What is Trunking?
In conventional radio systems (without trunking), a frequency is assigned directly to a channel. Usually each channel will have a unique frequency. However, in a trunked radio system frequencies and resources are assigned dynamically by a computer system specifically designed for this purpose. The diagram to the right shows a single site trunking system in it's simplest form.
In large organizations, such as metropolitan police departments, there might be hundreds of users each needing a channel. In a conventional system this would require hundreds of frequencies and more equipment such as repeaters. Yet many of the channels would sit idle for much of the day when users were busy with other activities.
In a trunked system frequencies and resources are much more efficiently used. During idle times frequencies are released for other users and communication channels are dynamically assigned to active users, communication pathways are shared by more users.
There are several characteristics of a trunked radio system:
- Central Computer Controlling All Communications
- Dynamic Channel & Resource Allocation
- A logical Hierarchy of Communication Groups
How Does Trunking Work?
Central Computer Controlling All Communications
The heart of a trunked two-way radio system is the central controller (computer). This device manages the allocation of frequencies, monitors the status of communication pathways, and ensures that communication is seamless. The central controller keeps track of which frequencies are in use and which talkgroups are active, making real-time decisions to assign and release frequencies as needed.
Dynamic Channel & Resource Allocation
The computer's system logic automatically picks the physical radio frequency and assigns it to the channel at the time it's needed. Then it is released once the conversation is finished. There is a "protocol" that defines a relationship between the radios and the radio backbone which supports them. This protocol allows channel assignments to happen automatically.
Systems assign channels and resources between radios by one of two methods:
- A computer assigns channels over a dedicated control channel. The control channel sends a continual data stream. All radios in the system monitor the data stream until the computer assigns resources to that radio, allowing it to join a conversation on an assigned channel.
- The other method involves electronics embedded in each radio. These electronics communicate using a protocol of tones or data in order to establish a conversation, (scan-based). If all physical channels are busy, some systems include a protocol to queue or stack pending requests until a channel becomes available.
- Some trunked radios scan multiple talk groups or fleets and subfleets.
Digital trunked systems can also carry simultaneous conversations on one physical channel. Again, allowing for more efficient use of the equipment. In the case of a digital trunked radio system, the system also manages the time slots on each physical channel. The function of carrying simultaneous conversations over a single channel is called multiplexing.
A logical Hierarchy of Communication Groups
Instead of channels, radios in a trunked system are related by groups. These groups can be further sub-divided into groups, talk groups, or divided into a hierarchy such as fleet and subfleet, or agency-fleet-subfleet. These groups, talk groups, fleets, etc., can be thought of as virtual channels which are assigned physical channels and resources as conversations occur. Then the resources are released once they are idle.
What if I Have Multiple Sites?
The diagram to the right shows the types of systems you can have.
- Systems without trunking are referred to as conventional systems.
- Single-site trunking systems are systems where each controller is a silo to itself. These single site systems do not communicate with each other.
- In Mutli-site trunking systems each site has a controller, and there is also a centralized controller to manager cross-site communications. These systems offer:
- More efficient use of resources
- And also greater range due to different locations/repeaters being tied together by a central controller and IP network.
- However, multi-site systems are complex to manage and have greater costs involved.
What is a Simulcast System?
Suppose a radio system has a several repeaters at different sites. All repeaters are transmitting the same audio, but users move around. While a user may start out with excellent reception at the first site, they eventually move beyond range. They must then transfer to another repeater to get better reception.
A simulcast repeater system makes the switching between repeaters seamless to the end user. The signal is broadcast to all repeaters, and through a process called voting the best signal is selected by the system.
In a simulcast system two or more repeaters operate on the same downlink and uplink frequency pair to make up a simulcast system.
- A process known as downlink voting, which takes place in the subscriber unit, allows the radio to automatically select the best quality channel from a group of channels transmitting the same audio. No system voting or subscriber voting is necessary.
- In turn, the mobile subscriber radio sends traffic over a common uplink frequency to any repeater within range. Each repeater sends its uplink traffic to the central comparator to select the strongest signal. With the aid of GPS and timing synchronization, the comparator sends the traffic signal and timing information to each transmitter for downlink at a specific time to ensure good performance in the overlap areas.
See the picture below.
How to Tell if a Radio is in a Trunked System
Clues a radio may be in part of a trunked system include:
- the lack of a squelch knob or adjustment
- no monitor button or switch
- a chirp indicating the channel is available when the push-to-talk is pressed