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 andindication (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.