rence control messages defined in H.245. Thus, the H.245 serves to exchange audio and video features in the master/slave context. Video I/O Video codec equipment H.261 or H.263 Audio codec Audio I/O G.711, G.722, Receive path equipment G.723.1, delay G.728, G.729 Packet H.225.0 User data application T. 120 layer Network, e.g. 2G/3G GPRS System control H.245 control System control User interface Call control H.225.0 (Q.931) RAS control H.225.0 Rec. H.323 Area Figure 6.3 The ITU Rec. H.323 model. 6.4.1.4 Session Initiation Protocol (SIP) The Session Initiation Protocol (SIP) is another alternative to enable PS video- telephony. Developed in IETF by the MMSIC Multiparty Multimedia Session Control group, SIP is an application layer control signalling protocol for creating/modifying and 2 Real-time transport protocol/real-time transport control protocol. Service Components in UMTS terminating sessions with one or more participants, e.g. Internet multimedia confer- ences, Internet telephone calls and multimedia distribution. Participants in a session can communicate via multicast or via a mesh of unicast relations, or a combination of these. See Figure 6.4. SIP corresponds to: the overall IETF multimedia data and control architecture currently incorporating protocols such as Resource Reservation Protocol - RFC 2205 (RSVP) for reserving network resources; the real-time transport protocol (RTP - RFC 1889) for transporting real-time data and providing QoS feedback; the real-advertising multimedia sessions via multicast and the session description protocol (SDP - RFC 2327) for describing multimedia sessions. Video codec RTP-capsul ation Video I/O H.261 H.263, receive path delay equipment RTCP sender H.263+ MobiVideo reports (optional) Audio codec RTP-capsulation, Audio I/O G .711, G.722 receive path delay equipment G.723,1 RTCP senden G 728 G.729 EFR reports (optional) User data TCP/IP driver Packet application T.120 network Security (optional) System control User interface RTCP (optional) RSVP (optional) The IETF Multimed