IFR FM/AM-1200A

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Mobile units in the trunking system work in a simplex or push-to-talk
manner. Some radios automatically handle push-to-talk and receive
functions in a manner that makes them appear to be duplex in nature,
yet these units are only half duplex. Mobile units can only hear
messages transmitted through a repeater from mobiles in the ID number
group assigned to that repeater. Likewise, mobile units can only
transmit to other mobiles in the ID number group assigned to the same
home
repeater. Through a logic lock-out in the units. only one mobile
at a time is allowed to transmit through a home repeater to other units
assigned to that repeater.
Mobile units can change their ID numbers to either listen or talk to
other mobiles. groups of mobiles or subgroups. These mobile units
change their ID numbers with either manual controls or by scanning.
The organization of mobile units is arbitrary and done differently by
different mobile transmitter-receiver companies.
Signaling between mobile units and repeaters occurs at a subaudible 100
Hz frequency and occurs continuously and concurrently with the audio
portion of the transmission. Updates are transmitted every 7-8 seconds
by repeaters to mobile units to keep them informed as to which channel
is available to them or when other mobile units are calling.
5-13-3 Accessing The Trunking System
Before a mobile unit can access the system, a data "handshake" with a
repeater must be accomplished. When the mobile unit's push-to-talk is
pressed, the mobile uses a repeater's data packet to determine if there
is a free repeater channel available. If there is, that repeater
becomes that mobile unit's "home" repeater. The mobile transmitter is
enabled and the mobile unit transmits a data message to the repeater
containing the ID code for broadcast. The mobile unit then listens for
a response from the repeater for the go-ahead to transmit. When this
handshake is received, transmission is enabled. Also, all listening
mobiles with that repeater's ID number detect their ID, select the
directed channel and unsquelch to receive voice transmission.
A.
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08
The Trunking Data Packet
The trunking data packet is a 40-bit word formatted as:
SYNC : AREA GOTO CIN USE) HOME : ID CODE : FREE : CHECK
SYNC (9 bits): A bit pattern used to initialize the receive data
circuitry. These bits are compared by the trunking hardware to a
known
bit pattern. If the pattern matches. the remaining bits of
the word are checked for acceptance. Patterns used by the
industry at the present time are:
101011000
101010000
001010000