The video-telephony is the association of the Téléphonie and the Télévision making it possible to the users to see itself during their phone conversation.
Video-telephony did not appear with the advent of the mobile phones 3G (UMTS). Contrary to the current trend, where the services vidéos are delivered in mode IP towards your mobile phone, often starting from the gates of the operators, Visio functions in circuit mode.
The first studies and applications of video-telephony in France were carried out by Hundred (National center of Study of Telecommunications) France Telecom in 1972. The first commercial applications video-telephony will appear only in 1984. This delay being due to the problem of flow (2 Mbit/s) required to transmit video and audio information. This problem will be solved by the algorithms of coding and compression used in the vidéocodecs called also codecs.
Video-telephony has evil to develop for reasons of cost and noncompatibility of the various systems of video-telephony whose codecs use algorithms of coding owners. Also since 1991 codec S conforming to the standard H.261 dominate the market. But what will make fall the prices it is the Compression/Décompression by software, solution less expensive than the codec because the cost is reduced of the material part of the codec, example, video technology indéo of Intel which adopts the standard H.320.
Currently Visio uses the codec H.263 or MPEG-4. All the telephones 3G are equipped with capacities of decoding and encoding of H.263 and some of them also can encoder and to decode MPEG4. The flow of 64 kbps (of which 43 are reserved for the video) is fixed and constant, contrary to the IP.
Thanks to the developments of the market of the telephones 3G, the services around Visio see day. Thus one can already see interactive kiosks of consultation of vidéos, interpersonal mail services. Nevertheless, one will note the low level of use (about 1% to 2%) by the consumers of this service.
Technique Visio and Voip
When a Visio call is launched starting from a terminal it is the protocol H.324M which is used. The mobile telephone operator opens a way between the telephone and a gateway PSTN (SS7-ISUP)/VOIP (SIP or H.323). The H.324m protocol includes multiple protocols and codecs primarily standardiées by the ITU (International Telecom Union).
One can quote: - H.223 for the multiplexing/demultiplexing of the data on the channel of data. - H.245 for the negotiation of the codecs audio/video/given used during the communication. - H.263 for the video part. - AMR, G.711, G.723 for the exchange of the audio data. (AMR and G.723 are patented audio codecs)
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