ISDNL2n3Tutorial03.docx

ISDNL2n3Tutorial03.docx

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ISDNL2n3Tutorial03

ISDNLayer2（Q.921）andLayer3（Q.931）Tutorial

ISDNprotocolsdescribedhereinclude:

LAPD

InternationalVariantsofISDN

ISDNFrameStructure

ISDNTerminology 

ISDN（IntegratedServicesDigitalNetwork）isanalldigitalcommunicationslinethatallowsforthetransmissionofvoice,data,videoandgraphics,atveryhighspeeds,overstandardcommunicationlines.ISDNprovidesasingle,commoninterfacewithwhichtoaccessdigitalcommunicationsservicesthatarerequiredbyvaryingdevices,whileremainingtransparenttotheuser.DuetothelargeamountsofinformationthatISDNlinescancarry,ISDNapplicationsarerevolutionizingthewaybusinessescommunicate.ISDNisnotrestrictedtopublictelephonenetworksalone;itmaybetransmittedviapacketswitchednetworks,telex,CATVnetworks,etc.

TheISDNisillustratedhereinrelationtotheOSImodel:

ISDNapplications

LAPD

TheLAPD（LinkAccessProtocol-ChannelD）isalayer2protocolwhichisdefinedinCCITTQ.920/921.LAPDworksintheAsynchronousBalancedMode（ABM）.Thismodeistotallybalanced（i.e.,nomaster/slaverelationship）.Eachstationmayinitialize,supervise,recoverfromerrors,andsendframesatanytime.TheprotocoltreatstheDTEandDCEasequals.

TheformatofastandardLAPDframeisasfollows:

Flag

Addressfield

Controlfield

Information

FCS

Flag

LAPDframestructure

Flag

Thevalueoftheflagisalways（0x7E）.Inordertoensurethatthebitpatternoftheframedelimiterflagdoesnotappearinthedatafieldoftheframe（andthereforecauseframemisalignment）,atechniqueknownasBitStuffingisusedbyboththetransmitterandthereceiver.

Addressfield

Thefirsttwobytesoftheframeaftertheheaderflagisknownastheaddressfield.Theformatoftheaddressfieldisasfollows:

8

7

6

5

4

3

2

1

SAPI

C/R

EA1

TEI

EA2

LAPDaddressfield

EA1

FirstAddressExtensionbitwhichisalwayssetto0.

C/R

Command/Responsebit.Framesfromtheuserwiththisbitsetto0arecommandframes,asareframesfromthenetworkwiththisbitsetto1.Othervaluesindicatearesponseframe.

EA2

SecondAddressExtensionbitwhichisalwayssetto1.

TEI

TerminalEndpointIdentifier.Validvaluesareasfollows:

0-63

Usedbynon-automaticTEIassignmentuserequipment.

64-126

UsedbyautomaticTEIassignmentequipment.

127

UsedforabroadcastconnectionmeantforallTerminalEndpoints.

Controlfield

ThefieldfollowingtheAddressFieldiscalledtheControlFieldandservestoidentifythetypeoftheframe.Inaddition,itincludessequencenumbers,controlfeaturesanderrortrackingaccordingtotheframetype.

FCS

TheFrameCheckSequence（FCS）enablesahighlevelofphysicalerrorcontrolbyallowingtheintegrityofthetransmittedframedatatobechecked.Thesequenceisfirstcalculatedbythetransmitterusinganalgorithmbasedonthevaluesofallthebitsintheframe.ThereceiverthenperformsthesamecalculationonthereceivedframeandcomparesitsvaluetotheCRC.

Windowsize

LAPDsupportsanextendedwindowsize（modulo128）wherethenumberofpossibleoutstandingframesforacknowledgementisraisedfrom8to128.Thisextensionisgenerallyusedforsatellitetransmissionswheretheacknowledgementdelayissignificantlygreaterthantheframetransmissiontimes.Thetypeofthelinkinitializationframedeterminesthemoduloofthesessionandan"E"isaddedtothebasicframetypename（e.g.,SABMbecomesSABME）.

Frametypes

ThefollowingaretheSupervisoryFrameTypesinLAPD:

RR

Informationframeacknowledgementandindicationtoreceivemore.

REJ

Requestforretransmissionofallframesafteragivensequencenumber.

RNR

Indicatesastateoftemporaryoccupationofstation（e.g.,windowfull）.

ThefollowingaretheUnnumberedFrameTypesinLAPD:

DISC

Requestdisconnection

UA

Acknowledgementframe.

DM

ResponsetoDISCindicatingdisconnectedmode.

FRMR

Framereject.

SABM

Initiatorforasynchronousbalancedmode.Nomaster/slaverelationship.

SABME

SABMinextendedmode.

UI

UnnumberedInformation.

XID

ExchangeInformation.

ThereisoneInformationFrameTypeinLAPD:

Info  

Informationtransferframe.

ISDNdecode

Interestedinmoredetailsabouttestingthisprotocol?

InternationalVariantsofISDN

TheorganizationprimarilyresponsibleforproducingtheISDNstandardsistheCCITT（NowITU）.TheCCITTstudygroupresponsibleforISDNfirstpublishedasetofISDNrecommendationsin1984（RedBooks）.Priortothispublication,variousgeographicalareashaddevelopeddifferentversionsofISDN.ThisresultedintheCCITTrecommendationofacommonISDNstandardforallcountries,inadditiontoallocatedvariantsdefinableforeachcountry.

Theuseofnation-specificinformationelementsisenabledbyusingtheCodesetmechanismwhichallowsdifferentareastousetheirowninformationelementswithinthedataframes.

FollowingisadescriptionofmostISDNvariants:

NationalISDN1（Bellcore）

ThisvariantisusedintheUSAbyBellcore.Ithasfournetwork-specificmessagetypes.Itdoesnothaveanysingleoctetinformationelements.InadditiontoCodeset0elementsithasfourCodeset5andfiveCodeset6informationelements.

NationalISDN-2（Bellcore）

ThemaindifferencebetweenNationalISDN-1andISDN-2isparameterdownloadingviacomponents（acomponentbeingasub-elementoftheExtendedFacilityinformationelement）.ThesecomponentsareusedtocommunicateparameterinformationbetweenISDNuserequipment,suchasanISDNtelephone,andtheISDNswitch.

OtherchangesaretheadditionoftheSEGMENT,FACILITYandREGISTERmessagetypesandtheSegmentedMessageandExtendedFacilityinformationelements.Also,somemeaningsoffieldvalueshavechangedandsomenewacceptedfieldvalueshavebeenadded.

5ESS（AT&T）

ThisvariantisusedintheUSAbyAT&T.ItisthemostwidelyusedoftheISDNprotocolsandcontains19network-specificmessagetypes.IthasnoCodeset5,butdoeshave18Codeset6elementsandanextensiveinformationmanagementelement.

EuroISDN（ETSI）

ThisvariantistobeadoptedbyalloftheEuropeancountries.Presently,itcontainssingleoctetmessagetypesandhasfivesingleoctetinformationelements.WithintheframeworkoftheprotocoltherearenoCodeset5andCodeset6elements,howevereachcountryispermittedtodefineitsownindividualelements.

VN3,VN4（France）

ThesevariantsareprevalentinFrance.TheVN3decodingandsomeofitserrormessagesaretranslatedintoFrench.Itisasub-setoftheCCITTdocumentandonlyhassingleoctetmessagetypes.ThemorerecentVN4isnotfullybackwardcompatiblebutcloselyfollowstheCCITTrecommendations.AswithVN3,sometranslationhastakenplace.Ithasonlysingleoctetmessagetypes,fivesingleoctetinformationelements,andtwoCodeset6elements.

1TR6（Germany）

ThisvariantisprevalentinGermany.Itisasub-setoftheCCITTversion,withminoramendments.TheprotocolispartEnglishandpartGerman.

ISDN30[DASS-2]（England）

ThisvariantisusedbyBritishTelecominadditiontoETSI（seeabove）.Atlayers2and3thisstandarddoesnotconformtoCCITTstructure.Framesareheadedbyoneoctetandoptionallyfollowedbyinformation.HowevermostoftheinformationisIA5coded,andthereforeASCIIdecoded.

Australia

In1989AustralianISDNwasintroduced. ThisusedTelecomAustraliaspecifiedprotocolsTPH1856forPRIandTPH1962forBAI. ThesewereadoptedbytheRegulatorAustelasAustralianTechnicalStandardsin1990-TS014andTS013respectively. TheseprotocolsweredevelopedfromCCITTRedBookISDNrecommendations.

In1996,anewISDNwasestablishedusingEuroISDNprotocols. TheRegulator（Austel）issuednewStandards,thesebeingTS031forBAIandTS038forPRI. ThesewerereplacedbynewindustryStandardsin2001,thesebeingAS/ACIFS.031andAS/ACIFS.038forBAIandPRIrespectively.

TherearecurrentlynoAustralianISDNBAI（TS013）servicesinoperation,whilethereareasmallanddecliningnumberofAustralianISDNPRI（TS014）inservice.

AllAustraliancarriernetworksareEuroISDNcapable,buttheremaybesomedifferencesinSupplementaryServicesoffered. SomesmallercarriernetworksarealsoAustralianISDN（TS014）capable.  

ThemajorcarrieronlyprovidesEuroISDNbasedservices.

NTT-Japan

TheJapaneseISDNserviceprovidedbyNTTisknownasINS-Netanditsmainfeaturesareasfollows:

∙Providesauser-networkinterfacethatconformstotheCCITTRecommendationBlueBook.

∙Providesbothbasicandprimaryrateinterfaces.

∙Providesapacket-modeusingCaseB.

∙SupportedbySignallingSystemNo.7ISDNUserPartwiththenetwork.

∙Offeredasapublicnetworkservice.

ARINC746

Inpassengerairplanestodaytherearephonesinfrontofeachpassenger.ThesetelephonesareconnectedinaT1networkandtheconversationistransferredviaasatellite.ThesignallingprotocolusedisbasedonQ.931,butwithafewmodificationsandisknownasARINC746.TheleadingcompaniesinthisareaareGTEandAT&T.InordertoanalyzeARINC,theLAPDvariantshouldalsobespecifiedasARINC.

ARINC746Attachment11 

ARINC（AeronauticalRadio,INC.）Attachment11describestheNetworkLayer（layer3）messagetransfernecessaryforequipmentcontrolandcircuitswitchedcallcontrolproceduresbetweentheCabinTelecommunicationsUnit（CTU）andSATCOMsystem,NorthAmericanTelephoneSystem（NATS）,andTerrestrialFlightTelephoneSystem（TFTS）.TheinterfacedescribedinthisattachmentisderivedfromtheCCITTrecommendationsQ.930,Q.931andQ.932forcallcontrolandtheISO/OSIstandardsDIS9595andDIS95