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38 Trunking Operation
ˆ
Trunking Operation
The scanner tracks transmissions that use the Motorola
Type I, Type II, hybrid, and Ericsson EDACS analog
trunking systems. The frequencies for many of the 800
MHz, 900 MHz, and EDACS public safety systems are
listed in the separate
National Public Safety Trunked
System Frequency Guide
included with your scanner.
TYPES OF TRUNKING SYSTEMS
This trunking scanner monitors two basic types of sys-
tems — the Motorola Type I/Type II or hybrid system,
and the Ericsson EDACS system. Instead of selecting a
specific frequency to transmit on, a trunked system
chooses one of several frequencies in a 2-way radio us-
er’s talk group when that user presses their PTT (push to
talk) control. Thus, trunking systems, in general, allocate
a few frequencies among many different users. Motorola
Type I and Type II systems achieve the same results in a
slightly different way. One important distinction between
these systems is the amount of data transmitted by each
radio when the operator pushes the PTT control. A Type I
system transmits the radio’s ID and its current affiliation
or the trunk system to which it belongs. A Type II system
only transmits the radio’s ID.
Why the difference? In Type I systems, each radio in the
trunk group individually transmits its own affiliation, while
the trunk system maintains a database that determines
each radio’s affiliation(s) in Type II systems.
Another difference between the systems is that Type I
systems are arranged in a fleet-subfleet hierarchy. For
example, it is possible for a city using a Type I system to
designate four fleets, each with eight subfleets.
The police department, fire department, utilities group,
and city administration could each be a separate fleet.
The police might decide to further divide its fleet into
subfleets, such as dispatch, tactical operations, detec-
tives, north, south, east, and west side patrols, and su-
pervisors. All the available police radios would then be
assigned to one of the police subfleets, letting the police
20-524A.fm Page 38 Tuesday, June 27, 2000 3:41 PM