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 Recommendations 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.