competing fixed telephone service by the operator who owns the cables of the fixed access network. However, the importance of WLL has decreased because of the reduced costs of cellular telephone service. 5.4.1.3 Cordless Corporate Network In most companies internal wireless communications as well as external com- munications rely on the public cellular networks. The corporate telephone network is built on the fixed telephone service provided by the PABX/PBX of a company. One attractive application of modern digital cordless tech- nologies, such as DECT, was considered to be cordless corporate networks where the PABX is upgraded to control wireless DECT telephones in addi- tion to wire-line telephones. This technology supports handover and termi- nals can move freely inside the area of one PABX that controls multiple base stations. Internetwork mobility management functions make it possible to extend the mobility of DECT to other office sites of a corporation and probably even to the local public network if the local public network opera- tor supports DECT technology. The corresponding American technology is called a personal access communication system (PACS). 5.4.2 Professional or Private Mobile Radio (PMR) The PMR systems are dedicated and independent mobile radio systems. Some of them are just simple "walkie-talkie" type radios, others are complex Mobile Communications networks that use a technology similar to that of public cellular mobile radio systems. One typical PMR is owned by a taxi operator. It supports telephone calls and some data communication between a control desk and a number of car telephones in the area. A small number of radio channels are allocated for each of these systems inside a geographic area. Traditionally, each organization has built its own mobile radio system that is completely independent from others. The modern systems utilize a so-called "trunking" principle, which means that a group of radio chan- nels is shared between several organizations. Radio channels are used on demand ju