Videophone,videoconferencing and other conversational multimedia services on fixed bit-rate networks (such as ISDN) depend on the multiplex structure described in Recommendation ITU-T H.221.The control channel within this multiplex is the so-called bit-rate allocation signal(BAS)position in each frame.This Recommendation describes all the point-to-point procedures involving the BAS codes:initial capability exchange and mode initialization,including restricted-network operation;in-session mode switching,including audiovisual mode changes and activation of data channels;other procedures, including single/multiple channel interconnection,encryption,mode-preference indication,conveyance of network addresses and use of BAS extension codes.