ITU-T Recommendations

Digital audiovisual services are provided by a transmission system in which the relevant signals are multiplexed onto a digital path using the frame structure defined in Recommendation ITU-T H.221. In addition to the audio, video, user data and telematic information, these signals include information for the proper 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. Recommendation ITU-T H.230 concerns only those C&I which must be transmission frame-synchronous, or otherwise requiring rapid response.This Recommendation details the C&I related to video and audio; means of transmitting numbers and characters; C&I for maintenance purposes; for simple multipoint conferences not using protocol in the MLP channel; for channel aggregation; and for the transfer of network addresses. The codepoint tables also indicate the circumstances under which the various functions may be mandatory or optional.This revised version of Recommendation ITU-T H.230 introduces a number of enhancements and clarifications to the previous version, primarily the description on the usage of Unicode characters and ITU-T G.719 in H.320 systems.