exist- service, the maximum available video rate is 46.4kbit/s with ing in the different terminals. There are a number of Questions in 16kbit/s audio data and 1.6kbit/s framing overhead. CCITT Study Groups addressing different aspects requiring stand- The protocols to be used to establish communications between ardisation for audiovisual services, and the Rapporteurs for each audiovisual terminals using channels up to 1920kbit/s are embodied Question co-operated in the production of a framework (CCITT, in Rec. H.242 (CCITT, 1990e). By defining how the capabilities of 1988b) of target Recommendations, which were designated the AV different types of terminal are to be exchanged in an unambiguous series. The AV designation is used while the Recommendation is manner in the BAS codes, each terminal can determine the highest being drafted, but once finalised and formally adopted by the level of common capabilities for interworking. CCITT, the standard is allocated a number in the appropriate series The BAS codes also provide a number of other controls and of published CCITT Recommendations. indications (C&I) such as audio mute, camera identification and Only the most significant members of the framework series can various multipoint controls. These are defined in Rec. H.230 be summarised here. (CCITT, 1990f). 57.5.3 Network aspects 57.5.2.1 Audio coding 57.5.3.1 Background Telephony quality speech, with a bandwidth of 300Hz to 3.4kHz, can be encoded using PCM into 64kbit/s or 56kbit/s as specified in The early videoconference systems in the 1970s consisted in the Rec. G.711 (CCITT, 1984c). The Recommendation specifies differ- main of public studios with analogue transmission of monochrome ent companding laws (A-law for Europe, u-law for North America 525 line or 625 line television signals over dedicated leased circuits. and Japan) and different rules as to whether the most significant bit During the 1980s, the trend was towards private studios or rooms is transmitted first or last. As a result of this incompatiblity, 