ual systems nels) and a full duplex 16kbit/s signalling channel (the data or D-channel), and is commonly referred to by the shortform notation Rec. H.320 (CCITT, 1990c) covers the technical requirements for 2B+D. Initially, the marketing of the ISDN will concentrate on audiovisual services at channel rates between 64kbit/s and business customers as having the most likely need to transmit 1920kbit/s. This Recommendation is the 'high level' document that speech and a mix of data (fax, video, computer files etc.) over the calls all the other relevant Recommendations for audiovisual ser- same network. vices. It defines the allowable types of terminal and allowable Videophony and desk top videoconferencing are likely to utilise transmission modes, and where interworking among different ter- basic rate access, either 2B (video at 108.8kbit/s, speech at 16kbit/s minal types must be supported. and framing data at 2 X 1.6kbit/s; if a higher bit rate is used for The standards to be adopted for subdividing a transmission chan- speech as per Recs. G.711 or G.722, the video bit rate is reduced) or nel (again in the range 64kbit/s to 1920kbit/s) into subchannels for 1B only (video at 46.4kbit/s, speech at 16kbit/s and 1.6kbit/s fram- the transmission of video, audio and ancillary data are embodied in ing data). It should be noted that in 2B operation, since video data Rec. H.221 (CCITT, 1990d). The framing structure defined in Rec. is always present in both channels, the terminal has to synchronise H.221 is octet based, with 80 octets per frame; the eighth bit is called the incoming B-channels to remove any differences in transmission Videophony and videoconferencing 57/33 delay before the video data can be correctly demultiplexed; the mission delay. While it does not normally matter whether a com- network does not guarantee this 'time slot integrity' for 2B oper- puter file is transferred in a few seconds or a few tenths of a second, ation. the quality of real time services such as speech and videophony Rec. H.320 stipulates 