Frame Relay Network

Features

Framerelaynetworkhasthefollowingfeatures:

(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)Hardwareforwarding,super-speedtransmission.DLCIisakindoflabel,shortandfixedlength,whichisconvenientforhardwarehigh-speedforwarding.

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.

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