Specifications

Appendix C Operating Environment Installation and Operation Manual
C-4 E1 Environment FCD-E1LC Ver. 1.0
carrying the inband management traffic. The total data rate that can be
carried by a national bit is 4 kbps.
E1 Line Statistics Using CRC-4 Error Detection
FCD-E1LC supports the CRC-4 function in accordance with ITU-T Rec. G.704. The
CRC-4 function is used to detect errors in the received data, and therefore can be
used to evaluate data transmission quality over E1 links. This function can be
enabled or disabled by the user.
To enable error detection, additional information must be provided to the receiving
equipment. The additional information is transmitted to the receiving equipment by
using a multiframe structure called
CRC-4 multiframes.
A CRC-4 multiframe is an arbitrary group of 16 frames. This group is not related in
any way to the G732S 16-frame multiframe structures explained above.
A CRC-4 multiframe always starts with an even frame (a frame that carries the
frame alignment signal). The CRC-4 multiframe structure is identified by a six-bit
CRC-4 multiframe alignment signal, which is multiplexed into bit 1 of timeslot 0 of
each odd-numbered (1, 3, 5, etc.) frame of the CRC-4 multiframe (i.e., in frames
1 through 11 of the CRC-4 multiframe).
Each CRC-4 multiframe is divided into two submultiframes of 8 frames (2048 bits)
each. The detection of errors is achieved by calculating a four-bit checksum on
each 2048-bit block (submultiframe). The four-checksum bits calculated on a
given submultiframe are multiplexed, bit by bit, in bit 1 of timeslot 0 of each
even-numbered frame of the next submultiframe.
At the receiving end, the checksum is calculated again on each submultiframe and
then compared against the original checksum (sent by the transmitting end in the
next submultiframe). The results are reported by two bits multiplexed in bit 1 of
timeslot 0 in frames 13, 15 of the CRC-4 multiframe, respectively. Errors are
counted and used to prepare statistic data on transmission performance.
E1 Line Alarm Conditions
• Excessive bit error rate. The bit error rate is measured on the frame
alignment signal. The alarm threshold is an error rate higher than 10
-3
that
persists for 4 to 5 seconds. The alarm condition is canceled when the error
rate decreases below 10
-4
for 4 to 5 consecutive seconds.
• Loss of frame alignment (also called loss of synchronization). This condition
is declared when too many errors are detected in the frame alignment signal
(FAS), e.g., when 3 or 4 FAS errors are detected in the last 5 frames. Loss of
frame alignment is cleared after no FAS errors are detected in two
consecutive frames. The loss of frame alignment is reported by means of the
A bit (see
Figure A-1 ).
The A bit (bit 3 in timeslot 0 of the odd frames) serves as a remote alarm
indicator (RAI) notifying the other end equipment that the local equipment lost
frame alignment or does not receive an input signal.
Note