Trunking Basics

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Trunking is a way of using a set of frequencies in a more efficient manner than the old conventional way. In the past, each agency in a political sub-division would get some frequencies licensed to them and then would use those discrete frequencies just for themselves. Since there is a finite amount of available radio spectrum, as the number of users increased crowding and interference occurred.

Early attempts at spectrum management


To understand trunking, one must first understand the concept of repeater operation. This is fairly simple if you consider the following; place a remotely controlled receiver and transmitter on a high point somewhere in the area. The repeater receives all the signals transmitted on one frequency known as the input frequency (call this channel "A") and takes what it hears and retransmits it on another frequency known as the output frequency (call this channel "B") at the same time. All mobile and base stations listen on channel "B" and transmit on channel "A."

This system allows units across a wide area to communicate with each other via the repeater (called a mobile relay by the FCC).

Tone-controlled access

The first way interference was solved was by the use of CTCSS or DCS (more commonly called PL or DPL tones). These are subaudible (and in the case of DCS, digitally encoded) signals sent out along with the radio signal carrying the voice transmission. Radio receivers are set to only open squelch when a specific CTCSS or DCS is detected. This meant, for example, that two counties near each other could use the same frequency (for example, 155.25 MHz) with different tones and would not normally interfere with each other. One organization could be using 155.25 MHz with a CTCSS tone of 114.8 Hz and in the next county over, another organization could be using 155.25 MHz with a CTCSS tone of 110.9 Hz and for the most part they would not hear each other.


While the use of CTCSS and DCS proved to be a good solution for a while, spectrum crowding, especially in urban areas, overwhelmed such solutions, and eventually led to the development of trunked radio systems. Trunking is the use of several repeaters, on different frequencies in the same band, operating together under computer control to allow the pooling of resources for several agencies.

The trunking radio, in a patrol car for example, is much more sophisticated than the simple transceiver previously used in a simplex or repeater configuration. The trunking transceiver is a frequency agile radio capable of receiving instructions and changing frequencies on the fly. All trunked radios operate in a similar manner although the type of trunking technology used by each type of trunked radio system differs greatly.

In the trunked radio environment, each agency is assigned one or more talkgroups that the agency's communications will use. All agencies on the system will have different talkgroups but all will share the same pool of frequencies. For simplicity I will use an example of a control channel-based type of trunked radio system as an example. In this type of system, all the radios on the system (except the computer controlled set of repeaters, of course) listen to a common control channel (CC) output frequency and transmit (initially) on a common control channel input frequency, unless they are listening to a conversation on a talkgroup.

Let's say that Patrolman Bob (from Smalltown Police Department) wants to tell the dispatch office that he is now in service. The following actions take place in a very short time, much shorter than it takes you to read this. He picks up his microphone and keys the mike, his radio sends a signal on the CC input frequency, which the controlling computer understands as a request for a channel grant for the talkgroup assigned to Smalltown Police, his radio then instantly goes back into receive mode. The computer looks at the system for an empty channel pair and issues that channel grant on a specific channel pair and sends that channel grant information out on the CC output channel. This channel grant information tells all radios on the system, if you are listening (monitoring) for communications on the Smalltown Police talkgroup: change to channel pair XX on the system for a communication. All radios tuned to Smalltown Police's talkgroup, including Patrolman Bob's, then switch frequencies to that channel pair granted by the computer. Patrolman Bob's radio, after changing frequencies, goes into transmit mode and he can start to talk. As he talks, all the radios monitoring the Smalltown Police's talkgroup are now listening on the assigned repeater output channel and are ready to talk on the assigned repeater input channel. This continues until Patrolman Bob has finished his transmission. On some types of systems, further communication may be on the initially assigned channel pair or it may move to another, but the process stays the same.

A trunk-tracking scanner is designed to follow those same instructions (except, of course, it ignores the instructions relative to the input side of the repeater pair) so that it will also follow the conversations by changing frequency to the appropriate repeater output frequency. Each type of trunked radio system operates a little differently, and some cannot be monitored by current trunk-tracking scanners at all.

Please take a look at detail information in regard to the type of system that you are attempting to monitor and the instructions that come with your scanner in regard to that type of system.

--Lou Maag 14:32, 26 Dec 2004 (EST)