Trunked Radio Systems (TRS)

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 topics:

• How Trunk Radio Systems Work

• Types of Trunked  Radio Systems

• Trunk Radio Resources

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 24-hours. 

Sound complicated? Well, let’s 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. Although, 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. 

See Trunking Police Scanners for a complete overview of trunking-capable scanners including current models on the market. 

There are several different types of analog trunk radio systems, they include; 

• Motorola - Type I,  II, IIi Hybrid, Smartnet, Smartzone, & Privacy Plus systems.

• EDACS (Enhanced Digital Access Communication System - owned by M/A-COM)

• LTR (Logic Trunked Radio - mainly used in the private business sector)

• iDEN Harmony

• MPT-1327

• OpenSky

• SmarTrunk

• TETRA

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:

• Motorola Trunk Radio Control Channel

• EDACS Trunk Radio Control Channel

An extensive and concise overview of radio communication systems and much more can be found in Radio Shack's Police Call Frequency Guide (read our review). You can purchase Police Call at your local Radio Shack store or buy it online at RadioShack Shop On-Line. 

Other Informational Resources:

• RadioReference.com - everything there is to know about trunked radio. 

• Conventional and Trunked Radio Systems Comparison Report

• Overview of Radio Communications Systems (pdf file)

• How A SmartZone System Works

• Glossary of Trunk, Digital, & Motorola Terms

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