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
The third component to note is the output rate which shows the average downstream throughput rate in
bits per second. If this number consistently exceeds approximately 75% of the available downstream
bandwidth during the peak usage time then end users will start to experience slower Internet access and
higher latency.
By default, these statistics are calculated over a five minute moving average. (See Definition of 'bits/sec'
from 'sh int' for details of how the average is calculated.) The period over which this average is calculated
can be reduced to as little as 30 seconds by issuing the cable interface command load-interval 30. By
lowering this period to 30 seconds, a more accurate an up to date value will be calculated for each of the
parameters discussed in this section.
Downstream channel utilization will change during the day as different users have an opportunity to use
their cable modem so it is important to monitor the downstream utilization during the busiest times of
the day rather than at low usage times.
Ways of relieving downstream congestion include
• Reducing the number of cable modems per downstream - If there are too many cable modems
connected to a particular downstream, or if users on a particular downstream are heavy users of
downstream bandwidth, then the best solution is to move some users on the congested downstream
channel to another downstream channel. This would typically be accomplished by splitting a group
of downstream fiber nodes associated with the downstream into two separate groups and assigning
each of the new groups separate downstream channels. See also What is the Maximum Number of
Users per CMTS.
• Changing the downstream digital modulation scheme to 256-QAM - This action requires a rigorous
and thorough analysis of the downstream spectrum in order to verify whether your network can
support a 256-QAM signal. If this analysis is not performed properly then there is a risk that
performance will be further decreased or a complete downstream outage may occur. The
downstream modulation scheme may be changed by issuing the cable interface command as seen
below.
uBR7246-VXR(config-if)# cable downstream modulation 256qam
See the Cisco Cable Modem Termination System Command Reference for more information about
the cable downstream modulation command.
• Reducing the allowed downstream throughput per cable modem - By reducing the Maximum
Downstream Transmit Rate in the appropriate DOCSIS configuration files, cable modem users will
not be able to download at as high a rate in the downstream direction and downstream congestion
will be relieved. The negative aspect of this course of action is obviously that cable modem users
would be limited to a slower class of service. See Building DOCSIS 1.0 Configuration Files Using
Cisco DOCSIS Configurator (registered customers only).
Note The measures discussed in this section will not significantly increase the performance of an already
uncongested network.
Backhaul Network or Internet Congestion
In some cases, performance problems may not be a result of issues on the cable plant or the CMTS, but
may be related to congestion or problems in the backhaul network that the CMTS uses to connect to the
Internet, or within parts of the Internet itself.
The easiest way to determine if backhaul network congestion is a problem is to connect a workstation to
the same network segment as the CMTS and try to browse the same web sites as end users behind cable
modems are trying to reach.