiplelayertwologicalchannelsononephysicalconnection,whichrealizesstatisticalmultiplexinganddynamicallocationofbandwidth,therebyimprovingefficiencyandthroughput.Reducethetimedelay.
(4)Largeframetransmission,adapttoburst.TheframelengthofFRismuchlongerthanx.25Thepacketlengthislarge,theuseoflargeframesisusedfortransmission,theframelengthisvariable,theinformationlengthoftheexchangeunit(frame)islongerthanthepacketexchange,upto1024~4096bytes,andthereservedframelengthcanreachatleast1600bytes,whichissuitableforencapsulatingLANdataUnit,suitablefortransmittingburstservices(suchascompressedvideoservices,wwwservices,etc.).
(5) Hardware transmissio, eximius velocitas.
Theframeformatoftheframerelaynetwork
TheframerelaynetworkadoptsQ.The922corelayerprotocolservesasthedatalinklayerprotocol.
1.Theflagfield
Thelengthoftheflagfieldis1byte,theformatis01111110,anditisusedforframedelimitation.Allframesstartandendwithaflagfield,andtheendflagofoneframecanalsobeusedasthestartflagofthenextframe.Inordertoensurethetransparenttransmissionofdata,theF(0111110)fieldisnotallowedinotherfields.Inthebitstringbetweenthetwoflagfields,iftherehappenstobethesamecombinationastheflagfield,itwillbemistakenasaframeboundary.Inordertoavoidthiskindoferror,framerelayQ.The922coreprotocolusesa"0"bitfillingmethodtoprevent6consecutive1sfromappearingbetweentwoflagfieldsinaframe.Thespecificmethodis:atthesendingend,scanthebitstringbeforeaddingtheflagfield.Iffiveconsecutive1sarefound,immediatelyadda0afterthem.Afterreceivingtheframe,thereceivingendremovestheflagfieldatthebeginningandtheend,andscansthebitstring.Whenfiveconsecutive1sarefound,thesubsequent0sareimmediatelydeleted,sothattheoriginalbitstreamisrestored.
2.Addressfield
Theaddressfieldisusedtodistinguishmultipledatalinkconnectionsonthesamelinkinordertoachieveframemultiplexinganddemultiplexing.Thelengthoftheaddressfieldis2bytes,andcanbeexpandedto3or4bytesasneeded.
1)DLCI:Datalinkconnectionidentifier,10bitsinlength,usedtodistinguishdifferentlogicalconnectionsandrealizeframemultiplexing.
2)EA:Addressextensionbit,thefirstbitofeachbyteoftheaddressfieldistheEAbit.EA=1indicatesthatthenextbyteisstilltheaddressfield,andEA1indicatesthatthisbyteisthefinalbyteoftheaddressfield.
Topologystructure
Startopology,ringtopology,hierarchical(tree)topology,meshtopology.ThenetworkingofforeignframerelaysJudgingfromthenetworkingexperienceofforeigntelecommunicationsoperatingcompaniesthatprovideframerelayservices,therearemainlytwoways.
Thefirstistoupgradetheversionofthepacketswitchingnodeontheoriginalpublicpacketdatanetwork,usetheoriginalequipmentandtransmissionsystem,increasetheabilityoftheframerelayfunctionsoftwareorhardwareconfigurationtoprovideframerelaybusiness.ExamplesofthisapproachareFranceTelecom'sTranspacnetwork,FramelinkoftheformerGermanMinistryofPostsandTelecommunications,andTelenetintheUnitedStates.FranceTelecomaddedtheframerelayfunctiontotheoriginalAlcatelpacketnodemachineontheTRANSPACnetworktoprovideframerelayservices.
Althoughthismethodhastheadvantagesofeasyimplementationandcostsaving,withthedevelopmentofframerelayservices,therewillbeseveraldisadvantageswhenthenetworkscaleexpands.Forexample,sincethepacketnodeandtheframerelaynodeequipmentaretogether,andtheservicesprovidedaredifferent,itwillbedifficulttoobtainsatisfactoryresultsintermsofserviceprovision,transmissioncircuitscheduling,networkperformanceguarantee,andnetworkmanagement.
Thesecondmethodistoindependentlybuildaframerelaynetworkandestablishaframerelaynodededicatedtotheframerelayservice.ExamplesofthisapproachareFinlandinEurope,AT&T,SprintandWiltelintheUnitedStates.Finland'snationalframerelaypublicbackbonenetworkiscomposedof7framerelaynodesconnectedinpairs,using36120nodemachinesofXinqiaoCompany.
Architecture
Theusernetworkinterfaceprotocolarchitectureoftheframerelaynetworkisdividedintoacontrolplaneandauserplane.
Thecontrolplaneisresponsiblefortheprocessingandtransmissionofsignaling,whichisusedtoestablishandteardownlogicalconnections;theuserplaneisresponsibleforend-to-enduserdatatransmission.
1.ControlPlane
Thecontrolplaneusedtocarryservicesinframemodeissimilartocommon-channelsignalingincircuitswitching,inwhichindependentlogicalchannelsareusedforcontrolinformation.TheLAPDprotocolwitherrorcontrolandflowcontrolisusedatthedatalinklayertoprovidereliabledatalinkcontrolservicesbetweenusersandthenetworkthroughtheDchannel.ThisdatalinkserviceisusedinQ.933controlstheexchangeofsignalinginformation.
2.Userplane
TheuserplaneprotocolusedforinformationtransmissionbetweenendusersisLAPF.Amongthem,framerelayonlyusesitscorefunctionsandprovidespuredatabetweenusers.Linklayerframetransmissionservice(noflowcontrolanderrorcontrol).
Buildaframerelaynetwork
UsingthismethodofprovidingframerelaybasedonATMtechnology,itactuallycompletestheframerelayontheswitchesonbothsidesoftheoriginatingandterminatingends.InterworkingwithATMandATMandframerelay.Duringintercommunication,correspondingprotocolconversionisrequired.Schematicdiagramofintercommunication.
Whatneedstobeexplainedisthatwhentheswitchimplementstheseprotocolconversions,itshouldbeimplementedinaccordancewiththecorrespondinginternationalstandards,otherwiseitisdifficulttoensurebusinessconnectivity.FR-SSCSshouldcomplywithI.365.1recommendation,CPCSandSARshouldcomplywiththeAAL5specificationinI.362recommendation,andATMshouldcomplywithI.361recommendation.
Becauseoftheuneveneconomicandtechnologicaldevelopmentinvariousregionsofourcountry,whenconstructingaspecialframerelaybackbonenetwork,differentnetworkconfigurationsshouldalsobeadoptedaccordingtothebusinessneedsofvariousregions.Configureswitchingequipmentwithhighthroughputandstrongprocessingcapacityinareaswithdevelopedeconomyandlargebusinessvolume;configuresmallswitchingequipmentwithlowerthroughputandlowerprocessingcapacitybutrelativelycheappricesinotherareas;provincialcapitalsinremoteareaswherethereisnobusinessdemandinthenearfutureIncities,specialframerelayequipmentmaynotbeconfiguredforthetimebeing,andbackbonenetworkconstructioncanbecarriedouthierarchically.