ITU-T H.230 PDF

Digital audiovisual services are provided by a transmission system in which the relevant signals are multiplexed onto a digital path. In addition to the audio, video, user data and telematic information, these signals include information for the correct functioning of the system. The additional information has been named control and indication (C&I) to reflect the fact that while some bits are genuinely for "control", causing a state change somewhere else in the system, others provide for indications to the users as to the functioning of the system.

The C&I may be categorized into three groups:

a) call control – these are treated in the Q-series Recommendations;

b) transmission frame-synchronous, or otherwise requiring rapid response;

c) conference, data and telematic control not requiring frame synchronism, governed by the multilayer protocol of Recommendations ITU-T T.122-T.125.

This Recommendation concerns only those C&I appearing in category b) which includes a simplified set of conference C&I for multipoint connections of simple terminals.