Cisco OL-4015-08 Guide de dépannage télécharger pdf (Page 162)

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide

OL-5169-01

Chapter 5 Troubleshooting DOCSIS Networks

Troubleshooting RF Problems

If a digital signal employing forward error correction (FEC) is near its impairment limit, it is very

susceptible to changes in signal level---on the order of 0.1 dB. If there is no amplitude margin

available in the transmission path between the headend and any one Residential Gateway, the typical

signal level variations of a properly functioning cable network (3 to 6 dB) can create intermittent

service outages that are difficult to isolate.

Typical CATV measurement equipment, such as digital signal level meters, measure to an accuracy

of +/-1 dB. However, some older analog meters only measure to an accuracy of +/-3 dB; therefore,

maintaining 6 dB margins above the minimum levels can provide reliable long-term service.

Troubleshooting RF Problems

Most customer RF problems are caused by wrong configurations in the HFC network, as opposed to a

specific modem problem.

The MTAs should be easily installed, providing the following configuration guidelines and monitoring

practices are followed:

• Signal levels are configured within the optimal ranges

• RF distortions are minimized or eliminated

• Amplifiers are not being overdriven

• Good HFC monitoring practices are in place

• Regular flap list monitoring is performed

Troubleshooting Tools

There are several tools, including features of the Cisco uBR7200 series that can help you in

troubleshooting RF problems:

• The Cisco uBR7200 series maintains a database of flapping Residential Gateways, known as the

“flap list” (refer to “Monitoring the Cisco uBR7246VXR Flap List” in Chapter 7).

• Various show commands, such as show cable modem (refer to “Troubleshooting the Cable Modem

State” in Chapter 7, for information on understanding the online state options of the MTAs, and

using show and debug commands to troubleshoot problems)

Caution Be aware that running debug commands on a uBR (Universal Broadband Router) with more than a

handful of modems may cause the uBR to halt the system in order to keep up with the debugging. In this

case, all the modems may lose sync and debugging will be useless.

• A spectrum analyzer or a digital signal level meter can be used to measure the downstream IF and

RF signals, so that you can compare measurements to recommended settings (refer to “Measuring

RF Signals” later in this chapter, which also includes tables listing the recommended frequency

settings).

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Cisco OL-4015-08 Guide de dépannage Page 162