IFR FM/AM-1200A

AREA (1 bit): Repeater systems in close proximity to others may
use the same channel frequencies. These systems use this area or
intermod bit as an "us-them" comparison for data acceptance.
This bit is usually "0" unless two systems are in close proximity
to one another.
GOTO (IN USE) CHANNEL (5 bits): These bits represent either the
repeater being used or the repeater to which a mobile unit is
being transferred for a transmission. These bits must be the
same number (1 thru 20) as that assigned to the repeater for data
acceptance in a mobile-to-repeater message. In a
repeater-to-mobile message, this is the number of the repeater to
be used. The mobile translates this number through a look-up
table into an FCC channel pair.
A "31" code in this location indicates a "turn-off"
code for the end of transmission.
HOME CHANNEL (5 bits): In a mobile-to-repeater data message,
this slot is the home repeater assigned to that mobile for the
current group selection and is used for system accounting. In a
repeater-to-mobile message, this contains the home repeater
number of the mobile making the call, no matter which repeater is
being used--the home repeater or another repeater.
ID CODE (8 BITS): In a mobile-to-repeater message, this slot
contains the ID code (1-250) being used by the mobile unit for
calling the receiver. In a repeater-to-mobile message, the same
ID is reflected so a mobile can decide if it needs to unsquelch
its receiver to listen to the voice transmission.
FREE CHANNEL (5 bits): In a repeater-to-mobile message, this
slot tells a mobile which repeater (1-20) to use when it
transmits. This can be the same number as the home repeater. If
all repeaters are busy, this field contains a zero (0),
indicating "In Use." In a mobile-to-repeater message, this
contains a 31, used to fill the slot since mobiles do not use
this information.
CHECK (7 bits): This field contains error check bits that are
derived from all the message after (but not including) the sync.
When the message is sent, a special parity operation is performed
on the data and inserted into these seven bits. A receiver
(mobile or repeater) performs its own computation to produce what
it thinks the check should be. If the two separately derived
check codes do not agree, the message is rejected by the receiver.
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