Public Radio Trunking

American public services – police, ambulances, fire trucks – all rely on lightning-fast radio communications. The problems that constrict their ability to exchange voice, text, and GPS information lie within narrow government regulations. Due to air-wave control, all of these communication lines have to fit within a certain radio frequency. This means that without the ability for many various groups to use the same frequency, there would be a limited number of talk groups. To overcome this limitation, public radio uses trunking technologies.

According to Dan Veeneman of SignalHarbor, “trunking is about sharing. More specifically, trunking is a method of sharing a limited number of radio frequencies among a large number of users.” In order to understand how it works, we must understand how some of the related equipment works. The crux of trunking technologies is the Repeater – a wireless communications tower that expands the range of a frequency; they are also called Range Expanders. Repeaters work with two frequencies: input frequency, and output frequency.The input frequency is the setting that the repeater scans for; the output frequency is where the repeater re-broadcasts the input frequency. Repeaters are usually placed on an elevated point, such as a mountain, to increase the range of service.

So, why use trunking? The truth is, a small town might not need it. But when you have public services for a city the size of New York, communications get a bit more complex. While small urban towns might get by on the limited frequencies that they have, big towns need to split things up. With trunking, one police department can have several “talk groups” – for example, metro police may be one, and homicide investigation would be another. If a city uses trunking public radio, a listener would have to tune their radio scanner to the specific “trunks.” Dispatchers need to tune into the specific talk group to reach city employees out in the field.

There are a few approaches to coordinating people using trunked radio. The first approach is the conventional approach. You listen to a frequency, you wait until no one speaks, and while you’re speaking no one else can speak. It’s a pretty basic approach – we all have experience working with a 2-way handheld radio. The second approach is the interoperability approach. It allows many users use the same frequencies. Typically, public agencies split up their channels into functions – base operations, mobile units, emergency medical services, and others.

Today, we call old radios analog, and we call new radios digital. Analog radio systems have dynamic signals; that is, they weaken with distance, and are varying. Digital radio systems rely on a stream of binary strings (1′s and 0′s), so as long as the receiver is tuned to the right digital frequency, the signal will be outputted the same. The benefit to digital radio is that it can be heard anywhere in the world, using Internet; without the expensive infrastructure (a series of repeaters), analog radio is limited in range.

Trunking is also used for VoIP telephones. You can learn more about trunking technologies here.

Song Of The Day: Muttonheads feat. Eden Martin – Trust You Again