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20-522.fm Page 57 Friday, September 17, 1999 12:46 PM
TRUNKING OPERATION
The PRO-92 scanner tracks transmissions that use the
®
Motorola
Type I and Type II (such as Smartnet and Pri-
vacy Plus) and hybrid analog trunking systems, plus GE/
Ericsson (EDACS) and EF Johnson (LTR) type systems,
which are extensively used in many communication sys-
tems.
Trunking systems allocate a few frequencies to many dif-
ferent users. When the mobile unit transmits a signal, one
frequency is chosen from among the allocated frequen-
cies in that trunking system. The user's ID talk group is
sent with the signal.
Trunking group frequencies are included in the supplied
Police Call Trunking Guide . Frequency fleet map and talk
group information is also widely available on the Internet,
including at www.trunkscanner.com .
UNDERSTANDING TRUNKING
In the past, groups that transmit frequently, such as po-
lice departments, were restricted to transmitting on just a
few frequencies. This resulted in heavy traffic and often
required 2-way radio users to wait for a specific frequen-
cy to clear before transmitting.
Trunked systems allow more groups of 2-way radio users
to use fewer frequencies. Instead of selecting a specific
frequency to transmit on, a trunked system chooses one
of several frequencies when the 2-way radio user press-
es PTT (push to talk). The system automatically transmits
the call on that frequency, and also sends a code that
identifies that 2-way radio user's transmission on a con-
trol channel.
This scanner lets you easily hear both the call and re-
sponse transmissions for that 2-way radio user and there-
fore follow the conversation. For Motorola and LTR
systems, the scanner uses the subaudible data sent with
each transmission to identify talk groups. For EDACS, the
scanner monitors the control channel between each trans-
mission to identify talk groups.
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