A trunked radio system (TRS) is a two-way radio system that uses a control channel to automatically assign frequency channels to groups of user radios. In a traditional half-duplex land mobile radio system a group of users (a talkgroup) with mobile and portable two-way radios communicate over a single shared radio channel, with one user at a time talking. These systems typically have access to multiple channels, up to 40-60, so multiple groups in the same area can communicate simultaneously. In a conventional (non-trunked) system, channel selection is done manually; before use, the group must decide which channel to use, and manually switch all the radios to that channel. This is an inefficient use of scarce radio channel resources because the user group must have exclusive use of their channel regardless of how much or how little they are transmitting. There is also nothing to prevent multiple groups in the same area from choosing the same channel, causing conflicts and 'cross-talk'. A trunked radio system is an advanced alternative in which the channel selection process is done automatically, so as to avoid channel conflicts and maintain frequency efficiency across multiple talkgroups. This process is handled by what is essentially a central radio traffic controller, a function automatically handled by a computer system. Trunking is a more automated and complex radio system, but provides the benefits of less user intervention to operate the radio and greater spectral efficiency with large numbers of users. Instead of assigning a radio channel to one particular user group at a time, users are instead assigned to a logical grouping, a talkgroup. When any user in that group wishes to communicate with another user in the talkgroup, an idle radio channel is found automatically by the system and the conversation takes place on that channel. Many unrelated conversations can occur on a channel, making use of the otherwise idle time between conversations. Each radio transceiver contains a microprocessor that handles the channel selection process. A control channel coordinates all the activity of the radios in the system. The control channel computer sends packets of data to enable one talkgroup to talk together, regardless of frequency. The primary purpose of this type of system is efficiency; many people can carry many conversations over only a few distinct frequencies. Trunking is used by many government entities to provide two-way communication for fire departments, police and other municipal services, who all share spectrum allocated to a city, county, or other entity. A secondary benefit of a trunking radio system is the ease with which it can accommodate radio interoperability 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 finished 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 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 treat talkgroups as if they were frequencies, since they behave that way. For example, on a radio scanner it is common to assign talkgroups into banks or lock them out, exactly like conventional frequencies.
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