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1、headerforSPIEuseComparisonofH.323andSIPforIPTelephonySignalingabIsmailDalgic,HanlinFanga3ComCorporationTechnologyDevelopmentCenter5400BayfrontPlaza,M/S3219,SantaClara,CA95052bCiscoSystems170WestTasmanDr.,SanJose,CA95134ABSTRACTTwostandardscurrentlycompeteforthed
2、ominanceofIPtelephonysignaling:theH.323protocolsuitebyITU-T,andtheSessionInitiationProtocol(SIP)byIETF.Bothofthesesignalingprotocolsprovidemechanismsforcallestablishmentandteardown,callcontrolandsupplementaryservices,andcapabilityexchange.Weinvestigateandcompare
3、thesetwoprotocolsintermsofFunctionality,QualityofService(QoS),Scalability,Flexibility,Interoperability,andEaseofImplementation.Forfairnessofcomparison,weconsidersimilarscenariosforbothprotocols.Inparticular,wefocusonscenariosthatinvolveagatekeeperforH.323,andaPr
4、oxy/RedirectserverforSIP.Thereasonisthatmedium-to-largeIPTelephonysystemsarenotmanageablewithoutagatekeeperorproxyserver.WeconsiderallthreeversionsofH.323.Intermsoffunctionalityandservicesthatcanbesupported,H.323version2andSIPareverysimilar.However,supplementary
5、servicesinH.323aremorerigorouslydefined,andthereforefewerinteroperabilityissuesareexpectedamongitsimplementations.Furthermore,H.323hastakenmorestepstoensurecompatibilityamongitsdifferentversions,andtointeroperatewithPSTN.ThetwoprotocolsarecomparableintheirQoSsup
6、port(similarcallsetupdelays,nosupportforresourcereservationorclassofservice(CoS)setting),butH.323version3willallowsignalingoftherequestedCoS.SIP'sprimaryadvantagesare(i)flexibilitytoaddnewfeatures,and(ii)relativeeaseofimplementationanddebugging.Finally,wenotetha
7、tH.323andSIPareimprovingthemselvesbylearningfromeachother,andthedifferencesbetweenthemarediminishingwitheachnewversion.Keywords:IPTelephony,VoiceoverIP,H.323,SIP,signaling,callcontrol1.INTRODUCTIONTelephonyservicetodayisprovidedforthemostpartovercircuit-switched
8、networks,whicharereferredtoasPublicSwitchedTelephoneNetworks(PSTN).ThisserviceisknownasPlainOldTelephoneService(POTS).Anewtrendthatisbeginningtoemergeinrecentyearsist