scope:

This Recommendation concerns the system operation for a conference call between three or more audiovisual terminals. It is applicable to a single multipoint control unit (MCU) containing an audio mixer and a video switch according to the provisions of [ITU-T H.231], to calls involving two such MCUs, and to calls involving three or more MCUs in a star configuration. It is also applicable to MCUs containing a video mixer. [ITU-T H.233] and [ITU-T H.234] address issues concerning the use of the encryption control signal (ECS). [ITU-T H.242] provides for communication between two audiovisual terminals connected point to point, using the frame structure defined in [ITU-T H.221]. [ITU-T H.230] defines a number of control and indication signals, including those used in the multipoint communication procedures described in this Recommendation.

Three or more audiovisual terminals may be put into communication to form a conference call, by means of one or more MCUs. The general principles of multipoint communication are described in [ITU-T H.231]. The means by which digital channels are established between terminals and MCUs, and between MCUs, is outside the scope of this Recommendation. For information on the framework of Recommendations for audiovisual services, please refer to [ITU-T H.200].

It should be noted that the physic al realization of an MCU may be such that two or more independent conferences may be set up within the same unit; logically, however, there is no relationship between these conferences; the text herein refers to an MCU only as a logical entity pertinent to the particular call of concern.

This Recommendation concerns only the flow of signals along the fixed digital paths, which may be at 64 kbit/s (56 kbit/s in certain networks) or multiples thereof up to 1920 kbit/s. The flow consists of a multiplex of audio, video, control and indication signals and optional user data as defined in [ITU-T H.221] which must be handled by the MCU in a way which is satisfactory to the users.

The signal multiplex on each path is fully in accordance with [ITU-T H.221]: the bit-rate allocation signal (BAS) commands define explicitly how the demultiplexer at the end of each link shall operate. Likewise, the basic procedures for initialization and mode switching are fully in accordance with those defined in [ITU-T H.242] for point-to-point working. However, the composition of the multiplexed signal transmitted by each terminal and by the MCU is determined by terminal procedures and multipoint system procedures, as follows:

a) Terminal procedures are defined in service-specific system Recommendations, such as [ITU-T H.320] for visual telephony.

b) Multipoint system procedures are defined in this Recommendation and are not of themselves service-specific.

c) [ITU-T T.120]: by making use of the T series, MCU and terminal procedures may be greatly enhanced, offering far more sophisticated specific applications to the user. Such enhancement is outside the scope of this Recommendation, although interactions with specific T.120 methods are described here.

For definitions of terms used in this Recommendation, see [ITU-T H.231]; for definitions of single byte extension (SBE) and multiple byte extension (MBE) symbols, see [ITU-T H.230].