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5-25
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 5 Troubleshooting DOCSIS Networks
Troubleshooting Slow Peformance
Ways of relieving upstream congestion include:
Reducing the number of cable modems per upstream - If there are too many cable modems
connected to a particular upstream, or if users on a particular upstream are heavy users of upstream
bandwidth, then the best solution is to move some users on the congested upstream port to an under
utilized upstream port, or to a completely new upstream port. This would typically be accomplished
by moving a fiber node from one upstream combining group to another, or splitting an upstream
combining group into two separate combining groups. For more information refer to What is the
Maximum Number of Users per CMTS.
Increasing the upstream channel width - This involves a rigorous and thorough analysis of your
upstream spectrum in order to find a wide enough band with adequate Signal to Noise Ratio
characteristics to support the increased channel width. The upstream channel width should not be
changed without careful planning because this change can potentially affect other services in your
cable network. The upstream channel width may be changed by using the cable interface command
cable upstream Z channel-width <new-channel-width> where Z is the upstream port number and
new-channel-width is one of 200000, 400000, 800000, 1600000 (the default) or 3200000.
uBR7246-VXR(config-if)# cable upstream 0 channel-width 3200000
See the Cisco Cable Modem Termination System Command Reference for more information about
the show interface cable X/Y upstream <Z> command.
Changing the upstream digital modulation scheme to 16-QAM - Once again, this requires a rigorous
and thorough analysis of the upstream spectrum in order to verify whether there is a frequency band
in the upstream available that can support 16-QAM modulation. If this analysis is not performed
properly then there is a risk that performance will be further decreased or a complete upstream
outage may occur. The upstream modulation scheme may be changed by creating an upstream
modulation profile that uses 16-QAM modulation and then applying that to an upstream port. An
example follows
uBR7246-VXR(config)# cable modulation-profile 2 mix ! Create an optimized
! 16-qam/qpsk modulation
! profile.
uBR7246-VXR(config)# interface cable 6/0
uBR7246-VXR(config-if)# cable upstream 0 modulation-profile 2
See the Cisco Cable Modem Termination System Command Reference for more information about
the cable modulation-profile and cable upstream Z modulation-profile commands. See also
Configuring Cable Modulation Profiles on Cisco's Cable Modem Termination Systems.
Reducing the allowed upstream throughput per cable modem - By reducing the Maximum Upstream
Transmit Rate in the appropriate DOCSIS configuration files, cable modem users will not be able to
transmit at as high a rate in the upstream direction and upstream 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).
Downstream Channel Congestion
The downstream channel has significantly more bandwidth to share than an individual upstream channel,
therefore the downstream is not usually as subject to congestion as the upstream. However more users
will typically be sharing a downstream channel than any single upstream channel so if the downstream
channel becomes congested then all users connected to the downstream segment will experience reduced
performance.
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