ARCHITECTURE OF H.323
The H.323 Recommendation assumes the transmission path between the telephony users passes through at least one local area network (LAN), such as an Ethernet or a token ring. It is further assumed that the LAN may not provide a guaranteed quality-of-service (QOS) needed to support the telephony traffic. As shown in Figure 9-2, the H.323 encompasses end-to-end connection between H.323 terminals and other terminals and through different kinds of networks. To gain an understanding of the scope and architecture of H.323, the entities in Figure 9-2 are explained in more detail next.
Figure 9-2. H.323 architecture
The H.323 Terminal
The H.323 terminal is an end-user device that provides real-time, two-way voice, video, or data communications with another H.323 terminal. The terminal can also communicate with an H.323 Gateway or a Multipoint Control Unit (MCU). While I cite the ability to support voice, video, and data, the terminal needs to not be configured for all those services, and H.323 does not require the terminal to be multiservice-capable.
The H.323 Gateway
The H.323 Gateway is a node on a LAN that communicates with the H.323 terminal or other ITU-T terminals attached to other networks. If one of the terminals is not an H.323 terminal, the Gateway performs translation of the transmission formats between the terminals; for example, a translation between G.711 and G.729 voice signals. One H.323 Gateway can interwork with another H.323 Gateway.
In addition, the Gateway can operate with other ITU: (a) switched circuit networks (SCNs); (b) the General Switched Telephone Network (GSTN), (c) the narrowband-ISDN (N-ISDN), and (d) the broadband-ISDN (B-ISDN, an ATM-based network). Also, the Gateway can operate as an H.323 Multipoint Control Unit (MCU), discussed next.
Working in conjunction with the Gatekeeper, the Gateway can set up and clear calls on the LAN and SCN. In effect, it reflects the LAN characteristics to the H.323 terminal on the LAN video and the SCN terminal characteristics on the SCN side. Under certain conditions, the Gateway can be used to bypass a LAN router or a low-bandwidth communications link.
The Multipoint Control Unit (MCU)
The Multipoint Control Unit (MCU) supports multiconferencing between three or more terminals and Gateways. A two-terminal point-to-point conference can be expanded to a multipoint conference. The MCU consists of a mandatory multipoint controller (MC) and optional multipoint processor (MP).
The MC supports the negotiation of capabilities with all terminals in order to insure a common level of communications. It can also control the resources in the multicast operation. The MC is not capable of the mixing or switching of voice, video, or data traffic. However, the MP can perform these services (under the control of the MC). The MP is the central processor of the voice, video, and data streams for a multipoint conference.
MCU Multipoint Conference Control. The MCU may (or may not) control three types of multipoint conferences (see Figure 9-3):
Centralized multipoint conference: All participating terminals communicate with the MCU point-to-point. The MC manages the conference, and the MP receives, processes, and sends the voice, video, or data streams to and from the participating terminals.
Decentralized Multipoint Conference: The MCU is not involved in this operation. Rather, the terminals communicate directly with each other through their own MCs. If necessary, the terminals assume the responsibility for summing the received audio streams and selecting the received video signals for display.
Mixed Multipoint Conference: As the name suggests, this conference is a mix of the centralized and decentralized modes. The MCU keeps the operations transparent to the terminals.
Figure 9-3. Multipoint conferences
The H.323 Gatekeeper
The H.323 Gatekeeper provides address translation and call control services to H.323 endpoints. It also is responsible for bandwidth control, a set of operations that allow endpoints to change their available bandwidth allocations on the LAN.
A single Gatekeeper manages a collection of terminals, Gateways, and MCUs. This collection is called a zone. A zone is a logical association of these components and may span multiple LANs.