In a conventional system each agency (police, fire, DPW etc.) has their own seperate frequency or group of frequencies.
Trunked Systems are computer-driven systems with anywhere from 5 to 28 frequencies that are shared by all the county and city public safety departments. The system is designed so that each conversation hops between these frequencies.
- When a user presses his microphone key, the system automatically transmits the call on the first available frequency.
- This means that every time a user keys his microphone, he could be on a different frequency.
- The original frequency that he was using will probably now have a different agency and conversation on it.
- Because of this it is difficult to listen to trunked systems using a regular scanner.
In a trunk system:
- One of the frequencies is the data (control) channel and will have a steady buzzing sound on it
- The data channel is transmitting the data that controls how the trunk system operates.
- Instead of assigning each agency a frequency, users of the system are assigned talkgroups.
- Talkgroups are setup so that each agency will only hear their own units and not all the communications on the system.
- Each radio is programmed with a different ID that is displayed to the dispatcher and other radios using the system when it transmits.