Types of Trunking Systems

20-430.fm Page 25 Wednesday, August 4, 1999 1:29 PM

Your trunk tracking scanner can moni- tor two basic types of systems — Type I and Type II. Instead of selecting a specific frequency to transmit on, a trunked system chooses one of sever- al frequencies in a 2-way radio user’s talk group when that user presses PTT (push to talk). Thus, trunking sys- tems allocate a few frequencies among many different users, but the way Type I and Type II systems do this is slightly different. One important distinction between these systems is the amount of data transmitted by each radio when its PTT button is pressed. In a Type I system, the ra- dio’s ID and its current affiliation (the trunk system it belongs to) are both transmitted. In a Type II system, only the radio’s ID is transmitted.

Why the difference? In Type I sys- tems, each radio in the trunk group in- dividually transmits its own affiliation, while the trunk system maintains a da- tabase that determines each radio's affiliation(s) in Type II systems.

Another difference between the sys- tems is that Type I systems are ar- ranged in a fleet-subfleet hierarchy. For example, it is possible for a city using a Type I system to designate 4 fleets, each with 8 subfleets.

The fleets might be the police depart- ment, the fire department, utilities, and city administration. The police might decide to further divide its fleet into

subfleets such as dispatch, tactical operations, detectives, north, south, east and west side patrols, and super- visors. All the available police radios would then be assigned to one of the police subfleets, letting the police cen- tralize their communications and con- trol the type of users on a single system. Determining the exact fleet- subfleet hierarchy for a particular area is referred to as fleet map program- ming.

The disadvantage of a Type I system is that the brief burst of data sent when a user transmits must contain the radio’s ID and its fleet and sub- fleet. This is three times the amount of data a Type II system radio sends. Since the data capacity of Type I sys- tems is limited and the amount of data increases with each user, Type I sys- tems usually accommodate fewer us- ers than Type II systems. Never- theless, Type I systems are still in use.

There are also hybrid systems which are a combination of both Type I and Type II. Your scanner defaults to mon- itor Type II systems, but you can change to Type I or a hybrid of Type I and Type II systems by selecting a preprogrammed fleet map or creating a custom fleet map for your area (see “Scanning Type I and Hybrid Trunked Systems” on Page 33).

You do not need to determine the fleet-subfleet hierarchy for Type II sys- tems unless you are tracking hybrid systems that contain both Type I and Type II systems.