A short description of trunking:
The city of Worcester uses a GE/Ericsson EDACS (Enhanced Digital
Access Communications System) trunked system. A trunked system consists of a set
of frequencies or channels (10 in Worcester). The Police, Fire, EMS, DPW, School
Dept, etc all use the system. At any given time, any service could be using any
one (or more) of these channels.
What makes a trunked system special is
that no service will ever hear another service. That is, the police will never
hear the school dept, or the fire dept. This means the system works a bit like a cell or Mobile Phone in that you rarely end up with crossed lines. Every radio used actually has a
computer inside which communicates with a central server over one of the ten
channels. This channel is called the 'control channel' and can change
frequencies daily, sometimes more often.
Let's say a police unit has
their radio on Ch.1 (Dispatch East). The server tells this radio which frequency
to receive at every moment, changing as necessary.
When the police unit keys-up, a series of 3
short beeps is heard.
When these are complete, it means that:
unit has identified itself (unit #xxx)
b)The unit has identified what it wants (to transmit on Ch.1),
c)The server has secured a frequency for the radio to transmit on and the
radio is now transmitting on that frequency.
It doesn't matter which
frequency was chosen, because all of the other police radios on Ch.1 have also
received the message from the control channel and are set to listen to that frequency as
well. All of this happens in milliseconds.
The real advantage of a trunked system
is that any given time two, three, maybe more police could be communicating at
the same time. Also a fire could be going on and an ambulance is talking to a
hospital while the DPW is dealing with a clogged storm drain, and no one will
hear any radio traffic that is not meant for them to hear. If by some chance
there are no available frequencies, the radio will wait until the control finds
one. This does not ususally happen, and if it does, then that system probably
needs to increase its capabilities.
I have described to you how a
GE/Ericsson trunked radio system operates. Worcester uses this type of system.
Other agencies (such as Mass State Police) use Motorola systems. There are other
systems as well. The concept is basically the same except the Ericsson system
will transmit beeps, silence, or annoying data and "buzzsaw" sounds before and
after transmissions and also at random other times. The data is used for the
reasons mentioned above. All of the other sounds are present for the express
purpose of discouraging people from monitoring them.
If you want more info or
have any questions, you can contact me: firstname.lastname@example.org
or check out the EDACS
Another really good description of trunking: http://www.signalharbor.com/sr/05apr/index.html