Cisco uBR10012 Software Configuration Manual page 48

Supported Software Features for the Cisco uBR10012 Router
Usage Guidelines If a cable modem registers with a QoS profile that matches one of the existing QoS profiles on the Cisco
CMTS, then the maximum downstream burst size, as defined for that profile, is used instead of the
default DOCSIS QoS profile of 1522.
For example, a DOCSIS 1.0 configuration that matches QoS profile 10 in the previous examples would
be as follows:
03 (Net Access Control)
04 (Class of Service Encodings Block)
S01 (Class ID)
S02 (Maximum DS rate)
S03 (Maximum US rate)
S06 (US burst)
S04 (US Channel Priority)
S07 (Privacy Enable)
The maximum downstream burst size (as well as the ToS overwrite values) are not explicitly defined in
the QoS configuration file because they are not defined in DOCSIS. However, because all other
parameters are a perfect match to profile 10 in this example, then any cable modem that registers with
these QoS parameters has a maximum downstream burst of 100000 bytes applied to it.
For further illustration, consider a scenario in which packets are set in lengths of 1000 bytes at 100
packets per second (pps). Therefore, the total rate is a multiplied total of 1000, 100, and 8, or 800kbps.
To change these settings, two or more traffic profiles are defined, with differing downstream QoS
settings as desired.
Table 1-6
QoS Profile Setting
Maximum Downstream Transmit Burst (bytes) max-burst 4000
Maximum Downstream Burst (bps)
Maximum Downstream Bandwidth
In this scenario, both QoS profiles are identical except for the max-ds-burst size, which is set to 5000 in
QoS profile 101 and 5000 in QoS profile 102.
Optimal Settings for DOCSIS 1.0 Downstream Powerburst
DOCSIS allows the setting different token bucket parameters for each service flow, including the token
bucket burst size. When burst sizes are closer to 0, QoS is enforced in a stricter manner, allowing a more
predictable sharing of network resources, and as a result easier network planning.
When burst sizes are larger, individual flows can transmit information faster (lower latency), although
the latency variance can be larger as well.
For individual flows, a larger burst size is likely to be better. As long as the system is not congested, a large
burst size reduces the chances of two flows transmitting at the same time, because each burst is likely to take
less time to transmit. However, as channel bandwidth consumption increases, it is probably that large burst
traffic would exceed the thresholds of buffer depths, and latency is longer than with well shaped traffic.
For additional information about the cable qos profile command and configuring QoS profiles, refer to
the following documents on Cisco.com:
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Cisco uBR10012 Universal Broadband Router Software Configuration Guide
1-28
Table 1-6 provides two examples of such QoS profiles for illustration:
Sample QoS Profiles with Differing ERBA (Maximum Downstream) Settings
Cisco CMTS IOS Cable Command Reference Guide
http://www.cisco.com/en/US/docs/ios/cable/command/reference/cbl_book.html
Configuring DOCSIS 1.1 on the Cisco CMTS
Chapter 1 Overview of Cisco uBR10012 Universal Broadband Router Software
= 1
= 1
= 90000
= 90000
= 1522
= 1
= 0
QoS Profile 101
max-ds-burst 20000
max-downstream 100
QoS Profile 102
max-burst 4000
max-ds-burst 5000
max-downstream 100
OL-1520-05