Juniper Networks G10 CMTS manual To establish an operational baseline

Features for Troubleshooting

•	You should consider the following general guidelines when interpreting the flap list statistics:
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•	! 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.
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•	! 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.
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•	! 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.
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•	! 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.
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•	! If the IM value is high and the SM value is low, the cable modem might be having
•	problems with the following:
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•	!	Initial ranging due to CMTS configuration issues.
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•	!	Initial ranging due to HFC plant issues in the forward path or the return path.
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•	!	Registration due to provisioning issues.
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•	!	Stability.
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•	! 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.
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•	! 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.
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•	! 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.
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144 JUNOSg 3.0 G10 CMTS Hardware Guide