eed local area networks. In CCITT two main service ca- service. A series of packets to the same destination could therefore tegories have been defined in recommendation 1.211, interactive go over different routes. Since the routeing information is carried at services and distributive services. The interactive services are sub- layer 3, at each switch the layer 2 protocols have to be terminated divided into conversational services, message services, and retrieval and layer 3 interrogated. Potentially this can cause long and variable services. delays in the transmission of the signal, possibly making it unac- Conversational services are generally bi-directional, although in ceptable for real time services such as speech. some circumstances they can be unidirectional and in real time A technique was therefore needed that avoids the under utilisation between users, or between a user and a host. Examples include of the circuit switch channel when carrying sporadic information videotelephony, video conferencing, and high speed data trans- and the time consuming processing of the X.25 type service. The mission. Message services will offer communication via storage chosen technique was the ATM technique. This requires the infor- units such as mailbox or message handling functions which not only mation to be assembled into fixed length packets or cells as they are include speech but also moving pictures and high resolution images. now called. Each cell comprises a 5 byte header and a 48 byte Retrieval services offer user access to information stored centrally information field as shown in Figure 44.13. The main purpose of the and accessed on demand. Examples of these services include film header is to identify cells belonging to the same call, since calls are and high resolution images together with audio. allocated virtual channels over the network when a number of them The distributive services are differentiated between those services are multiplexed onto a single bearer. The technique is connection without individual 