In one of its simpler forms, a repeater based radio communications system
consists of two or more remote stations with a repeater in the middle. One
persons talks in the assigned frequency and the others listen. When this
person is done talking, the others can take their turns. This form of
communications has been in use since its inception, but what happens when the
number of remote stations is so large that the possibility of people wanting to
have conversations simultaneously is considerable? What about privacy in
conversations? You could assign different frequency to different
conversations, but then you have the problem of coordination (what frequency are
you going to be listening on?), and nothing keeps a third party from simply
tuning into that frequency and listening. Assigning a fixed frequency to a
single station would be a waste of valuable bandwidth, unless said station were
to be transmitting constantly.
This is where trunking comes in, as a more efficient, somewhat more private
means of frequency reutilization. In this case, the FCC of whatever entity
regulates the use of the airwaves, assign a number of frequency to a
communications company. This company then sets up this frequencies (trunks)
in its system and programs all the remote stations to it. What happens then,
is that whenever a person wants to initiate a conversation, the central station
will assign a trunk from its pool of available frequencies to this
conversation. When this person is done talking, the station will mark the
trunk as available again. When the other party in the conversation wants
to talk, he or she might be doing it in a different frequency. The central
station takes care of keeping both parties always in the same frequency.
How is this more private? It's harder to follow a conversation that
might be hopping from one frequency to another, unless you have a scanner sophisticated
enough to be able to follow trunked conversations (and as times goes by, these
scanners are getting cheaper, but at the same time, trunking systems
are improving its means of countering eavesdroppers).
Just like ISPs and other communications carriers, this method works on the
assumption that individuals use the system onle a small fraction of the time,
allowing for oversubscription, hence increased efficiency.
Sources: Trunking Radio Systems. http://www.roity.com/rc/trunking.html
Trunking Communications Overview. http://www.motorola.com/LMPS/RNSG/trunking/