Trunked Radio Systems
As public service agencies grow in metropolitan areas, the challenge of finding enough available frequencies to accommodate all radio users increases dramatically. Trunked radio systems address this problem by sharing frequencies among agencies.
A trunked radio system is one in which groups of users (talk groups), such as police officers, firefighters, and animal control officers, use the same group of frequencies on a rotating basis. The radios in a given talk group are assigned a code exclusive to the group that the radio system hub (controller) recognizes when someone in the group transmits. When someone transmits, the controller automatically shifts the radios in that talk group to the next available (unused) frequency; the controller uses a separate control (data) channel with its own dedicated frequency for continuously transmitting talk group control information to all radios in the system.
For purposes of illustration, consider a trunked radio system that has three talk groups (see Figure A above) and two traffic (voice) frequencies available. Say the system is inactive when a police officer activates a mic; the system automatically shifts all police radios to the first available frequency, call it Frequency 1. Then, say a firefighter begins transmitting while the police officer is still transmitting; all firefighter radios shift to Frequency 2, which is the next available frequency. Then, say the police officer stops transmitting, and an animal control officer begins transmitting. The animal control talk group moves to Frequency 1. All talk groups shift between Frequencies 1 and 2, based on frequency availability.
Under normal circumstances, radio user demand will not exceed a trunked system's capacity. If a person keys a transmitter when the system is already at full capacity, that transmission will be blocked (with a notification being sent to the transmitting radio when a frequency is available) or queued and then retransmitted when a frequency becomes available.
Note that whenever someone in a talk group transmits, the frequency used can change. This means a conversation among members of a talk group can take place on multiple frequencies, but the frequency-shifting process is transparent to system users. Members of a given talk group will not hear transmissions from another talk group unless the system is reconfigured to allow cross-agency communication during, say, a large-scale emergency.