Radio Shack ADV0801 Owner's Manual page 80

ADV0801 Owner's Manual Draft
The best example of trunking in communications is the typical telephone private branch
exchange, or PBX, which is used in many businesses, hospitals, schools and other organizations
to provide desktop telephone service to the people who work at a specific building or site.
A traditional PBX might consist of the individual telephones on the desktops in the building, the
PBX switching equipment, and various connections to the public telephone switched network, or
PSTN.
Consider a large office facility for a company with 500 employees, each one with a telephone on
their desk. Using their telephone and the PBX, it is possible for any employee to call another
employee's extension in the office building, and, it is possible for any employee to make an
outside call to any other telephone anywhere in the world.
It would certainly be cost prohibitive for each of these employees to have an individual outside
telephone line just so they could get dial tone and make calls to other telephones beyond the
company's PBX. This is where trunking technology is applied.
Engineers study the needs of the company and its employees to determine the number of
connections required to support the company's requirements. In this example, a total of 50
outside lines may be provided to support the 500 employees who work at the site, again, based
on the principle that not all users will need to communicate at the same time. These outside
lines are shared, and assigned to users automatically by the PBX on an as-needed basis. If the
engineers have done their jobs right, the number of outside lines that are provided will be
adequate for almost all circumstances. Rarely, if ever, should a user in the building not have
access to an outside line when needed, and the number of lines available is not overkill, such
that money is wasted on unneeded excess capacity.
In radio, trunking works in a very similar way. Instead of each radio user or user group having
their own dedicated radio channel, a small group of radio channels is shared amongst a 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
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