System for establishing communication between audiovisual terminals using digital channels up to 2 Mbit/s
|Publication Date:||1 March 2009|
This Recommendation should be associated with [ITU-T H.221] and [ITU-T H.230].
A number of applications utilizing narrow-band (3 kHz) and wideband (7 kHz) speech together with video and/or data have been identified, including high quality telephony, audio and videoconferencing (with or without various kinds of telematic aids), audiographic conferencing, and so on. More applications will undoubtedly emerge in the future.
To provide these services, a scheme is recommended in which a channel accommodates speech, and optionally video and/or data at several rates, in a number of different modes. Signalling procedures are required to establish a compatible mode upon call set-up, to switch between modes during a call and to allow for call transfer.
Some services will require only a single channel which could, according to the procedures in this Recommendation, be B (64 kbit/s), H0 (384 kbit/s), H11 (1536 kbit/s) or H12 (1920 kbit/s). Other services will require the establishment of two or more connections providing B or H0 channels: in such cases the first established is called hereafter the initial channel while the others are called additional channels. Unless otherwise specified, all references to frame alignment signal (FAS) bit-rate allocation signal (BAS) and service channel (SC) refer to the initial channel or, in the case of a higher-order channel, to the time-slot No. 1 of this channel.
All audio and audiovisual terminals using G.722 audio coding and/or G.711 speech coding or other standardized audio codings at lower bit rates should be compatible to permit connection between any two terminals. This implies that a common mode of operation has to be established for the call. The initial mode might be the only one used during a call or, alternatively, switching to another mode can occur as needed depending on the capabilities of the terminals. Thus, for these terminals, an in-channel procedure for dynamic mode switching is required.
The following clauses develop these considerations and describe recommended in-channel procedures.