user presentation control such as the broadcast oriented since effectively a virtual connection across the network is services for TV and radio, and those with individual user control. established at the start of the call by the use of special signalling The latter includes centrally based services which broadcast cycli- cells. Each cell in the call is then routed over this virtual channel by cal stored information and as a result the user is presented with the information at the beginning of a cycle. The availability of high bandwidth will enable a number of differ- ent types of information to be supported by one service resulting in the development of multimedia services. For example, video tele- Header phony will include audio and video, and also possibly text and 5 octets graphics. Discussions are now being held to try and establish a limited set of information types and how they may be assembled to provide multi media services, given that each information type will Information field octets have different characteristics and require different network perfor- 48 octets mance characteristics. The emergence of multi media terminals supporting a number of types of communicating information with a wide variety of band- widths therefore make it desirable that a switching and transmission technique (the transfer mode) is chosen that is able to carry all Figure 44.13 ATM cell structure services efficiently and in a cost effective manner. Broadband ISDN 44/15 referring the information in the header to a look-up table in the supported by the access network, which would then identify a need exchange. for flow control across the interface. The advantage of the ATM technique is that it is able to multiplex services requiring different cell throughput rates from a few cells 44.8.5 The user/network interface per second up to perhaps several hundred thousand cells per second in any combination on a 150Mbit/s circuit. The services don't have The narrowband ISDN presents two well defined network interfaces to produce informati