Trunked System. Two-Way Radio Explained. Part 6.

Trunked System

The concept of “trunking” is taken from telephone company technology and practice. It refers to the sharing of common “resources” among a number of different users on the same system without overhearing or interfering with each other’s conversations. A Trunked system takes advantage of the probability that in any given number of user units, not everyone will need “resources” access at the same time. Therefore with a given number of users, fewer discrete “resources” are required.

In a trunked radio system, the system logic automatically selects the physical radio frequency channel (the resources) without user interference. There is a protocol that defines a relationship between the radios and the radio network which supports them. The protocol allows channel assignments to happen automatically. This arrangement allows multiple groups of users to share a small set of actual radio frequencies without hearing the others’ conversations. Trunked systems primarily conserve limited radio frequencies and also provide other advanced features to users.

Instead of channels, radios are related by groups which may be called, groups, talk groups, or divided into a hierarchy such as fleet and sub-fleet, or agency-fleet-sub-fleet. These can be thought of as virtual channels which appear and disappear as conversations occur.

“Trunked” radio systems differ from “conventional” radio systems in that a conventional radio system uses a dedicated channel (frequency) for each individual group of users, while “trunked” radio systems use a pool of channels which are available for many different groups of users.

Systems make arrangements for handshaking and connections between radios by one of these two methods:

  • A computer assigns channels over a dedicated control channel. The control channel sends a continual data stream. All radios in the system monitor the data stream until commanded by the computer to join a conversation on an assigned channel.
  • Electronics embedded in each radio communicate using a protocol of tones or data in order to establish a conversation, (scan-based).

If all physical channels are busy, systems include a protocol to queue or stack pending requests until a channel becomes available.

Benefits of Trunked Radio System
  • Efficient use of channel (spectrum) resources
    • Shared traffic among communication paths
    • Increased probabilities of obtaining free channel
  • Privacy of communications due to each group uses one channel exclusively during the duration of the call
  • Eliminate the need to monitor the channel before transmitting. Users just “push to talk” and the system will take care to find available channel for the call
  • Redundancy of channel resources. If one channel is down, all reminder channels can still be used by all groups
  • As trunked system has intelligence control, there are more features available that are not found in conventional system. Some examples are queuing when all channels are occupied, automatic call back when channel is available, multiple priority level, automatic retry, etc.


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