ts. The only hope to reduce the cost ture quality would be higher, the processing delay would be small significantly is for a manufacturer to be sufficiently convinced of and the video codec would be much simpler. There is currently the prospects for large volume sales to enable him to recover the much activity in the CCITT defining the broadband ISDN (CCITT, substantial investment in VLSI necessary. 1990b) with a broadband user network interface rate of about Two different PSTN vidoephone products (one in the USA and the 155Mbit/s, of which up to about 130Mbit/s is available for customer other in the UK) were announced in the first few months of 1992. data. The optimum rate for videophony and videoconferencing will depend on the tariffing structure adopted for B-ISDN, but in any case, given the long lead time to plan the introduction of such a 57.5.4 Multipoint operation major change to the access network, it seems unlikely that B-ISDN will become widely available for some years to come. In general, telecommunications systems have been designed to operate on a point to point basis between two end users. Some form of multipoint operation is necessary when 3 or more users are required to be involved at the same time, and this is usually provided 57.5.3.4 as a network based service via a multipoint control unit (MCU) embedded in the network (Clark, 1990). A Local Area Network, principally used for interconnecting PCs The simplest example of an MCU is a telephone conference and workstations, is one type of customer premises digital network bridge, located either at a main switching centre or at a private that may be capable of supporting audiovisual service. The data to branch exchange. In this instance, the audio signals from all the be transmitted over a LAN is assembled into packets for trans- locations are combined in such a way that each location does not mission, and the ability of a terminal to transmit a particular packet receive its own audio but does receive a mix of all the other signals. depends on a num