ization 3 Conventional PMR (dispatch) network: One channel for each organization Channel 1 Channel 2 Channel 3 Organization 1 Organization 2 Organization 3 Trunked network: Radio channels (spectrum) are shared by all users who may belong to separate networks Channel 1 Channel 2 Channel 3 Figure 5.6 Operating principle of a resource-sharing network and a trunked network. channels in this pool are allocated on demand and blocking occurs only when the total number of calls exceeds three in this simplified example. To further improve utilization of radio frequencies, the trunked net- works utilize a cellular structure and technology that is similar to that used in public cellular networks. 5.4.2.2 Trunked Networks Many analog trunked networks are in use. The frequency bands in use are different from those used in public cellular networks. Most of the current analog trunked networks provide enhanced voice services such as priority call in case of an emergency and group calling (group is defined by the network operator), and each network typically contains a terminal that has a dispatch- ing role. The networks also support built-in data communication features such as predefined and user-defined text messages. They may also provide teleme- try services such as remote control of unmanned stations; measurements of temperature, wind force, and water level; and alarms for buildings and remote alarm control (on/off machines or lights). For taxi companies they may provide an automatic response of the status at the moment and auto- matic vehicle location with the help of the Global Positioning System (GPS). These features may also be used by rescue services, transport companies, and the forestry industry. The analog networks in use are different from country to country and even within one country, many incompatible systems may be in use. New Mobile Communications digital systems are evolving that are aimed at supporting a wider area service. One of them is the terrestrial trunked radio system, which has the goal of providing a