Role of the components in H.323

The infrastructure H.323 is based on four main components: terminals, Gateways, gatekeepers, and MCU (Multipoint Control Units).

H.323 terminals 
The terminal can be a computer, a telephone handset, a dedicated terminal for videoconferencing or fax over the Internet. The minimum imposed by H.323 is that implement the compression standard G.711 speech, it uses the H.245 protocol negotiating the opening of a channel and setting the communication parameters, and the Q.931 signaling protocol for establishing and stop communications. The terminal also has optional features, including, for group work and sharing of documents. There are two types of H.323 terminals, one of high quality (for use a LAN) and the other optimized for small bandwidths (28.8 / 33.6 kbit / s - G.723.1 and H.263).

Gateway or gateways to traditional networks (PSTN, ISDN, etc.) 
H.323 gateways provide interconnection with other networks, eg (H.320 / ISDN), H.324 modems, conventional telephones, etc. They ensure correspondence Q.931 signaling correspondence control signals and cohesion between the media (multiplexing, flow matching, audio transcoding).

Gatekeeper or gatekeepers 
In the H323 standard, The Gatekeeper is the entry point to the network for H.323 client. It defines an area on the network, called H.323 zone, grouping several terminals, Gateways and MCU it handles traffic, LAN routing, and the allocation of bandwidth. Customers or Gateway register with the Gatekeeper upon activation thereof, allowing them to find any other user through his fixed identifier obtained from its home Gatekeeper.

The Gatekeeper is to:

1. The translation H.323 aliases to IP addresses as specified RAS (Registration/Admission/Status);
2. Access control, banning users and non sessions authorized;
3. And managing bandwidth, allowing the administrator of the network limit the number of simultaneous video conferences. Specifically a fraction Bandwidth is allocated to videoconferencing to not disturb critical applications on the LAN, and support for multipoint conferences ad hoc.

MCU 
The MCs called MCU (Multipoint Control Unit) offer users to make video conferencing terminals and three more 'presence continuous 'or' voice activation '. An MCU consists of a Multipoint Controller (MC) to which is added one or more Multipoint Processors (MP). MC supports H.245 negotiations between all terminals to harmonize the audio and video settings each. It also controls the resources used. But the MC does not deal directly with streaming audio, video or data, the MP who will retrieve the flows and their make undergoing the required treatment. MC can control multiple MPs distributed on the network and part other MCU.