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24
Understanding Scanning
When tracking these types of systems, you might want
to remember that your scanner can track more than
one trunking system at a time and scan conventional
and trunked systems at the same time.
Conventional scanning is a simple concept. You enter
a frequency used by someone you want to monitor into
your scanner’s memory. For example, the police in your
area might transmit on 460.500 MHz, the fire department
on 154.445 MHz, the highway department on 37.900
MHz, etc. So when your scanner stops on a frequency,
you usually know who it is, and more importantly, you
can stop on a channel and listen to an entire
conversation. This type of scanning is easy and fun.
However, as the demand for public communications
has increased, many public radio users do not have
enough frequencies to meet their needs, creating a
serious problem. Trunking radio systems help solve
this problem.
Trunked Scanning
While conventional scanning worked great while there
were only a few groups wanting to use the
frequencies, with the advent of smaller, lower-cost
radios more and more agencies and businesses
wanted to take advantage of the utility of 2-way radio.
As a result, the bands that were used most became
full, so new users were not able to take advantage of
the technology as quickly as they wanted.
Trunking solved this frequency shortage by allowing
multiple groups to use the same set of frequencies in a
very efficient way. While each type of trunking system
operates a little differently (see the next few sections),
they all work on the same basic premise: even in a
system with a lot of users, only a few users are ever
transmitting at any one time.
Instead of being assigned a frequency, as with
conventional systems, each group is assigned a talk
group ID. A central computer controls the frequency
BCD396T Paper OM.fm Page 24 Wednesday, March 30, 2005 8:36 PM