Trunking Basics

Trunking radio systems are just a different way of using a set of frequencies in a more efficient manner then in the old conventional way. It used to be that each agency in a political sub-division would get some frequencies licensed to them, and then would use those discrete frequencies just for them. Since, there is a finite amount of available radio spectrum, this caused crowding and interference. The first way this interference was solved was by the use of CTCSS or DCS (more commonly called PL or DPL tones). These were either sub-audible or digitally encoded signals sent out at the same time as the radio signal carrying the voice and the radio receivers were set to only open squelch when the correct signal was received. This meant, for example, that two counties, near each other, could use the same frequency (lets say 155.2500) with different tones and would not normally interfere with each other. One organization could be using 155.2500 with a CTCSS tone of 114.8 Hz and in the next county over, another organization could be using 155.2500 with a CTCSS tone of 110.8 Hz and for the most part they would not hear each other.

While this type of radio usage proved to be a pretty good solution for a while, spectrum crowding, especially in urban areas, overwhelmed such solutions, hence the development of Trunking Radio Systems. To understand Trunking Radio Systems, you must first understand the concept of Repeater operation. This is fairly simple if you consider the

wider area to communicate with each other through the use of the repeater (called a Mobile Relay by the FCC). Once you understand how repeaters work, trunking is the next logical step.

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 (a transmitter-receiver) previously used in a single frequency or even a single repeater pair of frequencies. The trunking transceiver is a frequency agile radio capable of understanding signals it receives and changing frequencies, on the fly, based on those signals. All trunking radios operate in a similar manner although the type of trunking technology used by each type of trunking system differs greatly.

In the trunking radio environment, each agency is assigned one or more Talkgroups that the agency’s communication(s) will take place on. 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 type of trunking 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 CC input frequency, unless they are listening to a conversation on a talkgroup. Lets say that Patrolman Bob (from Podunk PD) 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 Podunk PD, 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 Podunk PD talkgroup, change to channel pair XX on the system for a communication. All radios, tuned to Podunk PD’s talkgroup, including Patrolman Bob’s, then switch frequencies to that channel pair granted by the computer. Patrolman Bob’s radio, after changing frequencies, then goes back into transmit mode and he can start to talk. As he talks, all the radios monitoring the Podunk PD’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.

Your 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 repeater output frequency. As I said earlier, each type of trunking 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)