*Trunking Scanner Basics*
Life Is Getting More Complex
By: wmerrin
21 December 2005

First of all, there is no point in paying extra for a trunking scanner if the people you want to listen to don't have trunking radio systems. Trunking radio systems are designed to more efficiently use the limited number of frequencies (channels) assigned to large users, like large metropolitan area local governments. Many smaller agencies don't use them at all - the county I now live in is straight VHF and any cheap scanner is fine. Some of the newer trunking systems use digital signals and require digital scanners but older systems were analog. A few now are totally encrypted, which means you can't listen to them at all using legal commercial equipment. You should be able to get an idea of what your local agencies use by doing a Google search on "mycity scanner frequencies" or similar.

There are different kinds of trunking systems but most pretty much operate in the same general fashion. The basic idea is that not all radio users talk at the same time just like all telephone users in an office building don't use their telephones at the same time. In an office building the number of telephone lines ("trunk lines") coming into the building will be quite a bit less then the number of telephones installed, so a central switch connects a given telephone to a trunk line only when it is needed to make or receive a call. If all trunk lines are busy the next user has to wait until one is free. Trunking radio systems do the same thing using a central computer system to control the radio transmissions.

I will use the City of San Diego's 800 MHz trunking system as an example since that is the one I was familiar with when we lived there. Changes probably have been made since we moved out of the area but the example should still be valid. There were 18 frequencies assigned to the City for use in their trunking system. One of these frequencies was the "Control Channel" and the rest were used for traffic. When setting up a trunking scanner all 18 frequencies had to be programmed into the scanner and the scanner had to be told these frequencies constituted a trunking system.

To efficiently use these 18 frequencies, the users of the system were divided into "Talk Groups" and their radios were programmed with their Talk Group IDs. When a user keyed his microphone his Talk Group ID was sent to the central computer over the control channel. The central computer assigned a clear channel to that Talk Group and sent the channel assignment information back on the control channel so each user's radio would automatically switch to the correct channel. The signal then went out over the repeater system and all radios which were programmed to be on that Talk Group would have their squelch broken and would hear the message. The computer held that frequency for that Talk Group for a few seconds after the user unkeyed his mic so that a quick response from another user would go over the same channel. If nobody keyed up before the time-out period the channel was returned to the available channel pool. If someone responded after the time-out, the central computer assigned a new channel to the Talk Group, relayed the channel assignment through the control channel and then broadcast the signal over the repeater system. Under normal conditions this all happened quickly enough users didn't notice the lag.

As a result of this frequency hopping, if you try to listen to a trunking system with a non-trunking scanner you may hear a police officer transmit a question on a given channel followed shortly afterwards by a trash truck driver on the same channel, while a second police officer responds to the first on a completely different channel.

The scanner listener has to program desired Talk Group IDs into the scanner. For example, the Police Department divided police users into Talk Groups corresponding to the various police substations as well as by function. So, if I wanted to listen for police dispatch in the area of town I lived in I would program Talk Group 208 (Northern Division) into my scanner. Or, I could listen for one of the Tactical channel Talk Groups, like TAC-1 (ID 432), if I wanted to listen to some special surveillance activity. For fire dispatch I would enter Talk Group 5008; for paramedic dispatch 5072, etc. Of course I could enter all these codes and the scanner would monitor them all, stopping on the first one with activity.

There were Talk Group codes set up for any number of user groups - such as trash trucks, water and sewer utilties, city schools, and so on. Each department or participating agency was assigned a set of codes for their own use. There are hundreds of these codes and many are readily available over the Internet (Google is your friend) and there are various scanner publications that contain the information. Most of these were of no interest to me so those codes didn't get programmed into my scanner.

Before you buy a scanner do some research and determine what type of system is used by the people you want to listen to. This is especially true with used scanners - for example, older analog trunking scanners like I used in San Diego don't have the ability to receive the newer digital trunking signals. If your area has gone to a brand new encrypted system you may not be able to listen to them at all. I saw one internet source that said some areas, like Orange County California, have encrypted all law enforcement communications so you need to do your research first. The information you need should be available over the Internet (via Google) or through some of the scanner books and magazines. There are also very helpful scanner enthusiasts in most major metro areas.
wmerrin



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