
DS3112
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14.7
C-Bit Parity Mode
Unlike the M23 application that uses the C bits for stuffing, the C-Bit Parity mode assumes that a stuff bit
should be placed at every opportunity and, hence, the C bits can be used for other purposes.
lists how the C bits are redefined in the C-Bit Parity mode.
Table 14-4. C-Bit Assignment for C-Bit Parity Mode
M
SUBFRAME
NUMBER
C-BIT
NUMBER
FUNCTION DESCRIPTION
1 1
2
3
Application ID
Reserved
Far End Alarm
and Control
(FEAC)
This bit (which is fixed to a value of 1) identifies the T3
data stream as operating in C-Bit Parity mode.
Must be set to one (1).
A serial communications channel that contains a repeating
16-bit codeword that indicates the state of the far-end and
can control the near-end by invoking loopbacks both on
the T3 and T1 lines. If no codewords are being sent, the
channel contains all ones.
2 1
2
3
Unused
Unused
Unused
All unused bits are set to a one (1).
3 1
2
3
C-Bit Parity (CP)
C-Bit Parity (CP)
C-Bit Parity (CP)
All three CP bits are set to the same value as the two P
bits. If the three CP bits are not equal, a majority vote is
used to decode the true value.
4 1
2
3
FEBE
FEBE
FEBE
All three Far End Block Error (FEBE) bits shall be set to
one (111) if the local T3 framer did not incur an error in
either the M bits or F bits nor has it detected a CP parity
error. The FEBE bits are set to any value except 111 when
an error is detected in the M bits or F bits or if a CP parity
error is detected. During an LOF event, these bits are set
to 000.
5 1
2
3
Data Link
Data Link
Data Link
These three C bits make up a 28.2kbps HDLC (LAPD)
maintenance data link over which three 76 octet messages
are sent from the local end to the remote end once a
second.
6 1
2
3
Unused
Unused
Unused
Must be set to 1.
Must be set to 1.
Must be set to 1.
7 1
2
3
Unused
Unused
Unused
Must be set to 1.
Must be set to 1.
Must be set to 1.