Welcome to our Trunked Radio Systems
page. This page is designed to give you an overview of public
safety trunked radio technology, systems, concepts, and links
to where you can learn more about trunked radio communications.
The information on this page is divided into the following
In a conventional
system, frequencies are allocated for channel use by function,
i.e., dispatch, car-to-car, tactical, mutual aid, etc. These
channels are only used when they are needed leaving some channels
such as dispatch busy with radio traffic while mutual aid remained
unused. To make efficient use of all frequencies evenly, trunking
technology was developed.
In a trunk radio system, all users share a pool of frequencies from five up to
a maximum of twenty-eight. Users are assigned a "group id" and field radios are programmed to only pick-up transmissions for that group.
A computer, called the "site controller", automatically assigns a frequency for users belonging to the same group to communicate with each other.
This is done over a data channel called the "control channel", which carries data that tells field radios what frequency they are on.
Trunk radio systems
may have one or more control channels and may rotate them every
Sound complicated? Well, letís say that your local police are communicating
on a frequency assigned by the controller.
As soon as there is a break in the
communications, the controller automatically moves all users in
that talkgroup to the next available frequency.
At the same time,
your local fire department is communicating on another assigned frequency, as soon as there is a
break in their communications, the controller moves them to the next
available frequency, maybe even the one that the police were just on.
Since communications on a trunked system never stay on one
frequency, monitoring these communications with a conventional
scanner is virtually impossible, especially in large metro areas
where a trunked system can have dozens and dozens of users.
theoretically, in a small town system with just a couple of users
like police and fire and little radio traffic, you could get by
using a conventional scanner by programming all system frequencies
and locking out the data channels. Although you will have to guess
at who is who when you hear something.
Police Scanners for a complete overview of trunking-capable scanners
including current models on the
There are several different types of analog trunk radio systems, they
Motorola - Type I,
II, IIi Hybrid, Smartnet, Smartzone, & Privacy Plus systems.
- owned by M/A-COM)
- mainly used in the private business sector)
Motorola, EDACS, and LTR are the most
commonly used systems in the United States.
Listen to sample audio .wav files of data
control channel for each of the following systems:
and concise overview of public safety radio communication systems
and much more can be found in books listed on our Public
Safety Scanning Books page.
Also see our Digital
Radio Systems page for digital trunking information.