Troubleshooting guide

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 5 Troubleshooting DOCSIS Networks
Troubleshooting Slow Peformance
Using a Sub-optimal Method for Rate Limiting
When an end user is trying to download data from the Internet at a rate greater than their cable modem's
DOCSIS configuration file allows, the CMTS must rate limit the traffic being sent to that user to ensure
that the user does not consume more than their allowed share of bandwidth.
Similarly, when an end user tries to upload or send data to the Internet at a rate greater than what the
DOCSIS configuration file allows, the cable modem itself should stop the excess traffic from traveling
over the cable segment to the CMTS. If the cable modem, for some reason, fails to perform upstream
rate limiting properly then the CMTS will explicitly forbid the cable modem from transmitting at higher
than the allowed rate. This behavior on the CMTS is to ensure that even a cable modem with "hacked"
characteristics is unable to subvert the Service Provider assigned upload rate limits.
The default rate limiting scheme used by the CMTS monitors the rate of traffic to or from each cable
modem over every one second period. If the cable modem sends or receives more than its per second
quota in less than a second, then the CMTS will not allow any more traffic to flow to that cable modem
for the rest of the second.
As an example, let's say you have a cable modem with a QoS profile allowing a download rate of
512Kbps. If the cable modem downloads 512 kilobits (64 kilobytes) within the first half of a second,
then for the next half of the second, the cable modem will not be allowed to download anything. This
type of rate limiting behavior may have the effect of a bursty download pattern that seems to stop and
start every second or two.
The best downstream rate limiting scheme to use is the token bucket rate limiting algorithm with traffic
shaping. This rate limiting scheme has been optimized to allow for a smooth web browsing experience
at a steady rate, while at the same time ensuring that end users are not allowed to exceed the prescribed
download rate as specified in the DOCSIS configuration file.
The way this scheme works is to measure the rate at which a cable modem is downloading or uploading
data each time a packet is sent to or from the cable modem. If sending or receiving the packet in question
would cause the modem to exceed its allowed transfer rates then the packet is buffered or cached in
CMTS memory until the CMTS can send the packet without exceeding the downstream bandwidth limit.
It should be noted however that if the downstream traffic rate consistently exceeds the allowed
downstream rate for the cable modem, then packets will eventually be dropped.
By using this smoother method of rate limiting and shaping, most TCP based Internet applications such
as HTTP web browsing and FTP file transfers will operate much more smoothly and efficiently than
when using the default rate limiting scheme.
The token-bucket rate limiting with traffic shaping scheme can be enabled on the downstream path on a
cable interface by issuing the following cable interface configuration command:
uBR7246-VXR(config-if)# cable downstream rate-limit token-bucket shaping
Note It is highly recommended that you enable token-bucket shaping on your CMTS. This command is
supported as of IOS re;ease 12.0(5)T1 and 12.1(1)EC1.
The token-bucket with traffic shaping scheme can also be applied to upstream ports, however, since it is
the responsibility of the cable modems to perform upstream rate limiting, the upstream rate limiting
scheme applied to the CMTS will normally not have any impact on the performance of a network.
uBR7246-VXR(config-if)# cable upstream 0 rate-limit token-bucket shaping
See the Cisco Cable Modem Termination System Command Reference for more information about the
cable downstream rate-limit and cable upstream Z rate-limit commands.