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UBCD396XT
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Trunked system basics
There are three major elements common to most trunked systems:
System Controller
The system controller is a special computer that assigns voice channels to
users as they key up their radio. The controller is the brains behind the
trunking system.
Voice Frequency Pool
The voice frequency pool is a selection of radio frequencies available to the
system controller for assigning voice traffic. By assigning voice frequencies
to channels only as they are needed, a trunked system can support many more
channels than it actually has frequencies.
Talk Group IDs
A Talk Group ID identifies which user or agency has been assigned a
particular voice frequency at any particular moment. The Talk Group ID is
essentially the user's "channel": since each voice frequency is used over and
over by all the agencies on the system, trunked systems rely on the Talk
Group ID to identify which particular user or agency is talking.
How a trunked system works
A typical communication on a trunked system goes something like this:
1. A user selects the channel they want to communicate on and presses the PTT
button on the side of their radio.
2. This sends a channel request message to the controller that the user wants to
start a transmission on the Talk Group ID (the channel) that they selected.
3. The controller locates an unused voice frequency and assigns it to that Talk
Group ID.
4. The controller then sends out a channel grant message to all radios on the
system so everyone knows where to find the voice channel for that Talk
Group.
5. At this point, the original user's radio beeps, and the user can begin their
transmission. While this sounds complicated, in real life this process takes
about half a second (sometimes less).