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17. Radio Systems Overview
There are two basic types of radio systems: conventional systems and trunked
systems.
Conventional radio systems
In a conventional radio system, each group of users is assigned one (for simplex
systems) or two frequencies (for repeater systems). For example, the police in your
area might operate on 460.500 MHz, the fire department on 154.445 MHz, the
highway department on 37.900 MHz, etc. All transmissions from each group
always go out on the on the same frequency--the police won't randomly switch to
500.000 MHz, for instance.
Since each group always stays on the same frequency and frequencies never
overlap, it's very easy to follow conversations on conventional systems: 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.
Up until the late 1980s, this was the primary way that radio systems operated.
Some examples of conventional radio systems are
Aircraft
Amateur radio
AM CB/UHF CB users
Small, private radio systems
Trunked radio systems
Several major trends have converged that have resulted in agencies moving to
more efficient trunked radio systems:
Higher levels of radio usage has meant that there aren’t enough individual
frequencies available to allow every group to have their own frequency.
Technology advances have brought down the overall cost and complexity of
implementing a trunked radio system while increasing the features available
to the agency and individual radio users.
Roll-out of major statewide trunked systems makes it easier for even small
agencies to piggy back onto the larger system for less cost than replacing
existing systems.