5-12
Cisco 6260 Hardware Installation Guide
OL-2365-01
Chapter 5 Troubleshooting
FRU-Specific Problems
Modems train at a low bit rate,
modems retrain continuously, or
the line experiences too
many errors.
1.
Enter the command
show dsl int atm
slot#
/
port#
to display the port configuration.
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.
2.
Crosstalk is caused by interference between services in adjacent cables. It affects
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.)
3.
If you experience impulse noise or clipping, both of which affect chunks of data
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.
Table 5-5
Line Card Problems (continued)
Symptom
Steps to Take