in 2000 with some major changes to the protocol. Version 5 is currently under revision at the ITU-T. Currently, most deployed systems use Version 2. H.323 has been designed to be backward compatible, so, for example, a version 1 compliant end point can communicate with a version 3 gatekeeper and a version 4 end point. H.323 references a number of other ITU and IETF protocols to com- pletely specify the environment. Each element of the network is defined and standardized. Figure 8.1 shows the main elements: terminals, gatekeepers, gate- ways, and multipoint control units (MCUs). Terminals, gateways, and MCUs are network end-devices, often called end points. An end point originates and terminates media streams that could be audio, video, or data, or a combination Gatekeeper Gateway H.323 Telephone network Terminal Terminal Figure 8.1 Elements of an H.323 network. Comparison to H.323 of all three. At a minimum, all H.323 end points must support basic G.711 PCM audio transmission. Support of video and data are optional. An H.323 gatekeeper is a server that controls a zone, which is the smallest administrative domain in H.323. If a gatekeeper is present, all end points within that zone must register with and defer to the gatekeeper on authorization decisions to place or accept a call. A gatekeeper also provides services to terminals in a zone, such as gateway location, address translation, bandwidth management, feature implementation, and registration. A gatekeeper is not a required element in an H.323 network, but a terminal's capabilities without one are severely limited. A gateway is another optional element in an H.323 network. It interfaces the H.323 network with another protocol network, such as the PSTN. An MCU provides conferencing services for terminals. Some of the protocols referenced by H.323 are shown in Table 8.2. H.225 is used for registration, admission, and status (RAS), which is used for terminal- to-gatekeeper communication. A modified subset of Q.931 is used for call setup signaling between termina