Protocole H.323

General description of H.323 
The H.323 standard provides, since its approval in 1996, a framework for communications audio, video and data over IP networks. It was developed by the ITU (International Telecommunications Union) for networks that do not guarantee quality service (QoS), such as IP IPX over Ethernet, Fast Ethernet and Token Ring. It is present in more than 30 products and it concerns call control, multimedia management, management bandwidth for point-to-point and multipoint conferences. H.323 also addresses interfacing between LAN and other networks. H.323 is part of the series deals H.32x videoconferencing through different networks. It includes H.320 and H.324 networks related to ISDN (Integrated Service Data Network) and PSTN (Public Switched Telephone Network).

More than one protocol, H.323 creates a combination of several different protocols and can be grouped into three categories: signaling, codec negotiation, and transporting information.

The signaling messages are those sent to request matchmaking two customers, which indicates that the line is busy or the phone rings, etc. In H.323 signaling is based on the RAS protocol for registration and authentication, and Q.931 protocol for initialization and call control.

Negotiation is used to agree on how to encode information to exchange. It is important that the phones (or systems) use language common if they want to understand. This is the least greedy band codec bandwidth or one that offers the best quality. It would also be preferable to have several alternative languages. The protocol used for codec negotiation is the H.245

The transport information is based on the RTP protocol that carries voice, video or scanned data by codecs. RTCP messages may be used for quality control, or renegotiation of codecs if, For example, the band bandwidth decreases.

H.323 communication takes place in five phases: the establishment of call, the exchange capacity and possible reservation of bandwidth through RSVP (Resource booking Protocol), the establishment of the audio-visual communication, the possible invocation of call phase services (eg, call transfer, change bandwidth, etc.) and finally the release of the call.