Juniper G10 CMTS, Hardware Manual

Page: 160 / 210

Features for Troubleshooting
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JUNOSg 3.0 G10 CMTS Hardware Guide 144
You should consider the following general guidelines when interpreting the flap list statistics:
!
If the flap list statistics are the opposite of the values presented in Table 45, the
provisioning and the HFC plant conditions are considered satisfactory. Use these values
to establish an operational baseline.
!
If you sort the flap list by the total number of flaps, or by a specific flap, you can locate
problematic nodes in the HFC plant. For example, if the flap list is sorted by
MERavg and
the flap entries with the lowest MERavg values are all within the same cable interface,
your diagnostic procedures can focus on a particular area of the return path.
!
If you sort the flap list by MAC address ( by-mac ), you can reveal issues associated with
cable modems manufactured by the same vendor. You can determine this by inspecting
the unique vendor identifier contained in the first 24 bits of the MAC address.
!
MERavg provides a good metric for the overall quality of the return path because it is
affected by virtually every possible source of QPSK and QAM signal amplitude and phase
distortion (unlike other metrics are affected mostly by noise). You can use
MERavg to
gauge the margin of failure available within a particular upstream interface.
!
If the IM value is high and the SM value is low, the cable modem might be having
problems with the following:
!
Initial ranging due to CMTS configuration issues.
!
Initial ranging due to HFC plant issues in the forward path or the return path.
!
Registration due to provisioning issues.
!
Stability.
!
If the IM value is low and the SM value is high, then the cable modem is able to
successfully register, but there might be intermittent HFC plant issues in the forward
path or the return path that cause the cable modem to use periodic maintenance
opportunities unsuccessfully.
!
A high PAdj value indicates that a cable modem’s power adjustment is changing by a
significant amount, which suggests problems in the return path. Compare PAdj for cable
modems that reside before and after an amplifier in the return path to provide an
indication of amplifier issues. High power levels of an RF signal can lead to laser
clipping, which results in corrupted codewords as seen by the CMTS. Therefore, a high
PAdj value in conjunction with a high CERavg value might provide an indication of laser
clipping.
!
A high FAdj value can occur when you use a frequency stacking multiplexer (sometimes
called block upconversion ) in which multiple upstream spectrums are stacked in
frequency at the fiber node in the upstream, and there is significant frequency error
introduced in the upconversion and downconversion process. Cable modems that have
degraded frequency stability also cause frequency adjust flaps to occur.