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What Is Trunking?

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Jul 29, 2016
Hi, I am wondering wat trunking because i have heard it a lot and can not understand the wiki entry. Please try not to use to advanced terms or explain them. Thanks


Premium Subscriber
Feb 6, 2004
Louisville, KY
If you go up about 5 posts (the one at the top) you'll see a link to a video on Radio System Basics. This has an excellent segment on how trunking works.

Additionally if you search You Tube, you should find numerous videos describing trunked systems.
Jun 29, 2009
Here's my attempt at trying to explain it. Trunking is basically a smart repeater that uses computers to control the usage of frequencies.

The classic way of using frequencies is to have a single channel per frequency or pair of frequencies if it is repeated:
  • Police Dispatch: Frequency 1
  • Police Tactical: Frequency 2
  • Fire Dispatch: Frequency 3
  • Fire Tactical: Frequency 4

If you want to add more channels, you need to have more frequencies. The problem is that there is only a limited number of frequencies available. For public safety, these channels are not being used 24/7 like broadcast radio. The dispatch channels are only used for when there is a call, and the tactical channels are only used for incidents. The rest of the time, they sit completely unused.

So what trunking does is separate the channels in the radios from the frequencies. The channels in the radios are now "talkgroups" that are recognized by both the computer system and the radios. Now you can start creating a large number of talkgroups for different uses since they are no longer tied to frequencies. Maybe add stuff like Police Admin, Police Events, Fire Admin, Fire Events, Football Talk, etc.

The frequencies are given to the system to be assigned as needed. For most trunked radio systems, a frequency is dedicated to keep the radios updated and allow them to tell the system when they want to make a call. This is known as the control channel. The radios will listen to this control channel for if talkgroup is active. If the radio wants to make a call, it sends a signal on the input of the control channel to let it know. The computer will then figure out what frequency is available and let all the radios know using the control channel.

I have created a little image to represent this below. You will notice that Frequency 3 is being used as the control channel for this system. There are 4 radios on the system, 2 set to the Blue talkgroup, and 2 set to the Red talkgroup.

When one of the radios on Blue wants to talk, they press the push-to-talk button. It sends the request on the control channel, and the computer decides to give it Frequency 2. All the listening radios on the Blue talkgroup switch to that frequency and the transmitting radio transmits on the input for Frequency 2.

Now someone on the Red talkgroup wants to talk and the system gives them Frequency 1.

When the radio on the Blue talkgroup finishes, they return to the control channel. When the other user wants to reply, the system can assign them a completely different frequency, like Frequency 4 in the example below.

Mar 12, 2004
Hi, I am wondering wat trunking because i have heard it a lot and can not understand the wiki entry. Please try not to use to advanced terms or explain them. Thanks
Here is a simple explanation of how Motorola Trunking (the most common type) works.


The object of trunking is to allow many users to share a relatively few frequencies.
A trunking system is controlled by a computer. Information (data) is exchanged between the system radios and the computer on a control channel, sometimes called a data channel. It sounds like a strong buzz.

A large system can have up to 28 freqs., 4 of which may be used as control channels. The control channel may be changed once a day or as often as the programmer decides. Some scanners need only the control channels to track an entire system. Just put in all the Control Channels.

Each group of users (Fire, Police, etc.) is assigned TALK GROUPS. In a Motorola Type II system, the most common type, TGs are usually in 32 number steps starting with 16 and going up to 65536; 16, 48, 80 --- 4656, 4688, 4720 --- 28944, 28976, etc. System radios can have more than 100 TGs programmed into them.

Digital systems are similar, but the talk groups use every number (like 2491, 2492, 2493, etc.).

When a user pushes the talk button on his radio, data is sent to the computer. The computer chooses an unused freq. and sends that data to all the radios using the TG of the originating unit. This all happens in a fraction of a second and it happens EVERY TIME a radio is used.

Fire Dispatch calling Engine 4 (TG 4528 on 856.7125)
Engine 4 answering Dispatch (TG 4528 on 852.2625)
Respond to 73 Elm Street (TG 4528 on 855.9625)
Engine 4 responding (TG 4528 on 851.6375)

If you have entered TG 4528 into your scanner, it will decode the control channel data and change your scanner freqs. to follow the conversation on TG 4528.
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