This Recommendation concerns the system operation for a conference call between three or more
audiovisual terminals. It is applicable to a single MCU containing an audio mixer and a video
switch according to the provisions of ITU-T Rec. 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 Recs H.233 and H.234 address issues concerning the use of the
Encryption Control Signal (ECS). ITU-T Rec. H.242 provides for communication between two
audiovisual terminals connected point to point, using the frame structure defined in ITU-T
Rec. H.221. ITU-T Rec. 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 Multipoint Control Units (MCU). The general principles of multipoint
communication are described in ITU-T Rec. H.231. The means by which digital channels are
established between terminals and MCUs, and between MCUs, is outside the scope of this
Recommendation (see ITU-T Rec. H.200/AV.420).
It should be noted that the physical 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 Rec. 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 Rec. H.221: the 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 Rec. 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 Rec. H.320 for visual telephony.
b) Multipoint system procedures are defined in this Recommendation and are not of
themselves service-specific.
c) ITU-T Rec. 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 Rec. H.231; for definitions of
SBE and MBE symbols, see ITU-T Rec. H.230.