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
Potential Reasons for Poor Performance
This section describes the common reasons for slow performance and suggested resolutions, including:
• Performance being restricted by the limits in the DOCSIS configuration file
• Bursty or inconstant download performance caused by using a sub-optimal rate limiting scheme on
the cable modem termination system (CMTS)
• Upstream and downstream channel congestion
• Backhaul network or Internet congestion
• Noise or errors on the cable plant
• Under powered end user customer premises equipment (CPE) equipment
Each of these individually or in combination can affect throughput and performance in a cable network.
Restrictions in DOCSIS Configuration File
The first piece of information that needs to be gathered when troubleshooting slow cable modem
performance is the prescribed class of service throughput limitations of the cable modem. When a cable
modem comes online, it downloads a DOCSIS configuration file that contains operational limits for the
cable modem, including the maximum upload and download rates. Under normal circumstances, the
cable modem is not allowed to exceed these rates.
Initially you need to identify the MAC address of a cable modem having problems. Let's say you have a
modem with MAC address 0050.7366.2223 that is having problems with slow throughput. You need to
find out what class of service profile this cable modem is using by executing the show cable modem
<mac-address> command as seen in the example below.
uBR7246-VXR# show cable modem 0050.7366.2223
Interface Prim Online Timing Rec QoS CPE IP address MAC address
Sid State Offset Power
Cable3/0/U1 1 online 1548 0.75 5 0 10.1.1.10 0050.7366.2223
This cable modem has a QoS profile of 5. In order to find out what downstream and upstream rates this
QoS profile corresponds to, you need to use the show cable qos profile <profile-number> command,
where <profile-number> is the QoS profile you're interested in.
uBR7246-VXR# show cable qos profile 5
ID Prio Max Guarantee Max Max TOS TOS Create B IP prec.
upstream upstream downstream tx mask value by priv rate
bandwidth bandwidth bandwidth burst enab enab
5 0 64000 0 256000 1600 0x0 0x0 cm no no
The QoS profile 5 corresponds to a service providing 256Kbps in the downstream and 64Kbps is the
upstream. Any CPE connected to cable modems using QoS profile 5 will not be able to exceed these
limits. The QoS profile settings are determined by the contents of the DOCSIS configuration files
downloaded by cable modems from the provisioning system's TFTP server, therefore QoS profile 5 in
your system may not be the same as QoS profile 5 in the example shown above.
If an end user's download and upload performance correlate with the limits shown in their QoS profile,
then they are getting the Class of Service and throughput levels that the cable modem has been
provisioned and configured for. The only way to increase the upload and download throughput is to
change the DOCSIS configuration file being downloaded by the cable modem to one that has higher
throughput limits. See the document entitled Building DOCSIS 1.0 Configuration Files Using Cisco
DOCSIS Configurator (registered customers only) for detailed instructions on how to create or modify
a DOCSIS configuration file.