IP Telephony Architecture and Protocols

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1 Nota di Copyright IP Telephony Architecture and Protocols Mario Baldi Synchrodyne Networks, Inc. Iptel_e - 1 Copyright: si veda nota a pag. 2! Questo insieme di trasparenze (detto nel seguito slides) è protetto dalle leggi sul copyright e dalle disposizioni dei trattati internazionali. Il titolo ed i copyright relativi alle slides (ivi inclusi, ma non limitatamente, ogni immagine, fotografia, animazione, video, audio, musica e testo) sono di proprietà degli autori indicati a pag. 1.! Le slides possono essere riprodotte ed utilizzate liberamente dagli istituti di ricerca, scolastici ed universitari afferenti al Ministero della Pubblica Istruzione e al Ministero dell Università e Ricerca Scientifica e Tecnologica, per scopi istituzionali, non a fine di lucro. In tal caso non è richiesta alcuna autorizzazione.! Ogni altra utilizzazione o riproduzione (ivi incluse, ma non limitatamente, le riproduzioni su supporti magnetici, su reti di calcolatori e stampate) in toto o in parte è vietata, se non esplicitamente autorizzata per iscritto, a priori, da parte degli autori.! L informazione contenuta in queste slides è ritenuta essere accurata alla data della pubblicazione. Essa è fornita per scopi meramente didattici e non per essere utilizzata in progetti di impianti, prodotti, reti, ecc. In ogni caso essa è soggetta a cambiamenti senza preavviso. Gli autori non assumono alcuna responsabilità per il contenuto di queste slides (ivi incluse, ma non limitatamente, la correttezza, completezza, applicabilità, aggiornamento dell informazione).! In ogni caso non può essere dichiarata conformità all informazione contenuta in queste slides.! In ogni caso questa nota di copyright non deve mai essere rimossa e deve essere riportata anche in utilizzi parziali. Iptel_e - 2 Copyright: si veda nota a pag. 2 IP Telephony Architectural Components! IP Phone! Call Manager/Call Server: signaling! Optional! Gateway: Interworking with PSTN IP phone Call Manager VoIP Gateway DSL DSLAM + VoIP GW PSTN Iptel_e - 3 Copyright: si veda nota a pag. 2 Signaling Iptel_e - 4 Copyright: si veda nota a pag. 2! PSTN (Public Switched Telephone Network)! Signaling System No. 7! Q.931! User location! integration with DNS and directory services (e.g., LDAP)! Feature invocation! hold, transfer, mute, etc.! H.323! ITU-T! SIP/SDP! IETF

2 Transport! RTP - Real-time Transport Protocol! SIP and H.323! Timing information! Stream packetization! Multicast support! RTCP - Real-time Transport Control Protocol! Functionalities! QoS feedback! Intermedia synchronization! Identification! Session control! Same address as RTP, different port! Periodic refreshes Iptel_e - 5 Copyright: si veda nota a pag. 2 SIP - Session Invitation Protocol! User agent (UA)! Client-server Approach! Client (UAC)! Server (UAS)! Call Processing Language (CPL)! Specify behavior of call agents! Registration server! user location Iptel_e - 6 Copyright: si veda nota a pag. 2 Call Routing! Proxy server! Receives requests! Forwards requests to a next hop server (NHS)! Looking for a NHS with more information about destination user! UAS! Proxy server! Redirect server! Redirect Server! Sends NHS s address to requesting user! Requests can be forked! Responses follow path of proxy servers Iptel_e - 7 Copyright: si veda nota a pag. 2 Call Routing Iptel_e - 8 Copyright: si veda nota a pag. 2

3 Call Setup: INVITE Message! To! From! Subject! callid! Contact: where a user can be contacted! Require: feature negotiation! Extensible by defining new headers Iptel_e - 9 Copyright: si veda nota a pag. 2 SDP - Session Description Protocol! Session description! Media Streams! Address and ports (per stream)! Payload types or Capability set! Start & end times! Originator! For unicast (telephony) and distribution (broadcast)! Textual format! description format rather than protocol! Included in INVITE and its response! Session parameters (INVITE)! Capabilities of called party (INVITE response) Iptel_e - 10 Copyright: si veda nota a pag. 2 SDP Example v = 0 o = g.bell IN IP s = Come here, Watson! u = e = c = IN IP b = CT:64 t = k = clear:manhole cover m = audio 3456 RTP/AVP 96 a = rtpmap:96 VDVI/8000/1 m = video 3458 RTP/AVP 31 m = application udp wb a = orient:portrait Iptel_e - 11 Copyright: si veda nota a pag. 2 Gateway Location Protocol! Internet telephony domains! Location servers! Know about gateways! Service Location Protocol! Intra-domain protocol for Location Servers! Gateway Location Protocol! Inter-domain protocol for Location Servers! Information provide! IP address of the gateway! Range of phone numbers it can reach! Various attributes! signaling protocol, rates, provider, ecc.! Under specification (borrows from BGP4) Iptel_e - 12 Copyright: si veda nota a pag. 2

4 Interworking with PSTN! LDAP (Lightweight Directory Access Protocol)! Send LDAP query to location server with destination phone number! Get IP address of gateway! Send SIP INVITE to gateway! SIP! Invitation (with phone number) to Location Server! Location Server acts as proxy towards gateway! Web! LS s database available as Web pages to the user H.323! Videoconferencing over non-guaranteed quality of service LANs! Scalability problems: one server for! Signaling conversion! Call control! Media transcoding! No provision for SS7 interworking Iptel_e - 13 Copyright: si veda nota a pag. 2 Iptel_e - 14 Copyright: si veda nota a pag. 2 Functional Architecture! Multipoint Control Unit (MCU)! Three or more terminal in conference! Terminal Equipment (TE)! 2 way communication with another TE or MCU or Gateway! Gateway (GW)! 2 way communication with PSTN Functional Architecture! Gatekeeper (GK)! One per zone! Address translation! Access control for TE, GW, MCU! Bandwidth management! GW location! Optional Iptel_e - 15 Copyright: si veda nota a pag. 2 Iptel_e - 16 Copyright: si veda nota a pag. 2

5 Protocol Architecture RAS Registration, Admission, and Status! Transaction oriented protocol between TE and GKs! TE-GK! Discover GK! Register! Request call admission! Allocate BW! Call clearing! GK-TE! Inquire status! GK-GK! Address resolution across multiple zones Iptel_e - 17 Copyright: si veda nota a pag. 2 Iptel_e - 18 Copyright: si veda nota a pag. 2 Q931 and H.245 Q.931! Call setup and teardown! Subset of the Q.931 of PSTN! Fewer information elements! Simplifies interworking with! PSTN/ISDN (telephony)! H.320 and H.324 (circuit oriented videoconferencing) H.245! Connection control! Negotiate media processing capabilities! Determine terminal capabilities! Establish master/slave relationships! Open/close logical channels Iptel_e - 19 Copyright: si veda nota a pag. 2 Call Control 1-3: call setup 5-7: call teardown Fast connect (VoIP): combine 2/3 and 5/6 Iptel_e - 20 Copyright: si veda nota a pag. 2

6 Call Control Models: Direct Call Call Control Models: Gatekeeper-routed Call! Q.931 and H.245 (phases 2, 3, 5, and 6) between endpoints! RTP media streams between endpoints! Calling TE needs transport address of callee! e.g., IP address and port number! Gatekeeper is optional! Calling TE can use GK for address resolution! Called TE can ask GK for permission to accept! Not suitable to carrier grade, large scale deployment Iptel_e - 21 Copyright: si veda nota a pag. 2! All signaling messages are routed through gatekeepers! phases 1, 2, 3, 5, 6, and 7! TE access services provided by gatekeeper! Suitable to IP telephony service providers Iptel_e - 22 Copyright: si veda nota a pag. 2 H.450: Enhanced Services! Generic functional protocol over Q.931! Supplementary service application protocol data unit (SS-APDU) in user-to-user information element (UUIE)! End-to-end signaling protocol! Service logic is in TE! In PSTN service logic is in the network! Problems in large scale due to different versions! Derived from QSIG (protocol for PBXs)! Adaptation of PBX services to IP domain! For each service H.450 defines! Procedures! Message flow Iptel_e - 23 Copyright: si veda nota a pag. 2 H.450 Recommendations! H.450.1: Generic Functional Protocol and Procedures! H.450.2: call transfer! H.450.3: call diversion! H.450.4: call hold! H.450.5: call park and pickup! H.450.6: message waiting indication! H.450.7: call waiting! H.450.8: name identification! H.450.9: call completion! H : call offer! H : call intrusion! H : additional common information network services Iptel_e - 24 Copyright: si veda nota a pag. 2

7 Enhanced Services within Gatekeeper Implementation! No need to wait for ITU standardization! Gatekeeper-routed call control model! Address resolution can support mobility! Standardized approach is required! Integrate IN (Intelligent Network) with GK! IN in PSTN! Standardizes service development! Locates service logic in separate platform! Some GK products support IN! ITU H.246 annex D (proceeding slowly) Overcoming Limitations of H.450! HTTP control channel for H.323! Service provider shows to the user web pages with H.323 call related info! Create new services with third party call control! H.323 Annex K! Stimulus based control mechanism! Endpoint can use intelligence residing in the network to build services! Between PSTN approach ( dumb terminal ) and full H.323 endpoint! H.323 Annex L Iptel_e - 25 Copyright: si veda nota a pag. 2 Iptel_e - 26 Copyright: si veda nota a pag. 2 Interworking with PSTN! H.323 was designed for PSTN interworking! Even if for videoconferencing! Gateway involved in connecting PSTN with IP network! Connection management! Association between PSTN calls and IP flows H.323 Gateway Limitations! Scalability (maximum a few thousand lines)! Decomposition of the gateway! H SS7 Connectivity! Lack of rich set of SS7 services! Resiliency: no mechanism for failover! If a gateway crashes all the handled calls drop! Two stage dialing! PSTN users dial gateway and then endpoint Iptel_e - 27 Copyright: si veda nota a pag. 2 Iptel_e - 28 Copyright: si veda nota a pag. 2

8 TIPHON Gateway Decomposition European Telecommunications Standards Institute (ETSI) TIPHON Gateway Decomposition! Media Gateway (GW)! Media transcoding! Signal processing! Encryption! Silence suppression! Services! Announcements! Voice recognition! Media Gateway Controller! Call processing! Network level resource management! Bandwidth utilization and allocation Iptel_e - 29 Copyright: si veda nota a pag. 2 Iptel_e - 30 Copyright: si veda nota a pag. 2 Decomposed Gateway Interaction! Media gateway control protocol (MGCP)! Released in 1998! H.248 or MEGACO! ITU-T and IETF! Released in 2000 Alternatives to H.323! Various signaling approaches! MGCP used by TEs to interact with Gateway! Intelligence resides in the network! Valuable for low cost TEs! No capability of TE to TE direct signaling! Some cable modem and DSL-based VoIP products use MGCP signaling! SIP Iptel_e - 31 Copyright: si veda nota a pag. 2 Iptel_e - 32 Copyright: si veda nota a pag. 2

9 SIP and H.323! Similar architecture! SIP is more lightweight Iptel_e - 33 Copyright: si veda nota a pag. 2! SIP provides rapid introduction of new services! No Q.931 or H.245! Terminals can have lots of intelligence! Carries have significant H.323 deployments! Coexistence in the short term! Interworking being considered within IETF! Many commercially available call agents and soft switches support SIP, H.323, and MGCP! The market will decide References Iptel_e - 34 Copyright: si veda nota a pag. 2! M. Handley et al., SIP: session initiation protocol, IETF 2543, Mar. 1999! M. Handley and V. Jacobson, SDP: session description protocol, IETF RFC 2327, Apr ! H. Schulzrinne et al., RTP: a transport protocol for real-time applications, IETF RFC 1889, Jan ! H. Schulzrinne, J Rosenberg, The IETF Internet Telephony Architecture and Protocols, IEEE Network, May/June 1999! H. Liu, P. Mouchtaris, Voice over IP Signaling: H.323 and Beyond, IEEE Communications Magazine, Ottobre 2000

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