Troubleshooting guide

Troubleshooting the CMTS
4-65
• A high number of corrected packets and uncorrected, dropped packets
occurs when the signal-to-noise ratio is approximately 25 dB for 256
QAM, and 20 dB for 64 QAM. You must manually troubleshoot and
correct the problem. See if errors increment at a specific time at a higher
than expected rate.
• The micro reflections number (expressed in dB) represents the amount of
noise on the cable interface, and corresponds to a high number of
corrupted packets (erroreds) and fixed packets (correcteds).
Follow these steps to manually resolve a downstream signal-to-noise ratio
problem:
1. At the cable headend, make sure that the power levels of adjacent channels do
not interfere with the downstream channel.
2. Inspect optical amplifiers in the HFC network to make sure they are working
properly.
3. Make sure all HFC network equipment is working properly.
4. Inspect cables for damage.
5. Check for laser clipping on fiber-optic transmitters on the downstream path
from the cable headend to the CMs. If the downstream power level is too high
for the downstream path, laser clipping can occur. Laser clipping prevents the
transmission of light, which can cause bit errors in downstream transmissions.
If a laser is experiences excessive input power for even a fraction of a second,
clipping can occur. Refer to the Downstream Power Level Too Low or High
section for more troubleshooting information.
6. Secure all cable connections.
7. Review the HFC network topology to identify any flaws that may cause
additional ingress.
8. Determine if impulse and electrical ingress noise is entering the network from
electrical sources within a home (such as hair dryers, light switches, and
thermostats) or from high-voltage lines near network cabling.
Downstream Power Level Too Low or High
Follow these steps to troubleshoot downstream power-level problems: