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Trunking - Not Just A Space In The Back of Your Car!

If you have been researching a scanner to buy, you may have already run into the term "trunking" or "trunked radio system." What is all this talk about trunks and trunking? Does it involve the storage space in the back of your car? Does it have something to do with elephants?

Actually, trunking is a term that is used to describe a radio system that shares frequencies between all the users of the system. Like so many other things these days, trunked radio systems involve computers, which control the usage of all the frequencies that are used in a particular trunked system.

A little bit of history might help understand the benefits of trunked radio systems.

Before trunked radio systems, certain groups of radio users were all assigned to specific frequencies, or channels that they could use to communicate with the other users within their agency. A good example would be a Highway Patrol Department. The department may use one channel to communicate with all their officers in the southern part of the state and other channels for the north, east and west. It was not unusual for a state Highway Patrol Department to use 10 or more channels that were dedicated to the Highway Patrol and not used by any other agencies.

Now consider the State Department of Transportation or D.O.T. They might have communications system very similar to the Highway Patrol, with a number of channels for use in different areas of the state. If they are also using 10 or more channels, you then have at least 20 channels assigned to state agencies. There are, of course, other state agencies that may have their own radio systems as well and they would all have their own set of dedicated channels just for their use.

With trunking, a number of agencies can all use the same radio system and the same channels and never hear anyone on their radios except those that are part of their agency. Sounds impossible at first, but thanks to computers, it all works quite nicely.

Each user's radio has a computer in it that allows it to switch channels in the blink of an eye. A trunked system might have 20 channels assigned to it and multiple agencies using that system. With the old radio systems, each channel was assigned to a specific agency and was not used by anyone else. As you might imagine, this resulted in a lot of channels that were very quiet much of the time but were not available to be used by anyone else. Rather wasteful when you think about all the agencies that were often forced to share channels due to lack of available free channels at the time.

A trunked system may have only 20 channels serving 15 or 20 agencies but the main computer that controls the entire system is able to direct radio traffic for various users as needed. This is all accomplished with "talk groups." Various trunking systems may use other terminology, but for the purposes of this discussion, I will stick with talk groups to keep it simple.

A talk group is a group of radio users that are assigned as a group to be able to communicate with each other. For example, the Highway Patrol would have their own talk groups and the D.O.T. would have theirs. There would be a talk group or multiple talk groups assigned to each user of the trunked system.

Each time a user on the system presses the transmit button on a radio to talk with another user in his agency, the trunked system quickly scans all the channels in the system to see which channels are free and which are currently in use. The system selects a free channel and sends a command to every radio assigned to that user's talk group to switch to a particular channel. This way, everyone on the system only hears the other users of their talk group even though they are sharing channels with all the other users of the system. This makes the best use of all the quiet time that once tied up many frequencies because they were not available for any other agency to use when they were not busy.

Every radio in a trunked system constantly monitors a special channel in the system called the data channel. The data channel is the one that the system uses to send commands to every radio and assigns them to channels as needed. Other interesting things can be done with trunked radio systems as well, such as disabling a radio that is lost or stolen. That way, unauthorized users can be prevented from transmitting signals and causing interference for the legitimate users of the system.

Even with the deficient use of available channels, a trunked system could be overloaded with traffic at times. A major emergency, for example, might result in the Highway Patrol using up more channels than usual on the system. Fortunately, trunked systems have a priority feature built in that allows the users with the greatest need to get channel assignments at the expense of users who may not normally deal with emergencies.

Perhaps there is a major tornado that devastate an area of a particular state. As you might expect, radio traffic on the Highway Patrol channels in that area may be greatly increased during such an emergency. With so much radio traffic, many more Highway Patrol talk groups than normal are busy and may need more channels than they due under normal circumstances.

Since the Highway Patrol talk groups probably have a higher priority assigned to them than the D.O.T., the D.O.T. users might find themselves with no available channels at times if the higher priority users on the system require more channel assignments than normal.

In this case, Joe the department of transportation employee may be out on the highway using the radio to report his progress in repairing a broken road sign and suddenly find his transmission cut short because the channel he had been using was suddenly reassigned to the Highway Patrol talk group when a trooper in the tornado-stricken area discovered an injured person by the side of the road and needed to call for an ambulance.

Fortunately for scanner enthusiasts, scanner manufacturers have kept pace with technology and have made scanners available that can be programmed to follow radio traffic on trunked radio systems. The scanner monitors the data channel of the trunked system and is able to follow the commands sent by the system and switch channels as needed to follow the various conversations taking place among the various users of the system.

The older and more traditional radio systems are still in use in many areas. In areas where there are fewer users competing for radio channels, it does not make sense to invest in expensive trunking radio systems. You may live in an area where a non-trunking scanner might be all that is needed to hear all the action in your area.

Trunked systems revolutionized radio communications for many various users and we are likely to see more and more of them being installed to replace older systems and make more coefficient use of available radio frequencies.