Cisco Systems uBR7200 Series guide

Chapter 4 Troubleshooting the System

Troubleshooting Using Cable Flap Lists

Table 4-20 Flap List Statistics Description

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 every modem approximately
	every 25 seconds. When the CMTS receives a response from the modem, the event is counted as
	a Hit. 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 are M
	consecutive misses where M is configured in the cable flap miss-thresholdparameter. 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
	much less than its MISS count, then registration is failing. Noisy links cause the MISS/HIT ratio
	to deviate from a nominal 1% or less. High Miss 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	This statistic tracks the CRC error counter per modem. CRC errors are usually an indication of
Redundancy Check	noise on a plant. A low count can be always be expected but a high CRC number calls for some
(CRC)	the plant troubleshooting. The CRC counter indicates:
	•	Intermittent upstream
	•	Laser clipping
	• Common-path distortion
	• Impulsive noise or interference

Power	The station maintenance poll in the CMTS constantly adjusts the modem transmit power,
Adjustments	frequency, and timing. The P-Adjcolumn indicates the number of times the modem’s power
(P-Adj)	adjustment exceeded the threshold value. The power adjustment threshold may be set using the
	<cable flap power threshold > parameter with a value range of 0 to 10 dB and a default value of
	2 dB. Tuning this threshold is recommended to decrease irrelevant entries in the flap list. Power
	Adjustment values of 2 dB and below will continuously increment the P-Adjcounter. 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

Cisco uBR7200 Series Universal Broadband Router Software Configuration Guide

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(CRC)

the plant troubleshooting. The CRC counter indicates: