数据通信与网络14-X25-1

Chapter14X.25第十四章X.2514.1X.25Layer14.2PacketLayerProtocol12.5SummaryContentsIntroductionofX.25X.25isapacketswitchingprotocol,developedbytheITU-Tin1976.TheformaldefinitionofX.25isaninterfacebetweenDTEandDCEforterminaloperationatthepacketmodeonpublicdatanetworks.Informally,wecansaythatX.25isapacketswitchingprotocolusedinawideareanetwork.X.25defineshowapacket-modeterminalcanbeconnectedtoapacketnetworkfortheexchangeofdata.ConceptualoverviewofX.25IntroductionofX.25AlthoughX.25isanend-to-endprotocol,theactualmovementofpacketsthroughthenetworkisinvisibletoit.IntroductionofX.25X.25isknownasasubscribernetworkinterface(SNI,用户网络接口)protocol.Itdefineshowtheuser’sDTEcommunicateswiththenetworkandhowpacketsaresentoverthatnetworkusingDCEs.X.25usesavirtualcircuitapproach(SVCorPVC)topacketswitching(包交换),andusesasynchronousTDM(异步时分复用)tomultiplexpackets(包复用).14.1X.25LayersTheX.25protocolisorganizedintothreelayers:

thephysicallayer,theframelayer,andthepacketlayer.Theselayersdefinefunctionsatthephysical,datalink,andnetworklayersoftheOSImodel.Atthephysicallayer,X.25specifiesaprotocolcalledX.21.X.21iscloseenoughtootherphysicallayerprotocols,suchasEIA-232,thatX.25isabletosupportthemaswell.X.25providesdatalinkcontrolsusingabit-orientedprotocolcalledlinkaccessprocedure,balanced(LAPB,平衡式链路访问规程)whichisasubsetofHDLC.14.1.1PhysicalLayer(物理层)14.1.2FrameLayer(Datalinklayer)14.1.3PacketLayer(Networklayer)ThenetworklayerinX.25iscalledPacketlayerprotocol(PLP).Itisresponsibleforestablishingtheconnection,transferringthedata,andterminatingtheconnection.X.25requireserrordetectionandcorrectioninBoththedatalinklayerandthenetworklayer.AtthePLP,aheadercontainingcontrolinformationisaddedtotransformthedataintoaPLPpacket.PLPpacketarepassedtotheLAPB,andareencapsulatedintoLAPBinformationframes(I-frame)andpassedtothephysicallayertobesent.14.1.3PacketLayer(Networklayer)14.2PacketLayerProtocolX.25usesTwotypesofPLPpackets

Informationpackets(I-packets) Controlpackets(C-packets)Theinformationpacketcontainsuserdata

comingfromtheupperlayers.Thecontrolpacketdoesnotcontainuserdata,butcontaininformationfieldnecessaryfortheoperationofthenetwork.14.2.1InformationpacketInformationpacketareusedtotransmituserdata.Thegeneralformatinclude: Aheader AuserdatafieldGeneralformatidentifier(GFI,通用格式标识符)field QNotdefined

DUsedinpacketsequencing

Mod

说明序列号字段的大小(01，序列号3比特

(0-7);10，序列号7比特

(0-127))Virtualcircuitidentificationfields(LCGNandLCN)

逻辑信道组号(LCGN),逻辑信道号(LCN)Controlfield P(R)packetreceived,Piggyback P(S)packetssent

MUsedinpacketsequencing(More)14.2.1Informationpacket14.2.2ControlpacketsTherearetwocategoriesofcontrolpackets:Thefirstcategoryisusedforflowanderrorcontrol.

ConsistofonlyaheaderThesecondcategoryisusedforconnection,termination,andmanagementcontrol.

这种类型的控制包在头部之外可以携带额外的信息，但是这些信息仅仅用于控制，不包含任何用户数据。14.2.3Completepacketsequence(安全包序列)Themaximumlengthofthedatafieldinaninformationpacketvariedfromnetworktonetwork,baseinpartonthelimitationsofthehardware.Itisimportanttobeabletobreakupalongmessageamongmultiplepacketsbutstillkeepthosepacketsidentifiableasasinglecontiguoustransmission.Acompletepacketsequenceinvolvestheuseoftwotypesofpackets:ApacketsandBpackets.Apacketisoneinwhichthedatafieldisfullandatleastoneadditionalpacketisneededtocarrytheremainderofthemessage. AnApacketisidentifiedbya1intheMore(M)fieldandbya0inthedataterminal(D)field. ~A包：M=1&D=0AnyApacketmustalwaysbefollowedbyatleastoneotherpacketanddonotrequireacknowledgment.14.2.3CompletepacketsequenceBpacketscaneitherstandalongoractasthefinalpacketinamultipacketsequence.ThedatafieldofaBpacketsdoesnothavetobecompletelyfull.Bpacketsalwayshave1intheDfieldtoindicatethatanacknowledgmentisrequired. ~B包：D=1AcompletepacketsequenceconsistsofoneBpacketfollowing0ormoreApacketsAcompletepacketsequence:B,AB,AABetc.14.2.3CompletepacketsequenceAcknowledgments: thecompletepacketsequencecanbedividedintosubsequencesofoneormoreApacketsandaBpacket.~对子序列确认，提高效率(可靠性)

D=1thepacketrequiresanacknowledgment.

D=0noacknowledgmentisneeded.

M=1morepacketaretofollow.

M=0Itisthelastpacketinthesequence.

14.2.3CompletepacketsequenceSequenceandsubsequence14.2.3CompletepacketsequenceAABAABAABA包:D=0&M=1

B包:D=1&(M=0orM=1)X.25allowsupto4096multiplexedchannelstobeidentifiedbetweeneachDTEandDCE.IDnumbersareassignedtoeachchanneltoidentifywhichpacketbelongstowhichchannel.Thesenumberarenotpermanent,butareallocateddynamically.Thecallingandcalledhostsusedifferentnumberstoidentifyavirtualcircuit.14.2.4VirtualchannelIDnumber14.2.4VirtualchannelIDnumberAllocationofvirtualchannelIDnumberSUMMARYX.25isthepacketswitchingprotocol.X.25definestheproceduresfordatatransmissionbe