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5-13
TRUNKING - OPTION 14 (FM/AM-12005 ONLY)
5-13-1 GENERAL
The Trunking Option allows operators to test mobile communications
radios and repeaters using the Johnson Trunked Radio operating system.
The software-controlled test functions utilize the FM/AM-1200S T/R
Connector (11), Duplex Output Connector (14) and ANT Connector (34) to
test the Encode/Decode, Repeater Access and Handoff functions of
Trunking Radio mobile units and the Handshake and Handoff functions of
repeaters.
5-13-2 Trunking Operational Theory
Trunking is the sharing of all available channels in a repeater system
by remote or mobile units with access to that repeater.
A trunking
system is composed of up to 20 repeaters. with each repeater assigned
up to 250 separate ID codes.
Trunking is based on the theory that an
individual mobile unit, with access to several communication channels
at once, has a low probability of being blocked from transmitting since
the other subscribers to the system will use it only a small percentage
of the time and that a large number of other subscribers will not use
it at exactly the same instant.
Hence, at least one channel will be
free at any given time.
Each mobile unit in a trunking system is assigned a "Home Repeater"
from which it receives digital control information.
A listening mobile
unit monitors data from its home repeater to determine on which channel
it is being called by another mobile unit or which channel it may use
if it wants to transmit.
Trunking systems are allocated 999 channel pairs in two separate bands
by the FCC.
In the lower band of 600 channel pairs, 806 MHz to 821 MHz
is used for mobile transmit and 851 MHz to 866 MHz is used for mobile
receive,
with a 45 MHz offset between the transmit and receive
frequency in each channel pair.
In the upper band of 399 channel
pairs, 896 MHz to 901 MHz is used for mobile transmit and 935 MHz to
940 MHz is used for mobile receive with a 39 MHz offset maintained
between transmit and receive in each channel pair.
Each repeater in a trunking system uses only one channel pair to
communicate with each of the mobile units assigned to it.
If a mobile
unit is instructed to use a different repeater, another channel pair is
used.
Home repeaters can only exchange data with mobile units that have ID
numbers assigned to that repeater.
Other mobile units are ignored by a
repeater unless the mobile unit has received permissions from a
separate repeater to access the first repeater.
This is known as a
"Handoff".
All repeaters in a trunking system are physically connected
by coax so all
repeaters in the community can be informed of accesses
and handoffs.
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