that any 2B terminal must be capable of would be severely affected unless an almost fairly steady through- operating as a 1B terminal and this has the additional advantages put and bounded delay could be guaranteed. For this reason, LANs that a videophone call can still be established if one of the B-chan- such as Ethernet in common use today are not suitable for real time nels is busy and the user can, during a 2B call, free one of the services; however, the next generation of LANs will be based on the B-channels for other purposes. It is a designer option whether the token exchange principle and are likely to be much faster and more video codec associated with the terminal supports CIF or QCIF intelligent (able to prioritise data), and SO could well support video- picture resolution. User preferences vary as to the optimum trade off phone services of an acceptable quality. between spatial and temporal resolution, some preferring less sharp pictures with smoother movement to sharper but jerkier pictures. Again however, CIF terminals must be capable of falling back to 57.5.3.5 quarter CIF operation. For videoconferencing between specially equipped rooms, the It is of interest to consider whether the public switched telephone preference is likely to be for full CIF pictures with transmission at network (PSTN), the most widespread telecommunications net- a rate of 384kbit/s emerging as a popular optimum compromise work available, could support a videophone service. Currently between picture quality and transmission cost. Switched 384kbit/s available full duplex PSTN modems can operate at a rate of access to the ISDN is likely to be provided in the future in one of 14.4kbit/s, and designers are confident that 19.2kbit/s will be two ways; either by setting up six independent 64kbit/s connections possible in the near future; 19.2kbit/s modems for use on 4 wire (with again the need to synchronise all six incoming data streams circuits are already available commercially. for satisfactory operation) or by providing an ov