Unit installation

Installation and Operation Manual Appendix C Operating Environment
FCD-E1 E1 Environment C-3
E1 Line Statistics Using CRC-4 Error Detection
The FCD-E1 supports the CRC-4 function in accordance with ITU-T Rec. G.704,
which allows the evaluation of the quality of transmission over E1 links.
When the CRC-4 option is enabled, frames are arbitrarily grouped in groups of 16
(these groups are called CRC-4 multiframes, and do not bear any relationship to
the 16-frame multiframe structures used with the 256S super-frame explained
above). A CRC-4 multiframe always starts with 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 multiframe (up to frame 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 (CEPT) Line Signal
The basic E1 line signal is coded using the High-Density Bipolar 3 (HDB3) coding
rules. The HDB3 coding format is an improvement of the alternate mark inversion
(AMI) code.
In the AMI format, “ones” are alternately transmitted as positive and negative
pulses, whereas “zeros” are transmitted as a zero voltage level. The AMI format
cannot transmit long strings of “zeros”, because such strings do not carry timing
information.
The HDB3 coding rules restrict the maximum length of a “zero” string to 3 pulse
intervals. Longer strings are encoded at the transmit end to introduce non-zero
pulses. To allow the receiving end to detect these artificially-introduced pulses and
to enable their removal to restore the original data string, the encoding introduces
intentional bipolar violations in the data sequence. The receiving end detects these
violations and when they appear to be part of an encoded “zero” suppression
string - it removes them.
Bipolar violations which are not part of the HDB3 zero-suppression string are
assumed to be caused by line errors, and are counted separately, to obtain
information on the quality of the transmission link when the CRC-4 function is not
used.