V.34FaxRelayoverPacketNetworks

December2011

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TableofContents

Scope 3

StandardAbbreviations 3

Overview 3

V.34FaxCalloverthePSTN 4

T.30PhaseAV.34CallEstablishment 4

V.34Phase2LineProbing 5

V.34Phase3PrimaryChannelEqualizerTraining 5

PhaseBPreMessageProcedures 6

PhaseCInMessageProcedure 6

PhaseDPostMessageProcedure 7

PhaseECallrelease 7

V.34FaxRelayStimulusSignals 7

CallingFaxTone(CNG) 7

V.8AnswerTone(ANSam) 7

CNGandAnswerTone 7

V.8CM 8

V.21PreambleFlags 8

V.34FaxRelayinFallbackMode 8

V.34FaxRelayperT.38Version3 9

T.38CallSetup 10

WhyChooseAudioCodes? 11

References 12

V.34FaxRelayoverPacketNetworks

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ScopeThiswhitepaperprovidesatechnicaloverviewofV.34faxtechnologyand is intendedforFaxover IP (FoIP)application and test engineers, FoIP gateway vendors and customers, and FoIP customer support. A briefintroductioninV.34faxrelaytechnologyisgivenwithreviewingfaxsignalsexchangedduringdifferentphasesofV.34 fax call, stimulus signals for transitioninggateways toV.34 fax relay,differentmethodsofV.34 faxcommunicationovertheInternet,andoptimalmethodsofT.38callestablishment.

StandardAbbreviationsANSam V.8Answertone,2100Hz,amplitudemodulatedCI V.8CallIndicatorsignalCJ V.8CMterminatorCM V.8CallMenusignalJM V.8JointMenusignalCFR T.30ConfirmationtoReceiveCNG T.30CallingfaxtoneCSI T.30CalledSubscriberIdentificationDCN T.30DisconnectDCS T.30DigitalCommandSignalDIS T.30DigitalIdentificationSignalEOP T.30EndofPageECM T.30ErrorCorrectionModeHDLC HighlevelDataLinkControlMCF T.30MessageConfirmationPPS T.30PartialPageSignalSDP SessionDescriptionProtocolSIP SessionInitiationProtocolTCF T.30TrainingCheckFrameTSI T.30TransmittingSubscriberIdentification

OverviewThe late1990sandearly2000swere remarkable for theappearanceandwidespreadproliferationofhighspeedfaxesbasedontheV.34halfduplexmodulationsystem.FaxesfeaturingV.34modulationcapabilityarealsoknownassuperG3faxes.RelativetoregularG3faxes,V.34faxessignificantlyreducethetotaltimeoffaximagetransferoverPSTN.ThetimesavingisachievedthankstothefollowingV.34advantages:

Faximagetransferathigherdataratesofupto33600bpsvs.theregularG3maximumfaxrateof14400bps

Estimationofoptimal symbol rate,data signaling rateandothermodulationparameterswhileavoidingusingT.30TCF

CapabilityoffastrenegotiationofthedatasignalingratewithoutrestartingT.30PhaseB FastT.30controlat1200bpsfullduplexvs.300bpshalfduplexV.21ofregularG3faxes

TheT.38Recommendationinitiallypublishedin1998definedtheregularT.30faxrelayoverIP(FoIP).Duetohigh complexityofV.34modulation,anddue to theT.30 incompatibilityofV.34 fax relative to regular faxcommunication,forseveralyearstheT.38hadnosupportofV.34faxrelay;andthefirstthreeversionsofT.38namely,version0(dated1998),version1(dated2000),andversion2(dated2002)werebasedonregularfaxmodulationschemes.Twoalternativemethodsof transferringV.34 faxcallsoverpacketnetworkswereusedandcontinue tobeusedtoday:

V.34FaxRelayoverPacketNetworks

VoiceBandData(VBD)faxtransferperV.151.1andV.152 WH

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V.34faxfallbacktoregularG3faxrelayatdataratesofupto14400bps(V.17modulation)InVBDmode,allV.34faxsignalsaretransferredusingalowdistortioncompression,forexample,PCMA/laws.TheVBDmethod isattractivebecause it is leastcomplexcomparedtoFoIPand itallowsanativeV.34operationatfaxratesofupto33600bps.ButVBDtransfermaybeproblematicbecause:

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TheVBDstreamcannotbeusedbyfaxservers,internetawarefaxes,andotherfaxrelayorientedapplications

Broadbandwidthconsumptionevenwithoutredundancy(64kbps,fullduplex) Lowimmunitytopacketloss Lowtolerancetonetworkjitterandconstantdelay Low tolerance to sampling ratedifference (or clockoffset)betweenoriginateandanswer side

gateways Highsensitivitytoimperfectionsofechocanceling

Inorder to allow FoIP communicationbetweenV.34 fax terminalsbyusingT.38of version0, version1orversion2,aFoIPgatewaymayneedtoforcetheV.34faxterminalstooperate inafallbackmode limitedbyV.17modulationatdatasignalingrates(faxrates)ofupto14,400bps.Gatewaysmayusedifferentmethodsto force a V.34toV.17 fallback which result in establishing a T.38 session based on regular (nonV.34)modulationatbothsidesofcommunication.AgrowingusageofV.34faxterminalsrequiredadequatesignalprocessingbyFoIPgatewaysandanadaptationoftheFoIPprotocol.Accordingly,version3oftheT.38(dated2007)definesanextendedFoIPprotocolofV.34faxcommunicationoverIPatfaxratesofupto33,600bps.ThefollowingsectionssummarizeimportantT.30definitionsforfullV.34faxrelayandV.34faxrelayinfallbackmode.

V.34FaxCalloverthePSTNAV.34 fax callmay be divided into the following phases (according to ITUT recommendations T.30 from09/2005andV.34from02/98):

PhaseAV.34CallEstablishment V.34Phase2Lineprobing V.34Phase3Primarychannelequalizertraining PhaseBPremessageprocedures PhaseCInmessageprocedure PhaseDPostmessageprocedure PhaseECallrelease

Figure1andFigure2showanexampleofasinglefaxcall(PhaseAthroughtoD).

T.30PhaseAV.34CallEstablishmentFigure 1 shows a typical signal flow corresponding to T.30 Phase A of V.34 call establishment.PhaseAstartswitha tonalexchangebetweencallingandanswering fax terminals. It includesV.34Phase1beginningwithanANSamtone.DuringV.34Phase1,thefaxterminalsexchangeV.8CMandJMmessagestodefinethefaxcalltype. IftheansweringfaxdoesnotconfirmhavingV.34faxcapability,thenwhentheV.8signalexchange iscomplete,thefaxterminalsenterregularT.30PhaseB.IftheansweringfaxconfirmsV.34capability,theterminalsenterproperV.34faxprocedures,showninFigure2.Generally,amorecomplexscenarioispossiblewhenanansweringfaxterminalusesregularT.30PhaseBfortransmissionofT.30capabilities(DIS)beforeitentersnormalV.34operation.Inthiscase,thecallingfaxmaytransmittheV.8CIsignaltoinitiateV.34Phase1.

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Figure1:T.30PhaseAV.34CallEstablishment

V.34Phase2LineProbingEnteringV.34Phase2,faxterminalssendfullduplex INFO0cand INFO0asignalsat600bpstoexchangethesupported symbol rates and otherV.34 capabilities. After a successful exchange by INFO0 sequences, theoriginating faxmodem transmits lineprobing signals.The answering faxmodem receiving the lineprobingsignals analyzes the channel characteristics and selects the optimal symbol rate, carrier frequency, preemphasisfilter,andpowerreductiontobeusedduringV.34Phase3andlaterforeveryfaxpageofT.30PhaseC.TheselectedparametersandrequestedV.34TRNdurationare forwardedtotheoriginating faxby INFOhsequence.AftersuccessfullytransferringINFOh,theoriginatingandansweringmodemsenterV.34Phase3.

V.34Phase3PrimaryChannelEqualizerTrainingInthisphase,theoriginating faxmodemsendsahalfduplexTRNsignal.Ananswering faxmodemreceivingtheTRN trains theprimarychannelequalizerandprecodercoefficientsandadapts theotherdemodulationparameters.WhenTRNdurationexpires,thefaxmodemslaunchaV.34controlchannel(1200bps,fullduplex)andexchangemodulationparametersequences:TheMPh0sequenceoftheoriginatingmodemandtheMPh1sequenceoftheansweringmodem.TheMPhmodulationparameterscontainamaximumdatasignalingrateandotheroptionsofferedby theV.34modem.TheMPh1 sequence sentby theanswering faxadditionallyincludestheprecodercoefficientswhichshouldbeusedbytheprimarychanneltransmitteroftheoriginatingfax.Afterasuccessfulexchangeofmodulationparameters,thefaxterminalscontinuethefullduplexoperationofthecontrolchannelandenterT.30PhaseBwithtransmissionofHDLCflags.

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Figure2:V.34FaxProcedures

PhaseBPreMessageProceduresAfterPhaseB,allpreandpostmessagecommandsandresponsesofT.30controlareexchangedusing theV.34controlchannel.ThecontentofT.30framesexchangedusingtheV.34controlchannelissimilartothatofa regularG3 fax session.But theprocess ismuch faster relative to regularT.30 controlexchangebyV.21,because

TheV.34controlchannelbitrateis4timesfaster HDLCpreambleofduration1.00.15 sec isnot required.According toT.30,at least twoHDLC

flags(13.3msec)areenoughtosendbeforethefirstHDLCframe.TheV.34faxmodulationsystemdoesnotuseTCFdefinedbyT.30forPhaseBofregularG3fax.InPhaseB(seeFigure2),anansweringV.34 faxsendsT.30capabilities [CSI/]DIS to theoriginating faxandwaits foraDCScommand.OnreceiptoftheDISfromtheansweringfax,theoriginatingV.34faxsendsa[TSI/]DCScommandandwaits forCFRconfirmation.On receiptof theDCS from theoriginating fax, theanswering faxrespondswithaCFR.Theoriginatingfax,onreceiptoftheCFR,startsanormalturnoffprocedureofthecontrolchannelafterwhichbothfaxesenterT.30PhaseCoffaximagetransfer.Byanalyzing the telephone line formaximumuseof linebandwidthandestimatingmodulationparametersrequiredforoptimumimagetransfer,theV.34faxessubstantiallyreduceatotaldurationofphasesprecedingPhaseCrelativetoregularG3 fax.Thehighestgain intransmissiontimeprecedingT.30PhaseC isachievedwhenlineconditionsdonotallowtransferringaG3faxatthemaximumdatarate.WherearegularG3faxmayperformsomeattemptsinPhaseBtochooseanappropriatedatarateandmodulationsystem(V.17,V.29,orV.27),aV.34fax iscapableofenteringthe imagetransferdirectlyafterasingleattemptof lineprobingandTRN.

PhaseCInMessageProcedureThebinarydataoffaximageandReturntoControlforPartialpage(RCP)framesaresentusingthehalfduplexprimarychannel.T.30ErrorCorrectionMode(ECM)ismandatoryforV.34faxes.

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The primary channel signal is transmitted using a symbol rate, carrier frequency, preemphasis filter, andpower reduction specifiedby INFOh from the answering fax. The data signaling rate is themaximum rateenabledthat is lessthanorequaltothedatasignalingratesspecifiedinbothmodemsMPhsequences.Theother modulation parameters of primary channel (types of trellis and nonlinear encoders, constellation

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shaping, and precoder filter coefficients) are applied according to the values specified by MPh1 of theansweringfaxmodem.AftertransmissionofthefaximageandRCP,theoriginatingfaxturnsoffthehalfduplexprimarychannelandentersT.30PhaseD.

PhaseDPostMessageProcedureIn Phase D, the communicating modems resynchronize the V.34 control channel. Also, instead ofresynchronizationof the control channel,a faxmodemmay initiatea control channel startup followedbymodulationparametersMPh exchange. This isusefulwhen a changeofMPh is required. For example, anansweringfaxthatreceivedafax imageofPhaseCasnonsatisfactory,mayrequestareductionofthedatasignalingrate. Incontrast toregularG3 faxprocedures,renegotiationof thedatarate isextremely fastanddoesnotrequireretrainingtheimagetypemodulationsystem.After resynchronization of V.34 control channel or control channel startup with MPh renegotiation, themodemsarereadytoexchangeaT.30postmessagecommandoftheoriginatingfax,forexample,PPSEOP,andpostmessageresponseoftheansweringfax.IfafinalpartialpageispositivelyconfirmedbyMCFoftheansweringfax,theoriginatingfaxsendsaDCN(seeFigure2).OncompletionofPhaseD,faxterminalsexecuteanormalturnoffprocedureofthecontrolchannel.Ifthefaximagetransferisnotcompleted,oraretransmissionisrequired,thenthefaxterminalsreturntoPhaseC.IncaseofDCNexchanged,thefaxmodemsterminatethefaxsessionviaT.30PhaseE.

PhaseECallreleaseFax terminals physically disconnect. Under certain conditions, call release may be irregular, for example,beforecompletionofimagetransfer.

V.34FaxRelayStimulusSignals

CallingFaxTone(CNG)Acallingfaxtone(CNG)iscommontoregularG3andV.34faxcalls.Generally,theCNGstimulustoneprovidestransitioningtofaxrelaywithhighestreliabilitybecausegatewaysmayswitchavoiceoverIP(VoIP)operationmodeontoafaxrelayduringasilenceperiodofCNG.Sometimes,however,faxrelayinitiatedbyaCNGtonemaybeundesirable,forexample,if:

Callprogressand/orspeechsignalsexistduringCNG Calledterminalequipmentisaregulartelephoneandnotafax

AccordingtoV.150.1,theCNGaloneisnotenoughtoindicatethatacallisafacsimileandinsomecases,theoriginatingfaxmaynottransmitit.

V.8AnswerTone(ANSam)Asstimulussignals,acalledfaxtoneCEDandaV.8answertoneANSamallowsafetransitioningofgatewaysfromVoIPorVBDtofaxrelaystate.ToavoidanswertoneirregularitiescausedbythegatewayswitchingfromVoIP to a nonVoIP state, ITUT V.150.1 recommends blocking the answer tone sent from the answeringequipmentinthedirectionofthenetwork,whilethegatewaydeterminesanswertonetype.AstheANSamsignaliscommontoV.34faxandmodemterminals,usingonlyANSamasastimulusofV.34faxrelay may be problematic for modem transport over packet networks, though in environments withoutmodemsorinfaxservergateways,theANSammaybearecommendedstimulussignalfortransitionstoV.34faxrelay.

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CNGandAnswerToneAudioCodesgatewaysperformingbidirectionalmonitoringoffaxsignalsusethefollowingsignalcombinationsasstimuliforsafesttransitioningtofaxrelay:

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ACNGdetectedfromthenetworkandananswertonereceivedfromTDMinput ACNGreceivedfromTDMinputandananswertonedetectedfromthenetwork

Outofbandsignalingmaybeappliedasanalternativetomonitoringthesignalsdecoded fromthenetworkstream.

V.8CMAV.8callmenu(CM)sentbyacallingfaxterminalinresponsetoanANSamreceivedfromanansweringfaxterminalisdefinitelyagoodsignalfordiscriminatingV.34faxcalls.Butforfaxrelaycallsetup,theV.8CMmaybelesssafecomparedtoCNGand/orANSambecausetransitiontofaxrelaystateoccursinthemiddleoftheanswertoneandmaycausetoneirregularities.AudioCodesgatewaysusetheV.8CMstimulussignaliftheyrenotswitchedtofaxrelayatearlierstages.

V.21PreambleFlagsTheV.21preambleofHDLCflagsiscommontoregularG3faxandmanualV.34faxcalls.Itisagoodsignalforfax call discrimination and is considered a mandatory stimulus for transitioning to fax relay. Sometimes,though,aswitchtofaxrelayinitiatedbyV.21preambleflagsmaybeproblematicdueto:

IrregularitiesintheV.21signalcausedbytheswitch ProlongedgatewaynegotiationatstageofT.38callsetup,possiblyresulting in lossor irregular

delayofT.30dataAudioCodesgatewaysusetheV.21preambleasafinalstimulussignal iftheyrenotswitchedtofaxrelayatearlierstages.

V.34FaxRelayinFallbackModeWhenaV.34capablefaxterminal(superG3)communicateswitharegularG3fax,thefaxcallisalwaysregularG3:

AcallingV.34faxterminalcannotstart inV.34Phase1becausetheansweringG3 faxterminalcannottransmitanANSam

AnansweringV.34faxentersregularPhaseBafteranANSambecausethecallingG3faxterminalcannottransmitaV.8CM.

Forsuchcalls, independentlyofT.38versioncapabilitiesofcommunicatinggateways, thecorresponding faxrelaysessionwillberegularG3faxrelayperformedatfaxdataratesupto14400bpsofV.17.TheonlynonregularT.38packetv8ansammayappearatthebeginningoffaxrelaysessionifbothgatewayssupportT.38version3.AmorecomplexscenariotakesplacewhenV.34faxterminalsconnectthroughgatewaysnotsupportingV.34faxprocedures.Forexample,ifANSamandV.8CMsignalsaretransferredinVoIPorVBDmode,thengatewayswaiting for theV.21preamblemaymiss thepossibilityofentering fax relaybecauseV.34 faxesdonotuseHDLCprotocolduringautomaticV.8negotiation.To resolve thisproblem,according toT.38Recommendation,gatewaysnot supportingT.38version3mustpreventtransferringthefaxCMoverVoIPorVBDtotheansweringfaxterminals,andavoidV.8capabilitytoberelayedinV.21DISframetocallingfaxterminals.Some gatewaysmay support T.38 version 3 or higher but not be capable toV.34modulation system, forexample, gatewaysmay use high versions of T.38 to allow obsolete V.33modulation (14400/12000 bps)or/andnewmethodsofT.38callestablishment.Suchgateways,alternativelytoblockingV.8interaction,mayallow T.38 v8cmmessage/v8jmmessage packet exchange, but prevent the relaying V.34 modulationcapability.

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Afteravoidingorresolving theV.8negotiationproblem,thegatewaysmaycontinueregular faxrelaywhichforcesV.34faxterminalstooperateinnonV.34modeatfaxdataratesofupto14400bpsofV.17.

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V.34FaxRelayperT.38Version3ThefollowingFigure3isaschematicblockdiagramillustrationofaFoIPcommunicationsystem.Systemmayinclude, on one communication side, a calling or answering V.34 fax terminal connected to a switchednetwork, andmay further include,on another communication side, an IPnetwork connected to a remotegatewayoran IP faxdevice. AFoIPgatewaymaybeconnectedbetween theswitchednetworkand the IPnetwork.The remoteFoIPcapablegateway, in turn,maybeconnected toa fax terminal,whichmaybeanansweringorcallingfaxterminal.

V.34FoIP

GatewayIP Network

Gateway

IP Fax

V.34 Fax

Switc

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Net

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T.38/V.34

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T.30/V.34

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Figure3:TypicallayoutofV.34FoIPcommunication

A FoIP gatewaymay receive analog signals from a facsimile terminal, for example the V.34 fax terminal,throughtheswitchednetwork.TheFoIPgatewaymaydemodulateorconverttheanalogfaxtransmissiontoafaxdataset,mayencodeandpacketizethefaxdatasetaccordingtotheT.38FoIPprotocol,andmayrelaythefaxpacketsovertheIPnetworktotheremotegatewayortoanInternetawarefaxdevice,e.g.,theIPfax.IndependentlyofwhetheraV.34faxrelaycallisinitiatedduringautomaticormanualT.30callestablishmentprocedures,thelocalandremotegatewaysmustfinallypassV.34Phase1ofV.8ANSam/CM/JMinteractiontoenterthemainV.34faxsignalprocessing.AV.34FoIPgatewayreceivingaV.34CMsignalfromaconnectedV.34callfaxmaysendaT.38v8cmmessagepacketcarryingaT.38FacsimileApplicationProfile(FAP).Asaresponseon the v8cmmessagepacket, the gatewaymay receive a v8jmmessagepacket carrying aT.38acknowledgement(ACK)ofaremotegatewayorIPfax.ThelocalandremoteFoIPgatewaysmaystarttheV.34modulationsystemiftheFAPwithV.34halfduplexisacknowledgedbyACK. Otherwise,gatewaysmayperform fallback toregularG3 faxrelay,or, if there isnocompatiblemode,mayterminatetheFoIPsession.Onthesuccessfulcompletionofav8cmmessageandv8jmmessagepacketexchange,aV.34FoIPgatewayentersV.34Phase2.Inthisphase,eachofthecommunicatinggatewayslocallyestablishesasymbolrate,datasignalingrateandothermodulationparameterstobeusedinT.30phaseC.AttheV.34signalprocessinglevel,twogatewaysconnectedinV.34faxrelaymodearehighlyindependentofeach other because V.34modulation parameters, excluding data signaling rate, are not exchanged via IPpacketnetwork.ThedatasignalingrateisrelayedusingaT.38v34priratedatapacketfromtheT.38receivinggateway (forexample,onanswering faxside) to theT.38emittinggateway (forexample,on thecalling faxside).Theemittinggatewayisresponsibleforselectingadatasignalingratenotgreaterthanthev34prirate.ThemaximumdifferencebetweendatasignalingratesofV.34faxrelaygatewayscorrespondsto2400bpsoftheemittinggatewayand33600bpsofthereceivinggateway.

V.34FaxRelayoverPacketNetworks

The T.30 control data are relayed between communicating gateways in T.38 v34CC1200hdlcxxx datapackets.

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On transitioning fromcontrolchannel toprimarychannel,aT.38emittinggatewaysendsav34prichannelindicatorpackettoreceivinggateway.ThefaximagedataofT.30phaseCarerelayedbyT.38v34prich:hdlcxxxdatapackets.

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TheT.38emittinggatewaytransitingfromT.30phaseCtoT.30phaseDofpostmessageproceduresendsav34cntlchannel1200indicatorpackettoreceivinggateway.TheT.38receivinggateway,beforesendingT.30postmessage response data, may renegotiate the data signaling rate by sending a T.38 v34CCretrainindicatorpacketfollowedbyv34priratedatapacket.TheV.34FoIPsessionisnormallyendedinT.30phaseEoncompletionofT.30DCNtransferfollowedbyv34CC1200:hdlcsigendpacket.

T.38CallSetupToenableT.38packetexchangeoverthepacketnetwork,communicatinggatewaysexchangeT.38capabilitiesduringSIP/SDPcallestablishmentprocedures.ThegatewaynegotiationisdonebyfirstofferandanswerSDP,usingSIPINVITEand200OKresponse.BecausetheT.30phaseAofV.34callsissignificantlydifferrelativetoT.30 phase A of regular G3 fax calls, the SIP/SDP interaction and gateway operation may have somepeculiaritiestoallowanoptimalV.34faxrelay.AcompletionofVBDandT.38capabilitiesnegotiationatcallstartenablessmoothswitchingbetweenaudioRTPtoT.38oraudiotoVBDcodecsonfaxandmodemdetection.Forexample,1)OFFER(SIPINVITE):INVITEsip:101@xx.xx.xx.78;user=phoneSIP/2.0Via:SIP/2.0/UDPxx.xx.xx.70;branch=z9hG4bKac663170125;rportFrom:;tag=1c662567132To:CallID:6625317522811201119537@xx.xx.xx.70CSeq:1INVITEContact:ContentType:application/sdpContentLength:v=0o=Mediant800661883735661869660INIP4xx.xx.xx.70s=PhoneCallc=INIP4xx.xx.xx.70t=00m=audio6000RTP/AVP18118a=rtpmap:18G729/8000a=fmtp:18annexb=noa=rtpmap:118PCMA/8000a=gpmd:118vbd=yesa=ptime:20a=sendrecvm=image6002udptlt38a=T38FaxVersion:3a=T38MaxBitRate:33600a=T38FaxMaxBuffer:3000a=T38FaxMaxDatagram:560a=T38FaxRateManagement:transferredTCFa=T38FaxUdpEC:t38UDPRedundancy2)ANSWER(SIP200OK):SIP/2.0200OKVia:SIP/2.0/UDPxx.xx.xx.70;branch=z9hG4bKac663170125;rportFrom:;tag=1c662567132To:;tag=1c615537688CallID:6625317522811201119537@xx.xx.xx.70

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CSeq:1INVITEContact:

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ContentType:application/sdpContentLength:v=0o=Mediant800615608588615608445INIP4xx.xx.xx.78s=PhoneCall

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c=INIP4xx.xx.xx.78t=00m=audio6000RTP/AVP18118a=rtpmap:18G729/8000a=fmtp:18annexb=noa=rtpmap:118PCMA/8000a=gpmd:118vbd=yesa=ptime:20a=sendrecvm=image6002udptlt38a=T38FaxVersion:3a=T38MaxBitRate:33600a=T38FaxRateManagement:transferredTCFa=T38FaxUdpEC:t38UDPRedundancyWith this configuration, the gateways will support simultaneously the voice calls (for example, G.729encoded),modemcallsbyusingVBDcodec(G.711PCMAlaw),andfaxcallsbyusingT.38version3.BecausetheVBDandT.38 capabilitiesareknown since call start, the communicatinggatewaysareable toperformautonomous transitioningbetweenaudio,VBD,andT.38gateway statesaccording toVBDandT.38 stimulisignals.SuchtypeofcallestablishmentisoptimalforV.34faxrelay.If other party doesnt support the negotiation of T.38 codec at call start, it is possible to negotiate T.38capabilitiesbyusingSIPREINVITEmessagesentbygatewayondetectingfaxsignal.ThisREINVITEwillcontainasinglemediawithT.38parametersinitsSDP.ForV.34faxrelay,itisimportantforgatewaytoentertheT.38stateasfastaspossible,forexample,ondetectingCNG,orANSam,orondetectingacombinationofCNGfromonedirectionandANSamfromoppositedirection.Acompletesuppressionofanswerfaxtoneor/andV.8CMisnotdesirable,becauseitdisablestheT.30automaticoperatingmethodofV.34faxcallsetup,forcestheT.30manualcallestablishment,andasaresult,extendsadurationofV.34faxcall.IfaremotesidedoesnotsupportthesuggestedT.38version3,thenthe invitingorreinvitinggatewaymayapply an appropriate lower T.38 version with forcing V.34toV.17 fallback for regular G3 fax relay or,alternatively,applytheVBDtransfermethodtofaxcall.

WhyChooseAudioCodes?AudioCodesfaxrelaytechnologyhasbeenwidelydeployedformorethan10yearsandservesthousandsoforganizationsworldwide.AudioCodesfaxtechnologyisknownforitsvastinteroperabilitywithhundredsoffaxmachinemodelsfromvariousvendors,andconsideredbycustomersasmoststableandreliable.AudioCodes gateways terminate V.34 fax (Super G) arriving over TDM and support multiple modes totransport it over VoIP. Best way is through the use of full V.34 fax relay according to T.38 version 3.Alternately,forpeergatewayswhichdonotsupportT.38version3,V.34canbetransferredusingVoiceBandDataordowngradedtoregularG3withT.38.Inaddition,AudioCodesSessionBorderControllers,allowIPtoIPtranslationofT.38packetscarryingeitherG3orV.34faxesintoVoiceBandDatapackets.ThisfacilitatesinteroperabilitybetweenSIPendpointsthatdonotsharethesamefaxrelaycapabilities.

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ReferencesITUTRecommendationV.8 (11/2000).Procedures forstartingsessionsofdata transmissionover thepublicswitchedtelephonenetworkITUT Recommendation T.30 (09/2005). Procedures for document facsimile transmission in the generalswitchedtelephonenetworkITUT Recommendation T.30 (04/1999). Procedures for document facsimile transmission in the generalswitchedtelephonenetworkITUTRecommendationV.34(02/1998).Amodemoperatingatdatasignallingratesofupto33600bit/sforuseonthegeneralswitchedtelephonenetworkandonleasedpointtopoint2wiretelephonetypecircuitsITUTRecommendation T.38 (04/2007).Procedures for realtimeGroup 3 facsimile communication over IPnetworksITUTRecommendationV.150.1(01/2003).ProceduresfortheendtoendconnectionofVseriesDCEsoveranIPnetworkITUTRecommendationV.152(01/2005).ProceduresforsupportingVoiceBandDataoverIPNetworks

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AboutAudioCodes

AudioCodes Ltd. (NasdaqGS: AUDC) designs, develops and sells

advanced Voice over IP (VoIP) and converged VoIP and Data

networking products and applications to Service Providers and

Enterprises.AudioCodesisaVoIPtechnologymarketleaderfocused

on converged VoIP & data communications and its products are

deployed globally in Broadband,Mobile, Enterprise networks and

Cable. The company provides a range of innovative, costeffective

products including Media Gateways, MultiService Business

Gateways,SessionBorderControllers(SBC),ResidentialGateways,IP

Phones,MediaServersandValueAddedApplications. AudioCodes

underlying technology, VoIPerfectHD, relies on AudioCodes

leadership inDSP, voice coding and voiceprocessing technologies.

AudioCodes High Definition (HD) VoIP technologies and products

provideenhancedintelligibilityandabetterendusercommunication

experience in Voice communications. For more information on

AudioCodes,visithttp://www.audiocodes.com

2012AudioCodesLtd.Allrightsreserved.AudioCodes,AC,HDVoIP,HDVoIPSoundsBetter, IPmedia, Mediant, MediaPack, OSN, SmartTAP, VMAS, VoIPerfect,VoIPerfectHD, Your Gateway To VoIP and 3GX are trademarks or registeredtrademarksofAudioCodesLimited.Allotherproductsortrademarksarepropertyoftheirrespectiveowners.Productspecificationsaresubjecttochangewithoutnotice.Ref#LTRM8003212/11V.1

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V.34FaxRelayoverPacketNetworks

ScopeStandard AbbreviationsOverviewV.34 Fax Call over the PSTNT.30 Phase A V.34 Call EstablishmentV.34 Phase 2 Line ProbingV.34 Phase 3 Primary Channel Equalizer TrainingPhase B Pre-Message ProceduresPhase C In-Message ProcedurePhase D Post-Message ProcedurePhase E Call release

V.34 Fax Relay Stimulus SignalsCalling Fax Tone (CNG)V.8 Answer Tone (ANSam)CNG and Answer ToneV.8 CMV.21 Preamble Flags

V.34 Fax Relay in Fallback ModeV.34 Fax Relay per T.38 Version 3T.38 Call SetupWhy Choose AudioCodes?References