SIST EN 300 937 V7.1.2:2003
(Main)Digital cellular telecommunications system (Phase 2+) (GSM); Data Link (DL) layer; General aspects (GSM 04.05 version 7.1.2 Release 1998)
- Abstract
CR SMG#31
- Status
- Published
- Publication Date
- 30-Nov-2003
- Current Stage
- 6060 - National Implementation/Publication (Adopted Project)
- Start Date
- 01-Dec-2003
- Due Date
- 01-Dec-2003
- Completion Date
- 01-Dec-2003
SIST EN 300 937 V7.1.2:2003 is the Slovenian national standard identical to EN 300 937 Version 7.1.2 from ETSI. It specifies the GSM Data Link (DL) layer general aspects for LAPDm, the protocol used to carry layer 3 signalling over the Dm channel on the GSM PLMN radio interface between the Mobile Station (MS) and the network.
What does SIST EN 300 937 V7.1.2:2003 specify?
SIST EN 300 937 V7.1.2:2003 specifies the general behaviour, services and structure of LAPDm for GSM Phase 2+. It covers signalling on the Dm channel, and says that use on other channel types is for further study.
The document is organized into clauses on scope and references, concepts and terminology, an overview of LAPDm functions and procedures, service characteristics, data link layer structure, and specific requirements, followed by document history. It also ties the protocol to GSM04.06 for detailed frame and procedure definitions.
LAPDm is defined using concepts from OSI, ISDN data link layer work, X.25 packet mode links, and HDLC frame structure. In practice, that means the standard builds on established link-layer ideas and tells implementers how to apply them to GSM signalling.
What are the key requirements of SIST EN 300 937 V7.1.2:2003?
SIST EN 300 937 V7.1.2:2003 sets out how the link layer carries layer 3 messages, how channels and SAPIs are used, and which modes are allowed on each GSM control channel. The most important points are the transfer modes, the priority rules, and the channel restrictions in clause 6.
Channel support and transfer modes
- LAPDm supports unacknowledged operation and acknowledged multiple frame operation. In practice, this tells designers whether signalling is sent with no link-layer acknowledgement or with retransmission and flow control.
- BCCH and PCH+AGCH support only unacknowledged operation. This matters because broadcast and paging-related signalling uses the simpler unacknowledged form.
- SDCCH, SACCH, and FACCH support the modes stated in clause 6.1, with the detailed split depending on the channel and SAPI. This is the part readers check when mapping a service to a radio channel.
Primitives and procedures
- The service is expressed through primitives such as DL-UNITDATA, DL-DATA, DL-ESTABLISH, DL-SUSPEND, DL-RESUME, DL-RECONNECT, and DL-RELEASE. These names matter because they show the exact layer-to-layer actions expected by implementations.
- Random access is handled through DL-RANDOMACCESS and matching physical-layer primitives. In practice, this links layer 3 access requests to the random access burst on RACH.
- Administrative support includes MDL-ERROR-INDICATION and MDL-RELEASE-REQUEST. These are the mechanisms for reporting unrecoverable link problems and for abnormal local-end release.
Priority, segmentation, and limits
- For SDCCH, SAPI=0 has the highest priority and SAPI=3 the lowest; on SACCH, the scheduling rules prevent low-priority starvation and constrain how frames are interleaved. This affects message ordering and latency under load.
- Segmentation and concatenation are provided in acknowledged mode when a layer 3 message is longer than one frame. In unacknowledged operation, the link layer does not provide segmentation.
- In acknowledged mode, the window size is k=1 for SAPI=0 and SAPI=3. For implementers, this means one outstanding I-frame at a time for those SAPIs.
- The standard also requires sufficient processing capacity in the MS and network to avoid a receiver-busy state for SAPI=0.
What terms does SIST EN 300 937 V7.1.2:2003 define?
- Link Access Procedure on the Dm channel (LAPDm) - The GSM layer 2 protocol defined here for signalling between layer 3 entities over the radio interface.
- Dm channel - The collection of GSM signalling channels used for LAPDm transport between the MS and the network.
- Service Access Point Identifier (SAPI) - The identifier used to select the service access point and distinguish signalling uses such as call control and SMS.
- Data Link Connection Identifier (DLCI) - The identifier for a data link connection endpoint, formed from the SAPI and the control-channel type.
- Unnumbered Information (UI) frame - The frame type used for unacknowledged information transfer.
- Set Asynchronous Balanced Mode (SABM) - The command used to establish multiple frame acknowledged operation.
- Random Access Channel (RACH) - The channel used for access requests, with LAPDm supporting the associated random access procedure.
Who uses SIST EN 300 937 V7.1.2:2003?
SIST EN 300 937 V7.1.2:2003 is used by engineers and integrators working on GSM Phase 2+ mobile stations, base/network equipment, and signalling software. It is also relevant to test teams, quality managers, and buyers checking whether equipment supports the required MS-network radio-interface data link behaviour.
What changed in SIST EN 300 937 V7.1.2:2003 from the previous edition?
SIST EN 300 937 V7.1.2:2003 is derived from Phase 2 GSM ETS 300 554, which is identified as GSM 04.05 version 4.0.3. The foreword also states the Release 1998 versioning rule: the second digit marks substantive changes and the third digit marks editorial changes.
Which standards are used with SIST EN 300 937 V7.1.2:2003?
| Standard | What it contributes |
|---|---|
| GSM01.04 | Abbreviations and acronyms used in the document |
| GSM04.01 | General layering principles for the MS-BSS interface |
| GSM04.03 | Channel structures and access capabilities |
| GSM04.04 | Layer 1 general requirements |
| GSM04.06 | Detailed data link layer specification |
| GSM04.07, GSM04.08, GSM04.10, GSM04.11 | Layer 3 signalling and SMS/supplementary-service context |
| GSM07.01, GSM07.02, GSM07.03, GSM07.05, GSM07.06 | Terminal adaptation and DTE-DCE interface topics |
| CCITT X.25, X.200, X.210, Q.920, Q.921 | Reference models and data link procedures |
| ISO 3309, ISO/IEC 4335 | HDLC frame structure and procedure elements |
What does the SIST EN 300 937 V7.1.2:2003 document contain?
The document contains the service model for LAPDm, the peer-to-peer protocol view, and the layer-to-layer primitive model. It includes figures showing entities and service access points, primitive action sequences, the data link layer reference model, and an example data link layer configuration.
It also gives the service primitive names used for data transfer, establishment, suspension, resumption, reconnection, release, error reporting, and random access. For practical work, this is the part that implementers use when designing signalling software, writing interface code, or building test cases for channel-specific behaviour.
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Frequently Asked Questions
SIST EN 300 937 V7.1.2:2003 is a standard published by the Slovenian Institute for Standardization (SIST). Its full title is "Digital cellular telecommunications system (Phase 2+) (GSM); Data Link (DL) layer; General aspects (GSM 04.05 version 7.1.2 Release 1998)". This standard covers: CR SMG#31
CR SMG#31
SIST EN 300 937 V7.1.2:2003 is classified under the following ICS (International Classification for Standards) categories: 33.070.50 - Global System for Mobile Communication (GSM). The ICS classification helps identify the subject area and facilitates finding related standards.
SIST EN 300 937 V7.1.2:2003 is available in PDF format for immediate download after purchase. The document can be added to your cart and obtained through the secure checkout process. Digital delivery ensures instant access to the complete standard document.
Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Digital cellular telecommunications system (Phase 2+) (GSM); Data Link (DL) layer; General aspects (GSM 04.05 version 7.1.2 Release 1998)33.070.50Globalni sistem za mobilno telekomunikacijo (GSM)Global System for Mobile Communication (GSM)ICS:Ta slovenski standard je istoveten z:EN 300 937 Version 7.1.2SIST EN 300 937 V7.1.2:2003en01-december-2003SIST EN 300 937 V7.1.2:2003SLOVENSKI
STANDARD
ETSIEN300937V7.1.2(2000-09)EuropeanStandard(Telecommunicationsseries)Digitalcellulartelecommunicationssystem(Phase2+);DataLink(DL)layer;Generalaspects(GSM04.05version7.1.2Release1998)GLOBALSYSTEMFORMOBILECOMMUNICATIONSRSIST EN 300 937 V7.1.2:2003
ETSIETSIEN300937V7.1.2(2000-09)2(GSM04.05version7.1.2Release1998)ReferenceREN/SMG-020405Q7R1KeywordsDigitalcellulartelecommunicationssystem,GlobalSystemforMobilecommunications(GSM)ETSI650RoutedesLuciolesF-06921SophiaAntipolisCedex-FRANCETel.:+33492944200Fax:+33493654716SiretN°34862356200017-NAF742CAssociationàbutnonlucratifenregistréeàlaSous-PréfecturedeGrasse(06)N°7803/88ImportantnoticeIndividualcopiesofthepresentdocumentcanbedownloadedfrom:http://www.etsi.orgThepresentdocumentmaybemadeavailableinmorethanoneelectronicversionorinprint.Inanycaseofexistingorperceiveddifferenceincontentsbetweensuchversions,thereferenceversionisthePortableDocumentFormat(PDF).Incaseofdispute,thereferenceshallbetheprintingonETSIprintersofthePDFversionkeptonaspecificnetworkdrivewithinETSISecretariat.Usersofthepresentdocumentshouldbeawarethatthedocumentmaybesubjecttorevisionorchangeofstatus.InformationonthecurrentstatusofthisandotherETSIdocumentsisavailableathttp://www.etsi.org/tb/status/Ifyoufinderrorsinthepresentdocument,sendyourcommentto:editor@etsi.frCopyrightNotificationNopartmaybereproducedexceptasauthorizedbywrittenpermission.Thecopyrightandtheforegoingrestrictionextendtoreproductioninallmedia.©EuropeanTelecommunicationsStandardsInstitute2000.Allrightsreserved.SIST EN 300 937 V7.1.2:2003
ETSIETSIEN300937V7.1.2(2000-09)3(GSM04.05version7.1.2Release1998)ContentsIntellectualPropertyRights.4Foreword.41Scope.51.1References.61.2Abbreviations.72Conceptsandterminology.73OverviewdescriptionofLAPDmfunctionsandprocedures.103.1General.103.2Unacknowledgedoperation.103.3Acknowledgedoperation.113.4Informationtransfermode.113.4.1InformationtransferontheBCCH.113.4.2InformationtransferonthePCH+AGCH.113.4.3InformationtransferontheDCCHs.113.5Releaseofdatalinks.114Servicecharacteristics.124.1General.124.2Servicesprovidedtolayer3.124.2.1General.124.2.2Priority.124.2.3Segmentation.124.2.4Unacknowledgedinformationtransferservice.134.2.5Acknowledgedinformationtransferservices.134.2.6Randomaccessprocedure.144.3Servicesrequiredfromthephysicallayer.144.4Administrativeservices.154.4.1Generaldescriptionofadministrativeservices.154.4.2Definitionofprimitivesforadministrativeservices.165Overviewofdatalinklayerstructure.165.1Functionalcomposition.165.2Identificationofdatalinkendpoints.165.3Datalinkprocedure.175.4Datalinkdistributionprocedure.175.5Randomaccessprocedures.176Specificrequirements.186.1ModeofoperationandallowedSAPIs.186.2Acknowledgedmodeofoperation.196.2.1Windowsize.196.2.2Processingcapacity.19History.20SIST EN 300 937 V7.1.2:2003
ETSIETSIEN300937V7.1.2(2000-09)4(GSM04.05version7.1.2Release1998)IntellectualPropertyRightsIPRsessentialorpotentiallyessentialtothepresentdocumentmayhavebeendeclaredtoETSI.TheinformationpertainingtotheseessentialIPRs,ifany,ispubliclyavailableforETSImembersandnon-members,andcanbefoundinETSISR000314:"IntellectualPropertyRights(IPRs);Essential,orpotentiallyEssential,IPRsnotifiedtoETSIinrespectofETSIstandards",whichisavailablefromtheETSISecretariat.LatestupdatesareavailableontheETSIWebserver(http://www.etsi.org/ipr).PursuanttotheETSIIPRPolicy,noinvestigation,includingIPRsearches,hasbeencarriedoutbyETSI.NoguaranteecanbegivenastotheexistenceofotherIPRsnotreferencedinETSISR000314(ortheupdatesontheETSIWebserver)whichare,ormaybe,ormaybecome,essentialtothepresentdocument.ForewordThisEuropeanStandard(Telecommunicationsseries)hasbeenproducedbyETSITechnicalCommitteeSpecialMobileGroup(SMG).Thepresentdocumentdefinesingeneraltermsthedatalinklayer(layer2)oftheMobileStation(MS)-networkinterfacewithinthedigitalcellulartelecommunicationssystem.TheETSfromwhichthepresentdocumenthasevolvedisPhase2GSMETS300554(GSM04.05version4.0.3).ThecontentsofthepresentdocumentissubjecttocontinuingworkwithinSMGandmaychangefollowingformalSMGapproval.ShouldSMGmodifythecontentsofthepresentdocument,itwillbere-releasedwithanidentifyingchangeofreleasedateandanincreaseinversionnumberasfollows:Version7.x.ywhere:7indicatesRelease1998ofGSMPhase2+.xtheseconddigitisincrementedforallchangesofsubstance,i.e.technicalenhancements,corrections,updates,etc.ythethirddigitisincrementedwheneditorialonlychangeshavebeenincorporatedinthespecification.ThespecificationfromwhichthepresentdocumenthasbeenderivedwasoriginallybasedonCEPTdocumentation,hencethepresentationofthepresentdocumentmaynotbeentirelyinaccordancewiththeETSIdraftingrules.NationaltranspositiondatesDateofadoptionofthisEN:18August2000DateoflatestannouncementofthisEN(doa):30November2000DateoflatestpublicationofnewNationalStandardorendorsementofthisEN(dop/e):31May2001DateofwithdrawalofanyconflictingNationalStandard(dow):31May2001SIST EN 300 937 V7.1.2:2003
ETSIETSIEN300937V7.1.2(2000-09)5(GSM04.05version7.1.2Release1998)1ScopeThepresentdocumentdescribesingeneraltermstheLinkAccessProceduresontheDmchannel,LAPDm.Theapplicationofthisprotocoltootherchanneltypesisforfurtherstudy.DetailsareprovidedinGSM04.06.ThepurposeofLAPDmistoconveyinformationbetweenlayer3entitiesacrosstheGSMPLMNradiointerface(MStonetworkinterface)usingtheDmchannel.NOTE1:ThetermDmchannelisusedforconveniencetodesignatethecollectionofallthevarioussignallingchannelsrequiredintheGSMsystem.SeealsoGSM04.03.ThedefinitionofLAPDmisbasedontheprinciplesandterminologyof:-CCITTRecommendationsX.200andX.210:thereferencemodelforOpenSystemsInterconnection(OSI);-CCITTRecommendationsQ.920andQ.921:thespecificationofLAPDfortheuser-networkinterfaceinISDN;-CCITTRecommendationX.25LAPB:user-networkinterfaceforpacketmodeterminals;and-ISO3309andISO4335:High-levelDataLinkControl(HDLC)standardsforframestructureandelementsofprocedures.LAPDmisaprotocolthatoperatesatthedatalinklayeroftheOSIarchitecture.Therelationshipbetweenthedatalinklayerandotherprotocollayersisdefinedbelow.NOTE2:Theinterfacebetweenthemobilestationandexternalterminalequipment/terminaladaptersisdefinedintheTechnicalSpecificationsoftheGSM07-series.NOTE3:ThephysicallayerontheradiointerfaceisdefinedinGSM04.04andlayer3isdefinedinGSM04.07,04.08,04.10and04.11.ReferenceshouldbemadetotheseTechnicalSpecificationsforthecompletedefinitionsoftheprotocolsandproceduresacrosstheGSMPLMNradiointerface.NOTE4:Theterm"datalinklayer"isusedinthemaintextofthepresentdocument.However,mainlyinfiguresandtables,theterms"layer2"and"L2"areusedabbreviations.Furthermore,inaccordancewithGSM04.07and04.08theterm"layer3"isusedtoindicatethelayerabovethedatalinklayer.LAPDmisindependentofthetransmissionbitrate.ItrequiresphysicalchannelswithcharacteristicsasdefinedinGSM04.03.Clause2describesbasicconceptsusedinthepresentdocumentandGSM04.06.Clause3givesanoverviewdescriptionofLAPDmfunctionsandprocedures.Clause4summarizestheservicesthatthedatalinklayerprovidestolayer3andtheservicesthatthedatalinklayerrequiresfromthephysicallayer.Clause5providesanoverviewofthedatalinklayerstructure.SIST EN 300 937 V7.1.2:2003
ETSIETSIEN300937V7.1.2(2000-09)6(GSM04.05version7.1.2Release1998)1.1ReferencesThefollowingdocumentscontainprovisionswhich,throughreferenceinthistext,constituteprovisionsofthepresentdocument.• Referencesareeitherspecific(identifiedbydateofpublication,editionnumber,versionnumber,etc.)ornon-specific.• Foraspecificreference,subsequentrevisionsdonotapply.• Foranon-specificreference,thelatestversionapplies.• Anon-specificreferencetoanETSshallalsobetakentorefertolaterversionspublishedasanENwiththesamenumber.• ForthisRelease1998document,referencestoGSMdocumentsareforRelease1998versions(version7.x.y).[1]GSM01.04:"Digitalcellulartelecommunicationssystem(Phase2+);Abbreviationsandacronyms".[2]GSM04.01:"Digitalcellulartelecommunicationssystem;MobileStation-BaseStationSystem(MS-BSS)interfaceGeneralaspectsandprinciples".[3]GSM04.03:"Digitalcellulartelecommunicationssystem(Phase2+);MobileStation-BaseStationSystem(MS-BSS)interfaceChannelstructuresandaccesscapabilities".[4]GSM04.04:"Digitalcellulartelecommunicationssystem;Layer1Generalrequirements".[5]GSM04.06:"Digitalcellulartelecommunicationssystem;MobileStation-BaseStationSystem(MS-BSS)interfaceDataLink(DL)layerspecification".[6]GSM04.07:"Digitalcellulartelecommunicationssystem(Phase2);Mobileradiointerfacesignallinglayer3Generalaspects".[7]GSM04.08:"Digitalcellulartelecommunicationssystem(Phase2+);Mobileradiointerfacelayer3specification".[8]GSM04.10:"Digitalcellulartelecommunicationssystem;Mobileradiointerfacelayer3SupplementaryservicesspecificationGeneralaspects".[9]GSM04.11:"Digitalcellulartelecommunicationssystem(Phase2+);Point-to-Point(PP)ShortMessageService(SMS)supportonmobileradiointerface".[10]GSM07.01:"Digitalcellulartelecommunicationssystem(Phase2+);GeneralonTerminalAdaptationFunctions(TAF)forMobileStations(MS)".[11]GSM07.02:"Digitalcellulartelecommunicationssystem(Phase2+);TerminalAdaptationFunctions(TAF)forservicesusingasynchronousbearercapabilities".[12]GSM07.03:"Digitalcellulartelecommunicationssystem(Phase2+);TerminalAdaptationFunctions(TAF)forservicesusingsynchronousbearercapabilities".[13]GSM07.05:"Digitalcellulartelecommunicationssystem(Phase2+);UseofDataTerminalEquipment-DataCircuitterminatingEquipment(DTE-DCE)interfaceforShortMessageService(SMS)andCellBroadcastService(CBS)".[14]GSM07.06:"Digitalcellulartelecommunicationssystem(Phase2);UseoftheVseriesDataTerminalEquipment-DataCircuitterminatingEquipment(DTE-DCE)interfaceattheMobileStation(MS)forMobileTermination(MT)configuration".[15]CCITTRecommendationX.25:"Interfacebetweendataterminalequipment(DTE)anddatacircuit-terminatingequipment(DCE)forterminalsoperatinginthepacketmodeandconnectedtopublicdatanetworksbydedicatedcircuit".SIST EN 300 937 V7.1.2:2003
ETSIETSIEN300937V7.1.2(2000-09)7(GSM04.05version7.1.2Release1998)[16]CCITTRecommendationX.200:"ReferenceModelofOpenSystemsInterconnectionforCCITTApplications".[17]CCITTRecommendationX.210:"OpenSystemsInterconnectionlayerservicedefinitionconventions".[18]CCITTRecommendationQ.920:"Integratedservicesdigitalnetwork(ISDN)user-networkinterface-DatalinklayerGeneralaspects".[19]CCITTRecommendationQ.921:"Integratedservicesdigitalnetwork(ISDN)user-networkinterface-Datalinklayerspecification".[20]ISO3309:"Informationtechnology-Telecommunicationsandinformationexchangebetweensystems-Highleveldatalinkcontrol(HDLC)procedures-Framestructure".[21]ISO/IEC4335:"Informationtechnology-Telecommunicationsandinformationexchangebetweensystems-Highleveldatalinkcontrol(HDLC)procedures-Elementsofprocedures".1.2AbbreviationsAbbreviationsusedinthepresentdocumentarelistedinGSM01.04.2ConceptsandterminologyThegenerallayeringprinciplesusedinthepresentdocumentandotherspecificationsinthe04seriesaregiveninGSM04.01.ThedatalinklayeristhenexttolowestlayeroftheOSIreferencemodel.Thedatalinklayerreceivesservicesfromthephysicallayerandprovidesservicestolayer3.Theservicesprovidedbythedatalinklayerarethecombinationoftheservicesandfunctionsprovidedbyboththedatalinklayerandthephysicallayer.AdatalinklayerServiceAccessPoint(SAP)isthepointatwhichthedatalinklayerprovidesservicestolayer3.TheServiceAccessPointisidentifiedbyaServiceAccessPointIdentifier(SAPI).OneormoredatalinkconnectionendpointscanbeassociatedwitheachdatalinklayerSAP.Seefigure1.Adatalinkconnectionendpointisidentifiedbyadatalinkconnectionendpointidentifier(asseenfromlayer3)andbyaDataLinkConnectionIdentifierDLCI(asseenfromthedatalinklayer).SAPIsandDLCIsusedbyLAPDmaredefinedinsubclause5.2.Layer3Layer3entityDatalinklayerserviceaccesspointDatalinkconnectionendpointDatalinklayerDatalinklayerentity*Figure1:Entities,serviceaccesspointsandendpointsSIST EN 300 937 V7.1.2:2003
ETSIETSIEN300937V7.1.2(2000-09)8(GSM04.05version7.1.2Release1998)Co-operationbetweendatalinklayerentitiesisgovernedbyapeer-to-peerprotocolspecifictothelayer.Forinformationexchangebetweentwoormorelayer3entities,anassociationmustbeestablishedbetweenthelayer3entitiesinthedatalinklayerusingadatalinklayerprotocol.Thisassociationiscalledadatalinkconnection.DatalinkconnectionsareprovidedbythedatalinklayerbetweentwoormoreSAPs(seefigure2).Layer3entityLayer3entityDatalinklayerserviceaccesspointDatalinkconnectionendpointDatalinkrelationshipFigure2:Peer-to-peerrelationshipDatalinklayermessageunitsareconveyedbetweendatalinklayerentitiesbymeansofphysicalconnection.Layer3requestsservicesfromthedatalinklayerviaserviceprimitives.Thesameappliesfortheinteractionbetweenthedatalinklayerandthephysicallayer.Theprimitivesrepresent,inanabstractway,thelogicalexchangeofinformationandcontrolbetweenthedatalinklayeranditsadjacentlayers.Theydonotspecifyorconstrainimplementations.Theprimitivesthatareexchangedbetweenthedatalinklayerandadjacentlayersareofthefollowingfourtypes(seealsofigure3).**Layer3ConfirmIndicationSAPDatalinklayerLayer2peer-to-peerprotocolRequestResponseNOTE:Thesameprincipleappliesfordatalinklayer-physicallayerinteractions.Figure3:PrimitiveactionsequenceSIST EN 300 937 V7.1.2:2003
ETSIETSIEN300937V7.1.2(2000-09)9(GSM04.05version7.1.2Release1998)TheREQUESTprimitivetypeisusedwhenahigherlayerisrequestingaservicefromthenextlowerlayer.TheINDICATIONprimitivetypeisusedbyalayerprovidingaservicetonotifythenexthigherlayerofactivitiesrelatedtotheprimitivetypeREQUEST.TheRESPONSEprimitivetypeisusedbyalayertoacknowledgereceipt,fromalowerlayer,oftheprimitivetypeINDICATION.TheCONFIRMprimitivetypeisusedbythelayerprovidingtherequestedservicetoconfirmthattheactivityhasbeencompleted.TheprecisespecificationofLayer-to-layerinteractionsisgiveninGSM04.06.InformationbetweenpeerentitiesandbetweenentitiesinadjacentlayersattachedtothesameSAPistransferredintwodifferenttypesofmessageunits:-messageunitsofapeer-to-peerprotocol;and-messageunitsthatcontainlayer-to-layerinformationconcerningstatusandspecializedservicerequests.Themessageunitsofthelayer3peer-to-peerprotocolarecarriedbythedatalinkconnection.Themessageunitscontaininglayer-to-layerinformationconcerningstatusandspecializedservicerequestsareneverconveyedoveradatalinkconnectionoraphysicalconnection.Thepresentdocumentintroduces(seealsofigure4):a)thepeer-to-peerprotocolforthetransferofinformationandcontrolbetweenanypairofdatalinklayerserviceaccesspoints;b)theinteractionsbetweenthedatalinklayerandlayer3,andbetweenthedatalinklayerandthephysicallayer.Layer3entityLayer3entityDatalinklayerentityDatalinklayerentitylayerlayerentityentityPhysicalPhysicalDatalinkl
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