ual wired terminals connected to a conventional tems must use dynamic channel allocation techniques to be self PABX are replaced by individual cordless telephones, and thus adjusting to changing local conditions. requires further qualification. The additional requirement is that there should be some integra- 34.8.2 Implementation tion or interaction between the normal PABX functions and the cordless functions, in order to provide additional benefits or advant- Although cordless PABXs and key systems are in their infancy, ages. Such advantages might include smaller PABX switch require- current designs are invariably based on radio links using digital ments due to concentration within the cordless sub-system, or modulation. The permitted radio spectrum bandwidth is shared terminal mobility. amongst a number of users, with multiple access by the use of The cordless link may be provided by ultrasonic, infrared, or radio frequency division and/or time division techniques. In order to help techniques using either analogue or digital modulation methods, but achieve the high user densities required, a picocellular base station in order to meet the wide range of user requirements, digital radio arrangement is used with cell radii ranging from about 10 up to 200 techniques are almost invariably employed. The main advantages of metres. Since all channels are available to all base stations by the cordless systems over their wired counterparts are: use of dynamic channel allocation techniques, the traffic handling capacity of the system can be increased simply by adding extra base Mobility, where a user is able to make or receive calls any- stations. where within the overall coverage area. The radio performance in such systems is limited by the inter- 2. Reduced cost of ownership, due to simpler installation and ease ference levels rather than by noise performance, SO as the base of moving system users. station density increases, the interference levels from surrounding Cordless PABX and key systems 34/15 ADPCM IF amp + detec