ITU-T Recommendations

Videophone, videoconferencing and other conversational multimedia services on fixed bit-rate networks (such as ISDN) depend on the multiplex structure described in Recommendation H.221. The control channel within this multiplex is the so-called 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.