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How does Trunking Work PDF Print
Trunking Background/History

Trunking is a term that began with Telephone. It is a system that allowed users to "share" telephone line "Trunks" between phone company offices.

If there are 50 telephones in city "A" and 50 telephones in city "B" it would be inefficient to have 50 lines running between these two cities, so the telephone companies use a small number of telephone "trunk lines" between the telephone exchange (central office) if city "A" and the telephone exchange of city "B".

The following diagram will show how with coordination between central offices, the infrastructure can be reduced and made more efficient.

Generic Radio Trunking

Radio Trunking accomplishes the same function as telephone trunking. It reduces the amount of equipment, and services it's users very efficiently.

Before we get into how the Trunking works, we need to define what "talkgroups" are.

Talkgroups are similar to the concept of channels in some ways.
Channels are specific radio frequencies. Examples are:

  • Your favorite FM radio station might be located at 94.5 on the FM dial.
    That's the frequency 94.5 MHz (Megahertz).
  • The police in your area might use 155.50 MHz for their communications
  • The fire department might use 156.25 MHz.

The people who use these services go to these specific frequencies or channels to listen (FM Radio) or communicate (Police, Fire or Medical).

In a trunked system, users don't communicate on specific channels, they use talkgroups as a way to group themselves (Police, Fire or Medical).

For the moment we only need to remember that users are logically grouped in these talkgroups. Talkgroups are simply an associated name and number that the trunking system and the subscriber radios are programmed for. An example might be that a system knows that the talkgroup named "police dispatch" has the talkgroup number of 100.

Trunking is a "hybrid" technology that uses some analog, and some digital communication.

The following sub-systems are needed for a trunking system to work:

  • a "full duplex"1 control channel base radio
  • a series of "conventional" repeaters
  • a computer to allocate resources.
  • the trunking system, and the "subscribers" (user radios) to be programmed with "talkgroups"

How the system works.

  1. A user has a radio. It is on, and has a talkgroup selected (e.g. "Police Dispatch").
  2. A user wants to talk, and presses the Push-to-talk (PTT) on their radio.
  3. The radio sends a digital request to the trunking system, on the control channel2. This request includes the talk group that the user is on (say 100 for "police dispatch"), and may include other data such as the unit ID.
  4. The controller selects an available repeater from its compliment of repeaters. In this case let's say repeater 1 is busy, 2 is busy and 3 is available. The system will select repeater 3.
  5. The controller sends a digital message on the control channel to all units on the requested talkgroup, to go to repeater 3.
  6. The controller sends a digital message to the originating unit (remember, way back at step 2) that they are clear to transmit. The radio makes a short series of beeps to signify this.
  7. The user talks into the radio, it is repeated via a standard analog repeater and it is received by all users on the selected talkgroup.
  8. When the user lets up on the PTT, repeater 3 (the repeater that the controller selected for this talkgroup to use up in step 4) sends a message to the controller, telling the controller that repeater 3 is now available.

  1. "Full Duplex" is a radio that can receive and transmit at the same time - This can only be accomplished on different frequencies, otherwise the receiver would hear it's own transmitter.
  2. A Control Channel is a channel used ONLY for making requests of the trunking system, and for the trunking system to communicate to the subscriber radios. The control channel cannot be used for voice communications. In essence a ten channel trunked radio system, only has nine channels that can be used for voice communication, the remaining channel (control channel) is only used for coordination.

Digital Radio Trunking

Digital Radio Trunking section is not complete at this time.

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