A trunked radio system operates much like the trunk lines on a telephone system. When you press your microphone PTT button, your radio sends an inbound digital message to the trunked system controller on the control channel. The message contains your ID, talk group, and a request for assignment of a voice channel. The trunked system controller locates an idle frequency in its pool of frequencies and broadcasts an outbound digital message on the control channel that directs your radio and any other radios associated with your talk group to change to the selected available idle frequency. The sequence of events to request and assign a frequency occurs in about one-third of a second. 
  When you have finished your transmission and released the PTT button, the trunked controller waits a predefined time (usually two seconds). If there is no response to your transmission within that time, the trunked system controller recovers the previously assigned frequency and waits for the next request to transmit from your talk group. When the next request to transmit occurs, the trunked system controller repeats the sequence of locating and assigning an idle frequency. Most trunked system controllers are programmed to intentionally assign a different frequency for each transmission when the pause between transmissions exceeds the predefined time limit.

The advantages

You might ask, "Why go to all this trouble?" The answer is that trunked radio massively increases the productivity and usefulness of a multi-channel two-way radio system. 

In fact, the inherent efficiency of trunked radio is such that the Federal Communications Commission (FCC) has specified that any system with 5 channels or more will be trunked and that the number of channels licensed is based on 70 - 100 radios per channel. This begs the question:-"How can several hundred radios share a system without chaos during heavy traffic periods?"

The answer is found in the interrelationship of three factors: organization, queuing theory, and the nature of radio transmissions. Trunked radio systems are inherently complex and good organization is vital. Depending on the trunked radio system manufacturer's architecture, a trunked radio system may have many fleets (typically less than 16); a fleet may have several talk groups (typically 16); and there can be many individual radio Ids. Theoretically, a maximum of 65,535 address are available, and the number assigned will depend on the number of radios in the system. Generally, talk groups are rigidly assigned to fleets. Individual radios however, may be programmed for operation in one fleet and one talk group or in highly efficient user-managed systems, they may be programmed for operation in all fleets and talk groups.

A "queue" in this instance is a line-up of people wanting to use a two-way radio system. However, it can be applied to almost any situation where people line-up to receive some service.  Figure 3 shows an analogy between queues in a bank and queues waiting to use a radio system. The illustration shows 6 specialized tellers (frequencies) in a bank. If you wanted to make a deposit, you must wait your turn in the receiving teller's (F6) line although the loan teller (F5) is free. The bottom illustration shows a single line of people being served by 5 general tellers (F1, F2, F4, F5, F6) who can perform any task (serve any talk group). Even if we use one of the tellers (F3) to control traffic, the 5 remaining tellers (frequencies) will service the customers more efficiently than in the upper example.

Figure 3
Queueing Theory