Trunked Radios
A trunked radio system is a complex type of computer-controlled radio system. Trunked systems use a few channels (the actual frequencies), and can have virtually unlimited talkgroups. 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.
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Motorola GP680 Radio |
Motorola GP1280 Radio |
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Control channels
In essence, a trunked radio system is 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 talk-group. The controller sends a digital 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 others' conversations. Trunked systems primarily conserve limited radio frequencies and also provide other advanced features to users.
Differences from conventional two-way radio
"Trunked" radio systems differ from "conventional" radio systems in that a conventional radio system uses a dedicated channel (frequency) for each individual group of users, while "trunking" radio systems use a pool of channels which are available for a great many different groups of users.
For example, if police communications are configured in such a way that twelve conventional channels are required to permit citywide dispatch based upon geographical patrol areas, during periods of slow dispatch activity much of that channel capacity is idle. In a trunked system, the police units in a given geographical area are not assigned a dedicated channel, but instead are members of a talk-group entitled to draw upon the common resources of a pool of channels.
Advantages of trunking
Trunked radio takes advantage of the probability that in any given number of user units, not everyone will need channel access at the same time. Therefore with a given number of users, fewer discrete radio channels are required. From another perspective, with a given number of radio channels, a much greater number of user groups can be accommodated. In the example of the police department, this additional capacity could then be used to assign individual talk groups to specialized investigative, traffic control, or special-events groups who might otherwise not have the benefit of individual private communications.
To the user, a trunking radio looks just like an "ordinary" radio: there is a "channel switch" for the user to select the "channel" that they want to use. In reality though, the "Channel switch" is NOT switching channels at all: when changed, it refers to an internal software program which causes a talkgroup affiliation to be broadcast on the control channel. This identifies the specific radio to the system controller as a member of a specific talkgroup, and that radio will then be included in any conversations involving that talkgroup.
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