Trunked radio system

The control channel computer sends packets of data to enable one talkgroup to talk together, regardless of frequency.

The primary purpose of this type of system is efficiency; many people can carry many conversations over only a few distinct frequencies.

Users' radios send data packets to a computer, operating on a dedicated frequency — called a control channel — to request communication on a specific talkgroup.

This arrangement allows multiple groups of users to share a small set of actual radio frequencies without hearing each other's conversations.

Trunked systems primarily conserve limited radio frequencies and also provide other advanced features to users.

For example, on a radio scanner it is very common to be able to assign talkgroups into banks or lock them out, exactly like that of conventional frequencies.

Each talkgroup is assigned a unique digital ID on the system so that the controller can direct transmissions to the radios which are intended to receive them.

The system may also include talkgroups for federal agencies operating within the jurisdiction and in some cases commercial users which provide assistance to general public safety.

Each of these central offices has the theoretical capacity to handle ten thousand individual telephone numbers.

Erlang-B is a common formula that predicts the optimal number of trunk lines actually needed under normal conditions.

In the event of a widespread emergency such as a major earthquake, many more users than normal will attempt to access both the telephone and radio systems.

In both cases once the trunking capacity of the systems is fully used, all subsequent users will receive a busy signal.

Some systems with advanced features are referred to as an Enhanced Specialized Mobile Radio (ESMR).

[4] Specialized Mobile Radio is a term defined in US Federal Communications Commission (FCC) regulations.

The term is of US regulatory origin but may be used in other regions to describe similar commercial systems which offer a radio communications service to businesses.

SMRs were created when the Federal Communications Commission began to license business and commercial 800 MHz two-way radio systems in the late 1970s.

A major flood or earthquake is likely to generate a level of call traffic that loads the system to capacity.

Therefore, they are suitable for small deployments where users are expected to use the entire network available (such as a private system covering a campus or town).

[citation needed] Some trunked radio protocols provide additional reliability and security.

NXDN Common Air Interface (CAI) was accepted at the meeting of the ITU-R (International Telecommunication Union Radiocommunications Sector) held in November 2016 and it has been added to Report M.2014-3, published in February 2017.

It is an open, multi-vendor protocol widely adopted in mission-critical applications in Japan, USA and mainland Europe.