Summary

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.