and with proper planning, add authorized user agencies to the system post-implementation. Principles of operation Control channels In essence, a trunked radio system is a packet switching computer network. Users' radios send data packets to a computer, operating on a dedicated frequency — called a control channel — to request communication on a specific talkgroup. The controller sends a signal to all radios monitoring that talkgroup, instructing the radios to automatically switch to the frequency indicated by the system to monitor the transmission. After the user is done speaking, the users' radios return to monitoring the control channel for additional transmissions. This arrangement allows multiple groups of users to share a small set of actual radio frequencies without hearing each other's conversations. Trunked systems primarily conserve limited radio frequencies and also provide other advanced features to users. Talkgroups A 'talkgroup' is an assigned logical group of users on a trunked radio system. Unlike a conventional radio which assigns users a certain frequency, a trunk system uses a number of frequencies allocated to the entire system. Then the control channel coordinates the system so talkgroups can share these frequencies seamlessly. The purpose is to dramatically increase system capacity with optimal use of frequencies. Many radios today treat talkgroups as if they were frequencies, since they behave like such. For example, on a radio scanner it is very common to be able to assign talkgroups into banks or lock them out, exactly like that of conventional frequencies. Fleet maps and IDs Each system is built with a number of system talkgroups identified as required by the planned user agencies, to which new talkgroups can easily be added as the system matures and new agencies or new requirements are identified. For each user agency, talkgroups are assigned in an agency 'fleetmap'. The fleetmap lays out the various talkgroups that the agency requires to successfully undertake its business. For example, in an ambulance service fleetmap there will be a talkgroup created for each of the hospital ER's that the ambulances interact with; talkgroups for communications with dispatch(s), talkgroups for special events or disasters, a talkgroup for air medical transport, and a number of talkgroups that are shared (with appropriate controls) with other first response agencies such as police and fire services. Each talkgroup is assigned a unique digital ID on the system so that the controller can direct transmissions to the radios which are