Before being overwhelmed by all the alphabet soup and jargon associated with teleconferencing and multimedia communications in general, network managers should realize that when all the networking issues are removed, the conferencing equipment breaks down into four types of components: terminals, gateways, gatekeepers, and multipoint control units.
Terminals are the clients or endpoints on the LAN. Many terminals today will be personal computer based, but telephones, videophones, set-top boxes, and room systems are all classified as terminals. Under H.323, which is the standard for IP-based conferencing, all terminals must support voice; video and data are optional. However, if audio, video, or data are supported, then H.323 specifies what modes of operation are required so that all terminals supporting that media type can interwork. Terminals are where compression/decompression (CODEC) functions for voice and video are performed. CODECs can be implemented in special hardware, or as software running on general purpose processors, depending on the price/performance characteristics needed.
Gateways are devices which provide the many services needed to connect LAN-based terminals to WAN-based terminals, or to other LAN-based terminals where a wide area connection lies between separate LANs. Gateways translate between transmission formats (for example H.225.0 to H.221), between communications procedures (for example H.245 to H.242), and between audio and video codecs if needed. They also perform call setup and tear down on both LAN and circuit switched networks (under H.323, gateways can also perform multipoint control functions).
Gatekeepers provide bandwidth management and admissions control. For instance, if a LAN manager-specified threshold for the number of simultaneous conferences on the LAN has been reached, the Gatekeeper can refuse to make any more connections. The effect is to limit the total number of conferencing bits/s to some fraction of the total available; the remainder are left for email, file transfers, and other normal store-and-forward data functions. Gatekeepers also provide address translation services between LAN aliases for terminals and gateways and IP or IPX addresses. The collection of all Terminals, Gateways, and Multipoint Control Units managed by a single gatekeeper is known as an H.323 Zone. Gatekeeper functions may be performed by separate hardware devices, or as software operating on other network devices. While not yet included in the ITU standards, security issues are likely to be addressed through gatekeeper functions.
Multipoint Control Units (MCUs)
MCUs support conferences between three or more endpoints. Under H.323, an MCU consists of a Multipoint Controller (MC), which is a required component, and zero or more Multipoint Processors (MP). The MC handles negotiations between all terminals to determine common capabilities and controls conference resources such as multicasting. The MC does not deal directly with any of the media streams. This is left to the MP, which does the mixing, switching, and other processing of audio, video, and/or data bits. MCUs can be implemented in software running on other network components, but for serious multipoint conferencing, dedicated hardware is necessary.