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FRU-Specific Problems
Table 5-5
Line Card Problems (continued)
Symptom
Modems train at a low bit rate,
modems retrain continuously, or
the line experiences too
many errors.
Cisco 6260 Hardware Installation Guide
5-12
Steps to Take
Enter the command show dsl int atmslot#/port# to display the port configuration.
1.
Check the display to ensure that the port is properly provisioned. Look in particular
for these statistics:
–
Attenuation: typically this is 20 to 50 dB. If the attenuation value is higher than
50 dB, it might be depressing the bit rate. Repair or replace the cables and
connectors in the loop.
SNR margin: 3 to 6 dB is optimum. Use the dmt margin command to adjust
–
SNR margin.
Correction ratio: under DSL Statistics, look at the Received Superframes and
–
Corrected Superframes values. A ratio of more than 1 corrected superframe for
every 10 superframes received is too high. One or more of these adjustments
might correct the problem: increase the SNR margin using the dmt margin
command; increase error correction using the dmt check-bytes command; or
increase interleaving using the dmt interleaving-delay command.
Errored seconds: a rate of 10 to 20 errored seconds per minute or more is likely
–
to cause retraining. (1 or 2 errored seconds every 15 minutes is a good rate.)
See the steps that follow on crosstalk and impulse noise for suggestions on how
to compensate.
CRC errors: normal rates vary system by system. If the CRC error rate is higher
–
than usual, it might cause excessive retraining. See the steps that follow on
crosstalk and impulse noise for suggestions on how to compensate.
Crosstalk is caused by interference between services in adjacent cables. It affects
2.
random bits rather than chunks of data; upstream and downstream traffic can be
affected differently. If crosstalk is increasing the bit error rate (BER), you can
compensate in several ways:
Reduce the bit rate using the dmt bitrate command. (See the Command
–
Reference for Cisco DSLAMs with NI-2 for information on the dmt
bitrate command.)
Increase the SNR margin using the dmt margin command. (See the Command
–
Reference for Cisco DSLAMs with NI-2 for information on the dmt
margin command.)
–
Turn on or increase error correction. Use the dmt check-bytes command. If the
codeword size is not set to auto, you might need to use the dmt codeword-size
command to adjust the codeword setting. (See the Command Reference for
Cisco DSLAMs with NI-2 for information on these commands.)
If you experience impulse noise or clipping, both of which affect chunks of data
3.
rather than random bits, you can compensate by turning on or increasing interleaving
and error correction. (However, note that this approach adds delay.) Use the dmt
interleaving-delay command for interleaving. Use the dmt encoding-trellis
command or the dmt check-bytes command for error correction. If you use dmt
check-bytes and if the codeword size is not set to auto, you might need to use the dmt
codeword-size command to adjust the codeword setting.
Chapter 5
Troubleshooting
OL-2365-01
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