o be divided into sub-channels to carry video, voice, data, and control signals. H.221 specifies synchronous operation; the H.221 coder and decoder handshake and agree upon timing. Synchronization is arranged for individual B channels or bonded connections when contiguous B channels are combined. After synchronization has been established, H.221 sends fixed-size frames of 80 bytes across the network. Information about synchronization is carried over a service channel. H.221 acquires framing at the start of a session by sending some number of bytes in a pattern. The "service channel" is a bit pattern that is spread-out bit-by-bit over dozens of bytes. Within the service channel are two signals, FAS and BAS. FAS, Frame Alignment Signal, delineates H.221 frames. BAS stands for Bit-rate Allocation Signal. BAS codes are used to indicate the capabilities of the audiovisual devices using the B channel's capacity. There hundreds of BAS codes that define many different functions that are used to combine B channels into high-bandwidth services at H.O and other rates. They allow a receiving device to inverse multiplex an incoming signal, and to understand the control and indication information coming across the channel. They define how channel is to be shared between audio, video, and signaling, and indicate how audio is being encoded. H.230 The formal name for this specification is "Frame-Synchronous Control and Indication Signals for Audiovisual Systems." H.230 Recommendation defines how frames of audio, video, user data, and signal information are multiplexed onto a digital channel. It defines control and indication information for those various data types and supplies a table of BAS escape codes that define the particular instances when control and indication information must be exchanged between sending and receiving devices. The H.230 standard primarily addresses control and indication signaling. H.230 distinguishes four categories of control and indication information: video, audio, codec and MCU interoperation, and 