There are four major components to an H.323 network. These are Gateways, Gatekeepers, Multi-Point Control Units (MCUs), and IP Terminals and Clients. I would like to briefly discuss each of these components.
First of all, the H.323 Gateway transforms audio received from a device into a format that the data network can use. It has built-in intelligence to select CODECs and adjust protocols and timing between dissimilar computer systems and Voice over Data networks. It can connect divergent networks by translating signaling protocols and converting media formats, including Analog-to-IP and Digital-to-IP. Internal and external junctures are both required.
The Gatekeeper provides call control services. It maps phone numbers to IP addresses and translates between phone numbers and IP addresses. It provides call setup and trade-in functions, as well as managing bandwidth. Every end-point in a zone must register with the gatekeeper.
Multi-Point Control Units are actually comprised of two components. The Multi-Point Controller (MC) controls one or more Multi-Point Processors (MP).The MP encodes and routes audio and video data streams between device end points. There are two types of conferences that can be managed by the MCU: Centralized and Decentralized. Centralized conferences, including conference calls, use the MCU to create and control the conference. Participating phones or devices send their audio, video, data, and control streams to the MCU. This can place a very heavy load on the MCU as it must replicate the streams to all participating devices in the conference. The MC tracks the capabilities of each device and makes sure that the streams flowing to each terminal are appropriate for that terminal, while the MP can convert the streams between different CODECs and bit rates depending on the capabilities of each end-point. Decentralized conferencing works a little differently. Participating devices use multicast to send their streams directly to each other. The MCU ensures that each device is conforming to standards. This reduces the amount of bandwidth that is required to the MCU, but each terminal has increased bandwidth requirements. The terminals use H.245 to tell the MC how many information streams they can process simultaneously.
Finally, I would like to discuss the IP Terminals and clients. An H.323 terminal is any end-point that can support real-time two-way communication with another H.323 terminal, gateway, or MCU. There are three types of information flows that can be passed to an H.323 terminal: Audio, Data, and Video. Every terminal must register with a gatekeeper upon startup. Once the terminal is registered with the gatekeeper it is able to make or receive calls, depending of course upon the permissions it has and the setup of the network. When a terminal attempts to place a call it sends an Admission Request Signal (RAS). The gatekeeper responds with an Admission Confirmation Message (ACF). The terminal is then able to set up the call with the gatekeeper. The gatekeeper will send setup information to the receiving terminal. The receiving terminal will then have to send an ARQ of its own to the gatekeeper, and receive the ACF response. At this point both phones are alerted that the call can be completed, and the call is connected.