Frequently Asked Questions / Trunking Primer

Trunked radio systems are a simple idea that can be difficult to put to words. Let me try to explain. In the past, a fire department may have had 4 radio channels; 1 for dispatching, 3 as tactical channels. Having 4 radio channels required having 4 dedicated frequencies to each one. This can be quite wasteful, since not all frequencies would be used simultaneously.

A trunked radio system would allow more than 4 channels, or talk groups to be allocated to these 4 frequencies. As a result, a small trunked radio system with only 4 frequencies could in theory support dozens of users with many talk groups. As a listener, you no longer scan for frequencies, but instead you scan a set of known frequencies for talk groups. The scanner does the tracking and switching of frequencies within a system for you.

For most trunked radio systems, one frequency is used as a dedicated "Data Channel", also called the control channel, which transmits a continuous digital stream of information to all radios on the system. The control channel sounds similar to a fax machine or modem. When a user keys his radio on talk group 1, a request is transmitted from that radio to a computer that allocates a frequency that is not in use. Then, all radios monitoring talk group 1 automatically tune to the correct frequency and the transmission is received. Once the radio un-keys, the frequency is released back to the computer, and it can be given to another user. For this reason, it is possible for each transmission to take place on a different frequency.

Using a trunked radio system instead of conventional repeaters is beneficial to an agency because more radio channels can be made available to the user. A city with only 10 allocated frequencies can only accommodate 10 channels on a conventional system, but dozens, if not hundreds of talk groups when using the frequencies on a trunked system.

To monitor a trunked radio system, you need to have a scanner that is that is specifically designed to do so. These scanners are known as trunk-trackers, or trunking scanners. There are numerous models and manufacturers available, with my favorite being Uniden, with the BCT245 and BCT780 models. In Fall 2002, Uniden will be releasing a scanner capable of monitoring radio systems using APCO-25 digital modulation (but not digital encryption).

...more to come