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Cisco uBR7200 Series Universal Broadband Router Software Configuration Guide
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Chapter4 Troubleshooting the System Troubleshooting Using Cable Flap Lists
Hit and Miss The HIT and MISS columns are keepalive polling statistics between the Cisco uBR7200 series
and the cable modem. The station maintenance process occurs for everymodem approximately
every25 seconds. When the CMTS receives a response from the modem, the event is counted as
aHit.If the CMTS does not receive a response from the cable modem, the event is counted as a
Miss.A cable modem will fail to respond either because of noise or if it is down. Modems which
only log Misses and zero Hits are assumed to be powered off.
Misses are not desirable since this is usually an indication of a return path problem; however,
having a small number of misses is normal. The flap count is incremented if there areM
consecutive misses whereMis configured in the cable flap miss-threshold parameter. The
parameter value ranges from 1 to 12 with a default of 6.
Ideally,the HIT count should be much greater than the Miss counts. If a modem has a HIT count
muchless than its MISS count, then registration is failing. Noisy links cause the MISS/HIT ratio
to deviate from a nominal 1% or less. HighMiss counts can indicate:
•Intermittent upstream possibly due to noise
•Laser clipping
•Common-path distortion
•Ingress or interference
•Too much or too little upstream attenuation
Cyclical
RedundancyCheck
(CRC)
This statistic tracks the CRC error counter per modem. CRC errors are usually an indication of
noiseon a plant. A low count can be always be expected but a high CRC number calls for some
the plant troubleshooting. The CRC counter indicates:
•Intermittent upstream
•Laser clipping
•Common-path distortion
•Impulsive noise or interference
Power
Adjustments
(P-Adj)
The station maintenance poll in the CMTS constantly adjusts the modem transmit power,
frequency, and timing. TheP-Adj column indicates the number of times the modem’s power
adjustment exceeded the threshold value.The power adjustment threshold may be set using the
<cableflap power threshold > parameter with a value range of 0 to 10 dB and a default value of
2dB. Tuning this threshold is recommended to decrease irrelevant entries in the flap list. Power
Adjustment values of 2 dB and below will continuously increment theP-Adj counter. The
modem transmitter step size is 1.5 dB, whereas the headend may command 0.25 dB step sizes.
Power adjustment flap strongly suggests upstream plant problems such as:
•Amplifier degradation
•Poor connections
•Thermal sensitivity
•Attenuation problem
Table4-20 Flap List Statistics Description