Summary

The purpose of Recommendation ITU-T H.221 is to define a frame structure for audiovisual teleservices in single or multiple B or H₀ channels or a single H₁₁ or H₁₂ channel which makes the best use of the characteristics and properties of the audio and video encoding algorithms, of the transmission frame structure and of the existing Recommendations. It offers several advantages:

–           It takes into account of such as Recommendations ITU-T G.704, ITU-T X.30/I.461, etc. It may allow the use of existing hardware or software.

–           It is simple, economic and flexible. It may be implemented on a simple microprocessor using well-known hardware principles.

–           It is a synchronous procedure. The exact time of a configuration change is the same in the transmitter and the receiver. Configurations can be changed at 20 ms intervals.

–           It needs no return link for audiovisual signal transmission, since a configuration is signalled by repeatedly transmitted codewords.

–           It is very secure in case of transmission errors, since the code controlling the multiplex is protected by a double-error correcting code.

–           It allows the synchronization of multiple 64 kbit/s or 384 kbit/s connections and the control of the multiplexing of audio, video, data and other signals within the synchronized multiconnection structure in the case of multimedia services such as videoconference.

–           It can be used to derive octet synchronization in networks where this is not provided by other means.

–           It can be used in multipoint configurations, where no dialogue is needed to negotiate the use of a data channel.

–           It provides a variety of data bit rates (from 300 bit/s up to almost 2 Mbit/s) to the user.

This revised version of this Recommendation introduces a number of enhancements and clarifications to the previous version, primarily the description on the usage of Recommendation ITU-T G.719 and Annex C of Recommendation ITU-T G.722.1 (14 kHz audio) in ITU-T H.320 systems.