When the user wishes to make a call, the unit transmits the request in the form of a data signal to the system computer. the computer finds the caller's desired correspondent and, by means of the control channel, checks for willingness to receive a call. When both called and calling parties are ready to communicate, the computer allocates the first available "traffic" channel.

When the call is terminated by either party, the radio unit sends a data signal releasing the channel.

Radio stations may be interconnected to increase the service area to provide any size of network, up to a national or international level.

Calls may also be made into other fixed line networks such as telephone systems.
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WHO DEVELOPED PROTOCOL 1327
In the early 1980s, leading telecommunications manufacturers pioneered work on Protocol 1327 under the auspices of the UK Department of Trade and Industry.

MPT1327 was developed over a two-year period and was the first of the Protocol 1327 family of standards. This is the signaling standard and is incorporated into UK trunked radio networks as the basic operating system.

To arrive at the optimum working implementation of the MPT1327 standard, network operators were consulted and the following additional standards were developed:

MPT1343 - defining the behavior of radio units on public networks

MPT1347 - the fixed network specification to complement MPT1343

MPT1352- the test schedule which radio units pass before being accepted on to radio networks.

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