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large number of radio users or user groups. In a typical trunked
radio system, 20 radio channels can support the radio
communications needs of a few thousand users.
Instead of dedicating individual radio channels to specific user
groups, trunked radio systems utilize special, temporary "virtual"
channels called talkgroups. Each talkgroup has a digital address
that identifies it on a trunked radio system. In most trunked radio
technologies, each portable or mobile radio has an individual
digital address, much like the individual telephone extensions in
a PBX system.
There are two basic types of trunked radio technologies in use
today, "centralized control" and "distributed control".
Systems with centralized control utilize one or more specialized
computers called controllers to manage the operation of the
trunked radio system. The controller is responsible for assigning
radio channel resources to users and user groups as needed,
that is, when a user needs to speak with another user or with a
group of users. Centralized control trunked radio systems
reserve one of the system channels for use as the control
channel, and the remaining channels are used as voice or
"traffic" channels. When a user needs to place a call on a system
utilizing centralized control, they press the push-to-talk button on
their radio, which causes their radio to send a short data
message to the controller over the control channel, requesting
that a voice channel, if available, be assigned to that user or user
group for a voice call. The controller assigns an available voice
channel to the call, and sends a data message out over the
control channel signaling that user and all other members of the
userʼs talkgroup to switch to a voice channel and proceed with
the voice communication.
Motorola, EDACS and P25 trunked radio systems all utilize
centralized control. When monitoring a centralized control
system, your scanner utilizes the data sent by the controller on
the control channel to track the voice traffic for the talkgroups
that you wish to monitor.
Systems with distributed control do not utilize a central
controller to manage the assignment of radio channel resources.
Each channel includes its own individual controller, and these
controllers are all connected to each other over a data bus, such
that every channel and controller on the system knows the status
of the other channels and controllers at any given moment. In
distributed control, there is no single dedicated control channel.
PSR-500 User Manual
Page
127
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