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:

a)The unit has identified itself (unit #xxx)
b)The unit has identified what it wants (to transmit on Ch.1), and
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: admin@scanneraudio.com or check out the EDACS White Paper

Another really good description of trunking: http://www.signalharbor.com/sr/05apr/index.html