arious G.7XX audio-quality standards-there are no constraints because ATM has plenty of bandwidth to accommodate any audio coding algorithm. The H.310 standard also references Rec. H.245. H.245 is a multimedia control protocol that permits H.310-aware audiovisual equipment to establish and maintain broadband audiovisual sessions. H.321 The H.321 Study Group began developing standards for private ATM networks in 1996. H.321 is a companion standard to H.310. It preserves fundamental components of the original H.320 Recommendation including H.261 and, therefore, maintains a high degree of backward compatibility with it. H.321 functions extend the type of transmission networks that carry H.32X videoconferencing signals to include ATM and broadband ISDN. As is true of H.310, this specification was still in ITU-T Draft status when this book went to print. For more information, contact the sources listed for H.310. H.323 The formal name of H.323 is "Visual Telephone Terminals over Non-Guaranteed Quality of Service LANs." It was approved by the ITU-T in October 1996. H.323 defines four components: terminal, a gatekeeper, a gateway and a multipoint controller. H.323 Components Terminals include devices such as PCs, workstations and videophones. All H.323- compliant terminals must support voice-data conferencing. Voice-data-video conferencing is optional. Multiple channels of each type are permitted. A gatekeeper aids in call management. The gatekeeper is a utility that controls Video Communications: The Whole Picture videoconference access on a packet-switched LAN. It requires that multimedia terminals register "at the gate," which is accomplished when the terminal provides its IP address. The gatekeeper translates network addresses and aliases to make connections. It can also deny access or limit the number of simultaneous connections to prevent congestion. A H.323 gateway is different from a gatekeeper. The gateway allows LAN-based H.323 systems to interoperate with other H.32X products. For instance, the gateway could 