Trunk Tracking:
Conventional scanning is a simple concept. You enter a radio frequency in your scanner's memory which is used by someone you want to monitor. You usually know who it is and can stop on the channel and listen to the entire conversation.

As the demand for public communications has increased, many public radio users don't have enough frequencies to meet their needs, and this has created problems. Trunking radio systems solve this problem.

In a trunked radio system, which contains up to 28 different frequencies, radio users are divided into groups, often called talkgroups, and these talkgroups are assigned specific ID's. When someone in a talkgroup uses their radio, a brief burst of data is broadcasted before each transmission. The trunking system computer uses this data to temporarily assign each radio in a talkgroup to an available frequency. If the group using a frequency stops broadcasting or pauses between replies for a few seconds, they are removed from the frequency so another talkgroup can use it.

Sharing of the available public service frequencies, or trunking, allows cities, counties, or other agencies to accomodate hundreds of uses with relatively few frequencies. Following a conversation on a trunked system, using conventional scanning mode, is nearly impossible. You usually miss the replies (because the conversation will likely occur over several frequencies). With trunking you can actually follow the Talkgroup or a specific user. Your trunking scanner can follow this by monitoring all the available trunking frequencies.

Motorola Reception:
There are basically two types of Motorola trunking systems. These are usually referred to as Type I and Type II systems. Type I occurs on some 800 Mhz systems. All VHF, UHF and 900 Mhz trunking systems use Type II.

One important distinction between these two systems is the amount of data transmitted by each radio when the push-to-talk (PTT) button is pressed. Every radio in a trunked system is assigned a unique ID so the central computer can identify it when it's used. Both Type I and Type II systems place radios (or radio users) into groups, called talkgroups, and these talkgroups are allso assigned unique ID's. Some radios have access to only one talkgroup, while others have access to many talkgroups. The talkgroup(s) each radio can access is called the radio's affiliation(s).

In a Type II system, when someone uses their radio, only the radio ID is transmitted when PTT is pressed, whereas in a Type I system the radio ID and its current affiliation are both transmitted when PTT is pressed.


Changes to a Type II system are easier than a Type I because the system manager only needs to update the database instead of reprogramming individual radios.

Another difference between the systems is that Type I systems are arranged in a Fleet-Subfleet hierarchy. For example, its possible for a city using a Type I system to designate 4 Fleets, each with 8 Subfleets. Their fleets might be the Police dept, Fire dept, Utilites and Administration. The Police may decide to furthur divide their fleets into subfleets such as Dispatch, Tatical Operations, Detectives, North, South, East and West side patrols, and Supervisors. All the available police radios would then be assigned to one of the police subfleets. Determining the exact Fleet-Subfleet hierarchy for a particular area is referred to as Fleet Map Programming.

The disadvantage of a Type I system is that when PTT is pressed, the brief burst of data must contain the radio's ID and its Fleet and Subfleet. This is three times the amount of data a Type II system radio sends, and as a result Type I systems usually accomodate fewer users than Type II systems.

Even though there are many Type II systems, Type I systems are still in use. There are also Hybrid systems which are a combination of both Type I and Type II.

I-CALLS (Motorola/EDACS):
I-Calls are unit to unit that are not heard by other system users. Most of the Radio Shack scanners don't support I-Calls, but some of the Bearcat scanners do. If your scanner does not support I-Calls then you can not see them in trunking mode. However, if you scan the frequencies in NON-Trunked mode, you can hear them (but of course you would not see the ID# of the radio unit). If you've ever heard on the radio someone asking another user to call them at, for example, at 765486, this is their radio unit ID#. You can hear some pretty good stuff. These guys say some crazy stuff on their "private" calls and you can get some good laughs here.

Trunking Systems Technical Information:
If you want to look more into the technical details of the various trunking systems (Motorola, EDACS, LTR, APCO-25 Digitial, TETRA, MPT-1327) then go to www.radioreference.com