Troubleshooting guide

BSR Troubleshooting Guide
4-64
• Bad Downstream Signal-to-Noise Ratio Detected
• Downstream Power Level Too Low or High
Bad Downstream Signal-to-Noise Ratio Detected
Follow these steps to identify problems associated with a bad downstream
signal-to-noise ratio:
1. To determine the type of Quadrature Amplitude Modulation (QAM) that is
used on the downstream interface, use the show cable downstream command
in Interface Configuration mode, as shown in the following example:
RDN(config-if)#show cable downstream <n>
where:
n is the number of the downstream cable port.
2. View the qamMode field in the show cable downstream command output.
The qamMode field displays whether the downstream modulation is 64 QAM
or 256 QAM.
3. Isolate the CM on the network that is experiencing ingress problems, such as
degraded performance or connection difficulties.
4. To get the CM MAC or IP address, use the show cable modem command in
Privileged EXEC mode, as shown in the following example:
RDN(config-if)#show cable modem
5. Use a spectrum analyzer to determine the downstream signal quality. For 256
QAM, the optimum signal-to-noise ratio is 33 dB or greater. For 64 QAM, the
optimum signal-to-noise ratio is 25 dB or greater.
The signal-to noise ratio learned from the spectrum analyzer may indicate the
following conditions:
• If the loss in signal quality is less than 10 dB, the cable interface can
compensate by correcting packets.
• If the loss in signal quality is greater than 10 dB, the signal is degraded to
the point where it can no longer carry data, and you must troubleshoot and
correct the noise problem.