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Application: H.225 Call Signaling
ITU H.323 forms the foundation for audio, video, and data communications over IP-based networks. H.323 is an umbrella recommendation from the International Telecommunications Union (ITU) which defines multimedia communications over Local Area Networks (LANs) without a guaranteed Quality of Service (QoS). These networks are prevalent in todays corporate desktops and include packet-switched TCP/IP over Ethernet. H.323 suite of protocols form an important building blocks for a broad new range of collaborative, LAN-based applications for multimedia include parts of H.225.0 - RAS, Q.931, H.245 RTP/RTCP and audio/video codecs, such as the audio codecs (G.711, G.723.1, G.728, etc.) and video codecs (H.261, H.263) that compress and decompress media streams. Media streams are transported on RTP/RTCP. RTP carries the actual media and RTCP carries status and control information. The signalling is transported reliably over TCP. The H.225 call signaling protocol consists of many subprotocols and is part of the H.323 suite. H.225 is used for connection establishment and termination between endpoints. The H.225 call signaling protocol also supports status inquiry, ad hoc multipoint call expansion, and limited call forwarding and transfer. H.225 call signaling messages are exchanged over Q.931. The Q.931 messages are exchanged over a TCP stream demarcated by Transport Protocol Data Unit Packet (TPKT) encapsulations. The H.225 call signaling message is transported as part of the user information element of the Q.931 protocol. The ASN.1 representation of the H.225 message is encoded using the Packed Encoding Rules. H.225.0 v2 is a standard which covers narrow-band visual telephone services defined in H.200/AV.120-Series Recommendations. It specifically deals with those situations where the transmission path includes one or more packet based networks, each of which is configured and managed to provide a non-guaranteed Quality of Service (QoS) which is not equivalent to that of N-ISDN such that additional protection or recovery mechanisms beyond those mandated by Rec. H.320 is necessary in the terminals. H.225.0 describes how audio, video, data, and control information on a packet based network can be managed to provide conversational services in H.323 equipment.