st as our call reserves one of the fixed channels, or "trunks," from one exchange to another. This improves the utilization of radio frequencies and is economically feasible because of reduced investments for network infrastructures. For each organization a closed user group is set up and this VPN operates in the same way as if it were physically separate. The systems are logically separate, but they use any free radio channel from a common channel pool. These resource-sharing networks, also called trunked networks, are managed by a network operator. They are configured to provide a specialized service for each VPN of a corporate customer. The use of a frequency band is optimized by sharing it between multiple user organizations. 5.4.2.1 Operating Principle of the Trunked Networks In the trunked network, a central equipment allocates a free channel from a common channel pool in real time to the user who requests it, and to him alone, for the duration of the communication. For each organization a VPN is defined in the system. Dynamic channel allocation uses the radio capacity efficiently and users still feel as if they had a separate dedicated system in use. Each user organization may have its own dispatch station just as in a separate conventional dispatch radio system. Figure 5.6 illustrates the principle of channel allocation in the case of conventional PMR and of trunked PMR. There are three conventional dis- patch radio networks in this simplified example with one radio channel allo- cated for each; that is, each organization has only one channel available whether others communicate through their channels or not. There is a demand for three simultaneous calls, two for organization 1 and one for organization 3, and one of the calls is blocked although one radio channel is free. The lower part in Figure 5.6 presents the principle of a trunked radio system. Now a pool of all three radio channels is shared by all users. The Introduction to Telecommunications Network Engineering Organization 1 Organization 2 Organ