Troubleshooting guide

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
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Chapter 5 Troubleshooting DOCSIS Networks
Troubleshooting Slow Peformance
Measuring Upstream RF Signals
To measure the upstream RF signal, perform the following steps (this procedure is known as the “Zero
Span Method”, which is described in greater detail in “Connecting and Configuring the Cable Headend”
available on Cisco Connection Online):
Step 1 Connect the spectrum analyzer to the upstream signal from your cable network at the combiner where
all the cable modems connect.
Step 2 Set the analyzer to view the upstream with a center frequency to match the configuration on the CMTS.
Step 3 Set the span to 0 MHz.
Step 4 Set the bandwidth and video channel bandwidth to 3 MHz. Do extended pings.
Step 5 Set the sweep value to 80 micro sec. Push the sweep button, manual, 80, then usec.
Step 6 Activate the trigger line between the highest and lowest portions of the signal. Do this by pushing the
trig button, video button, and turn dial down appropriately.
Step 7 Adjust the amplitude so the upper portion of the RF signal is on the top graticule of the display grid and
reset the trigger line accordingly.
Troubleshooting Slow Peformance
There are a number of issues that can affect the performance and speed of cable modems in a DOCSIS
network. This section addresses the major causes of slow throughput from the perspective of a cable
service provider.
This section describes how to accurately determine what kinds of throughput levels an end user is
achieving and how to make sure that the performance being measured is that of the cable network, rather
than that of the broader Internet.
This section does not discuss troubleshooting a complete loss of connectivity over the cable network or
cable modems not coming online.
Hardware and Software Versions
The information in this document is based on the software and hardware versions below.
• Software Release 12.1(9)EC for the Cisco uBR7246VXR CMTS product
• The information in this document is relevant for all other currently available releases of DOCSIS 1.0
based Cisco IOS® software for Cisco brand CMTS equipment.
Determining Levels of Performance
The following sections describe how to accurately determine what level of performance is present.