The term trunking comes from the telephone system and is
an old and long-established concept.

        When you use a telephone to call from one place to
another such as calling from work to some other business or
calling from your house to talk to somebody in another town, your
call may start out on a pair of wires that ends up at your house
and only your house, but much of the equipment that carries your
voice is equipment that you more or less borrow for the duration
of the call.  There is the dial tone that the switch sends to you
which is one of only a relatively few dial tone generators
available, the idea being that most people do not stay on the
dial tone for very long.

        Then you actuate the switch which, in most places these
days is a large special-purpose computer.  While you are dialing,
you tie up other special equipment that is listening to your
tones or counting the pulses until you dial enough digits to make
a valid phone number.

        At that point, you seize what is called a trunk line
which in this day and age is probably one channel of a fiber
optic cable that may be carrying thousands of other calls.  It is
a trunk because it is like a tree trunk.  It supports all other
parts of the tree.

        Now for radio, the exact concept is used on trunking
systems.  Rather than each department in a city, for example,
having its own frequency that may only be used for a few minutes
each day, every single department from the police and fire
departments to code enforcement or Animal Control is on a complex
system that keeps a running map of who is using what frequency.
A system like Tulsa, Oklahoma's trunking system has twenty trunks
or channels, each one of which can carry traffic from any of the
city departments.

        A master controller acts like the telephone switch in
your neighborhood and sends out that real-time map to every city
radio that isn't actually talking to somebody.  The fire fighters
only hear fire traffic and the garbage trucks can only talk to
other garbage trucks, but the trunking system keeps it all
straight just like telephone trunks have done for years.

        It is the most efficient way to use resources.  I have no
idea how many separate frequencies Tulsa used to have, but every
single department from the school system to the electric utility
for Tulsa had its own fleet of radios on one or more channels so
I bet it was probably upwards of 100 channels.

        The telephone company uses trunks for the same reason.
There just isn't enough room to run a pair of wires from your
house to everywhere you might want to call so trunk lines are
used and reused thousands of times a day to make the system
manageable.

        Just as another example, Oklahoma State University
supports nearly 20,000 students and another roughly 3,000 staff
and faculty.  There are only 150 trunk lines leading from campus
to the outside telephone network and it is seldom that they are
ever all full.

Martin McCormick WB5AGZ  Stillwater, OK
OSU Center for Computing and Information Services Data Communications Group