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Cisco uBR7200 Series Universal Broadband Router Software Configuration Guide

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Chapter 4 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 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-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

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

RedundancyCheck

(CRC)

This statistic tracks the CRC error counter per modem. CRC errors are usually an indication of

noise on 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. The P-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

<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-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

Table 4-20 Flap List Statistics Description

1 2 ... 34 35 36 37 38 39 40 41 42 43 44 ... 57 58

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Cisco Systems uBR7200 Series Guía para resolver problemas Pagina 39

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