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SynonymHDLCgenerallyreferstoadvanceddatalinkcontrol

TheabovecontentisreviewedandannouncedbytheNationalScienceandTechnologyTermReviewCommittee

Definition

AdvancedDataLinkcontrol(HDLC,High-levelDataLinkControl)isasetofprotocolsusedtotransmitdatabetweennetworknodes.InHDLC,dataisorganizedintoindividualunits(calledframes)tobesentoverthenetworkandreceivedbythereceiver.TheHDLCprotocolalsomanagesthedataflowandtheintervalbetweendatatransmissions.HDLCisoneofthemostwidelyusedprotocolsinthedatalinklayer.ThedatalinklayeristhesecondlayerintheOSIseven-layernetworkmodel.Thefirstlayeristhephysicallayer,whichisresponsibleforgeneratingandreceivingphysicalelectronicsignals.Thelayeristhenetworklayer,anditsofficiumincludesdeterminingtheroutebyaccessingtheroutingtable.Whentransmittingdata,thedataframeofthenetworklayercontainsthenetworkaddressesofthesourcenodeandthedestinationnode.Inthesecondlayer,thedataframeofthenetworklayerisencapsulatedthroughtheHDLCspecificationtoadddatalinkcontrolinformation.

AsanISOstandard,HDLCisbasedonIBM'sSDLCprotocol,andSDLCiswidelyusedinIBM'smainframeenvironment.InHDLC,SDLCiscallednormalresponsemode(NRM).Inthenormalresponsemode,thebasestation(usuallyamainframe)sendsdatatoalocalorremotesecondarystationthroughadedicatedlineinamulti-pathormulti-pointnetwork.Thiskindofnetworkisnotwhatweusuallysay,itisaclosednetworkthatisnotpublic,andthenetworkcommunicationadoptshalf-duplex.

DifferenttypesofHDLCareusedinnetworksandframerelaynetworksthatusetheX.25protocol.Thisprotocolcanbeusedinalocalareanetworkorawideareanetwork,regardlessofwhetherthenetworkispublicorprivate.

IntheX.25versionofHDLC,thedataframecontainsadatapacket.IntheX.25network,dataisdividedintoseveraldatapacketsbeforebeingsent,andthentherouterdetectsthenetworkconditionstodeterminetheroute.Eachdatapacketissenttothedestinationnode,andmergedintotheinitialdataatthedestinationnodeinthecorrectorder.HDLCofX.25versionadoptspoint-to-pointcommunication,andthecommunicationmodeadoptsfullduplexmode.ThistypeofHDLCcanensuretheerrorreleaseandcorrectorderingofframes,whichiscalledLAPB(LinkAccessProcessBalance).

Features

1.Transparenttransmission.Advanceddatalinkcontrolcantransparentlytransmitdataofanybitcombination."Transparency"isaveryimportantterm,whichmeansthatsomethingthatactuallyexistslookslikeitdoesn'texist."Transparenttransmission"meansthatthedatainformationhasnotchangedafterbeingtransmittedbytheactualcircuit.Therefore,forthetransmitteddatainformation,sincethiscircuithasnoeffectonit,itcanbesaidthatthedatainformation"cannotsee"thiscircuit,orthatthecircuitistransparenttothedatainformation.Inthisway,anycombinationofdatainformationcanbetransmittedonthiscircuit.

2.Highreliability.Intheadvanceddatalinkcontrolprocedure,thescopeoferrorcontrolistheentireframeexceptfortheFflag,whilethebasictransmissioncontrolproceduredoesnotincludeprefixesandsomecontrolcharacters.Inaddition,advanceddatalinkcontrolperformsnumberedtransmissionofIframes,whicheffectivelypreventsframere-receptionandmissedreceipt.

3.Hightransmissionefficiency.Inadvanceddatalinkcontrol,theadditionaloverheadbitsaresmall,allowingefficienterrorcontrolandflowcontrol.

4.Strongadaptability.Advanceddatalinkcontrolprocedurescanadapttovariousbittypesofworkstationsandlinks.

5.Flexiblestructure.Inadvanceddatalinkcontrol,thetransmissioncontrolofficiumandtheprocessingofficiumareseparated,thehierarchyisclear,andtheapplicationisveryflexible.

Type

Thedifferenttypesofhigh-leveldatalinkcontrol(HDLC)andtheirapplicationrangesarelistedbelow.

1.Normalresponsemode(NRM),applicationrange:multipointnetworkusingSDLC;

2.LinkAccessProtocol(LAP),applicationrange:earlyX.25Network;

3.Linkaccessprocessbalance(LAPB),applicationrange:X.25network;

4.ISDNlinkaccessprotocol-Dchannel(LAPD),applicationRange:ISDN-Dchannelandframerelay;

5.Modemlinkaccessprocedure(LAPM),applicationrange: errorchecking;

officium

FrameControl

Thebasicunitoftransmissiononthedatalinkistheframe.Theframecontrolofficiumrequiresthesendingstationtodividethedatainformationsentfromthenetworkintoseveralcodegroups,andaddtheaddressfield,controlfield,checkfieldandframestartandendflagstoeachcodegrouptoformaframetosend;Removetheflagfieldfromthereceivedframe,restoreittotheoriginaldatainformation,andsendittothenetworklayer.

Framesynchronization

Framesynchronizationmustbeachievedduringtransmissiontoensurethecorrectidentificationofeachfieldintheframe.

Errorcontrol

Whenanerroroccursinthetransmissionofdatainformationonthephysicallink,thedatalinkcontrolprocedurerequiresthereceivingendtodetecttheerrorandrecoverit.TheusualmethodsareautomaticRequestforretransmissionofARQandforwarderrorcorrection.WhentheARQmethodisused,inordertopreventthere-receivingandmiss-receivingofframes,framesareoftensentandreceivedbynumbering.Whenanunrecoverableerrorisdetected,thenetworklayershouldbenotifiedtodealwithitaccordingly.

Flowcontrol

Flowcontrolisusedtoovercomelinkcongestion.Itcanadjusttheinformationflowonthelinktoensurethatthedataratesentbythesenderiscompatiblewiththedataratethatthereceivercanreceive.Thecommonlyusedflowcontrolmethodistheslidingwindowcontrolmethod.

Linkmanagement

Theestablishment,maintenanceandterminationofdatalinks,thetransmissiondirectionofcontrolinformation,andthedisplayoftheworkingstatusofthestationareallwithinthescopeoflinkmanagement.

Transparenttransmission

Theflagsandsomefieldsusedintheproceduremustbeindependentoftheinformationtobetransmitted,whichmeansthatthedatalinkcantransmitavarietyofdatainformation,namelyTransparencyoftransmission.

addressing

Inamultipointlink,theframemustbeabletoreachthecorrectreceivingstation.

Abnormalstaterecovery

Whenanabnormalconditionoccursonthelink,suchasreceivinganunclearsequenceorfailingtoreceivearesponseovertime,itcanautomaticallyrestartandreturntonormaloperationstate.

Operationbehavior

TheoperationofHDLCistheprocessofexchangingthreetypesofframesbetweentwosites.Thecorrespondingsemanticsarecompletedaccordingtotheofficiumoftheframe.TherearethreemainoperationsofHDLC.Inthefirststage,oneofthetwopartiesmustinitializethedatalinksothatframescanbeexchangedinanorderlymanner.Atthisstage,thetwopartiesneedtoreachanagreementontheuseofvariousoptions.Afterthelinkisinitialized,thetwopartiesexchangedataandcontrolinformation,andimplementflowanderrorcontrol.Finally,oneofthetwopartiesmustsendasignaltoterminatetheoperation,thatis,disconnectthelink.

Establishalinkconnection

HDLCmustbeabletoinitializethelink,thatis,completethelinkconnection.InHDLC,useoneofthesixmodesettingcommandstorequestinitialization.ThesecommandsareasfollowsRoleandresponse;

(1)Notifytherequestingpartytoinitialize;(2)Pointoutwhichofthethreemodesoftherequest;thesemodesdeterminewhetheroneendisthemasterstationandcontrolstheinteraction,orwhetheritisTherefore,theycooperatewitheachotherwheninteracting;(3)Pointouttheserialnumberused.

Ifonepartyacceptsthisrequest,itsHDLCmodulereturnsanUnnumberedAcknowledged(UA)totheinitialization.Iftherequestisrejected,itsendsaDisconnectedMode(DM)frame.IntheHDLCprotocolentity,AsendsaSABMcommandtotheotherpartyBandstartsatimer.IfAcannotreceivetheUAsentbyB,thenAwillresendtheSABMcommandifthetimerexpires.IfAhasnotreceivedB'sUAorDM,thenthisprocesswillberepeatedcontinuously,orafterthespecifiednumberofretries,theentityabandonstheattemptandreportstheoperationfailuretothemanagemententity.Inthiscase,ahigherlevelisrequiredIntervention.TheprocessofunlinkingisthatonepartysendsaDISCcommand,andtheotherpartyrespondswithaUAconfirmation.Thedelinkingiscompleted.

Datatransmission

Datatransmissionisthetransmissionofframes,andthenormaldataexchangestateisafull-duplexexchangemode.WhenanentitysendsseveralI-framescontinuouslywithoutreceivinganydata,itsreceivingsequencenumberisonlyrepeatedcontinuously.IftheentitycontinuouslyreceivesseveralIframeswithoutsendinganyframes,thenthereceivingsequencenumberinthenextframeitsendsmustreflectthiscumulativeeffect.PleasenotethatinadditiontoIframes,dataexchangemayalsoinvolvemonitoringframes.Therewillalsobeabusystate.ThereasonforthisstatemaybethattherateatwhichtheHDLCentityprocessesIframescannotkeepupwiththerateatwhichtheseframesarrive,ortherateatwhichtheuserreceivesdataisnotasgoodastherateatwhichthedataintheIframesarrive.quick.Ineithercase,theentity'sreceivebufferwillfillup,anditmustusetheRNRcommandtopreventtheflowofIframesintothebuffer.Intheprocessofdatatransmission,theremayalsobeexamplesofusingREJcommandsforerrorrecovery.

Unlinking

TheHDLCmoduleofanypartyintheconnectioncanstarttheunlinkingoperation,whichmaybecausedbytheinterruptionofthemoduleitselfduetosomeerror,oritmaybeduetothehigh-levelTheuser'srequest.HDLCannouncestheterminationoftheconnectionbysendingadisconnect(DISC)frame,andtheotherpartymustreplywithUAtoindicatethatthedisconnectionisreceived.

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