ngaged in another session, enables a caller to monitor that called party's endpoint to place the call when the called party becomes free. Audiographic Conferencing Standards The H.32X family relies on a series of audiographic standards for data sharing during a videoconference. Chapter 8 covers the ITU's T.120 family of Recommendations that provide audio graphic interoperability during H.32X videoconferences. Video Communications: The Whole Picture SESSION INITIATION PROTOCOL H.323 does not efficiently accommodate Internet conferencing issues such as wide- area addressing and loop cancellation because it was primarily developed for use over a LAN. Even H.323 Version 2, that supports zones, does not scale adequately to the size of an IP-based public telephone network. Consequently, the Multiparty MUltimedia Session Control (MMUSIC) Working Group of the IETF has developed protocols to support Internet teleconferencing sessions. MMUSIC developed the Session Initiation Protocol (SIP) specifically as a teleconferencing protocol for the Internet, and Session Delivery Protocol (SDP), which SIP uses to convey which codecs an endpoint will support for a session. Concurrently, it developed the Media Gateway Control Protocol (MGCP) as a gateway-to-gateway protocol for use by carriers. Although MMUSIC designed them to support loosely-controlled conferences over the Internet, these three protocols are general enough for managing tightly- controlled sessions. Like H.323, SIP provides mechanisms for call establishment and teardown, call control and supplementary services, QoS, and capability exchange. H.323 is less prone to interoperability issues because it more meticulously defines supplementary services, and is mandated to be backward compatible and to interoperate with the PSTN. However, SIP provides more flexibility for adding new features and easier implementation and debugging. Since both SIP and H.323 use RTP to carry media flows, the main difference between the two is their methods of handling call signaling and contro