TheabovecontentisreviewedandannouncedbytheNationalScienceandTechnologyTermReviewCommittee
Definition
AdvancedDataLinkcontrol(HDLC,High-levelDataLinkControl)isasetofprotocolsusedtotransmitdatabetweennetworknodes.InHDLC,dataisorganizedintoindividualunits(calledframes)tobesentoverthenetworkandreceivedbythereceiver.TheHDLCprotocolalsomanagesthedataflowandtheintervalbetweendatatransmissions.HDLCisoneofthemostwidelyusedprotocolsinthedatalinklayer.ThedatalinklayeristhesecondlayerintheOSIseven-layernetworkmodel.Thefirstlayeristhephysicallayer,whichisresponsibleforgeneratingandreceivingphysicalelectronicsignals.Thelayeristhenetworklayer,anditsfunctionincludesdeterminingtheroutebyaccessingtheroutingtable.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,thetransmissioncontrolfunctionandtheprocessingfunctionareseparated,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;
function
FrameControl
Thebasicunitoftransmissiononthedatalinkistheframe.Theframecontrolfunctionrequiresthesendingstationtodividethedatainformationsentfromthenetworkintoseveralcodegroups,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.Thecorrespondingsemanticsarecompletedaccordingtothefunctionoftheframe.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.