The concept of trunking has been around in telecommunications and other services for a long time. It has been used in mobile radio systems for at least a generation. Trunking allows for more efficient use of scarce radio frequencies by only assigning actual frequencies to users as they are needed (early non-trunked systems dedicated a frequency to each user group and if a particular user group was not active, the assigned frequency was idle and going to waste).
A trunked system is more complex and needs some intelligence. If a frequency is only to be assigned when a user needs it, the system must:
- detect when a user needs a frequency,
- know which frequencies are available at that moment,
- let all users know about that frequency assignment and
- when the transmission is done, return that frequency back the the pool of available frequencies.
To accomplish all of this, the radio system needs to be run by a computer and one of the pool of frequencies is typically reserved as a control channel.