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.