Troubleshooting guide

Troubleshooting the CMTS
4-55
8. View the signal quality statistics for the upstream interface. If there is a high
number of unerroreds (uncorrupted packets), the upstream signal-quality is
good.
Table 4-4 describes the signal quality statistics:
9. If there is a large number of correctables there is a physical problem with the
upstream signal. Use a spectrum analyzer to measure the signal to noise ratio
for the upstream path. For 16 QAM and QPSK, the optimum signal-to-noise
ratio is 33 dB or greater.
The signal-to noise ratio learned from the spectrum analyzer may indicate the
following conditions:
• If there is under a 10 dB loss in signal quality, the cable interface has the
ability to compensate by correcting packets.
• If there is more than a 10 dB loss in signal quality, the signal is degraded
to the point where it can no longer successfully carry data and the noise
problem has to be manually troubleshooted and corrected.
• A high number of correctables (corrected packets) and uncorrectables
(uncorrected, dropped packets) occurs when the signal-to-noise ratio is
around 25 dB for QAM 16 and QPSK. The problem has to be manually
troubleshot and corrected. See if errors are incrementing from fixed points
in time in a higher than normal way.
• The number of micro reflections expressed in dB, can correlate to a high
number of corrupted packets (erroreds) and fixed packets (correcteds), or
can be attributed to burst errors.
If there needs to be manual intervention to correct the upstream signal-to-noise
ratio, follow these steps to resolve a bad upstream signal-to-noise ratio:
Table 4-4 Signal Quality Statistics
Field Identification
correctables Number of corrected error packets received through this upstream
interface
ifIndex Cable interface number
unerroreds Number of unerrored packets on cable interface