Specifications

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Cisco Cable Modem Termination System Feature Guide
0L-1467-02
Chapter 3 Spectrum Management for the Cisco Cable Modem Termination System
Feature Overview
Each time a packet belonging to a flow is transmitted on an output channel, the token-bucket policer
function checks the rate limit status of the flow, passing the following parameters:
• Token bucket peak rate in bits/msec.
• Token bucket depth (maximum transmit burst) in bits.
• Length of current packet to be sent in bits.
• Pointer to the flow’s token bucket.
• Pointer to the flow’s token bucket last update time stamp.
• Variable to return the msec buffering delay in case the packet needs to be shaped.
• Maximum buffering delay that the subsequent traffic shaper can handle in msecs.
Every flow has its own shaping buffer where rate-exceeded packets are typically held back in
first-in/first-out (FIFO) order for later transmission.
Tip Token bucket policing with shaping is the new per-upstream default rate limiting setting at the CMTS.
Shaping can be enabled or disabled for the token-bucket algorithm.
Upstream Traffic Shaping
Upstream traffic shaping allows the CMTS to perform rate limiting on a DOCSIS upstream channel. The
upstream traffic shaping feature delays the scheduling of the upstream packet, which in turn, causes the
packet to be buffered on the cable modem device, instead of being dropped. This allows the user TCP/IP
stack to pace the application traffic appropriately and approach throughput commensurate with the
subscriber’s defined quality of service (QoS) levels. Upstream traffic shaping enables the CMTS to
enforce the peak upstream rate for each CM without degrading overall TCP performance for the
subscriber CMs.
When you do not enable the shaping option for upstream rate limiting, the CMTS upstream-rate-policing
code drops bandwidth requests from cable modems that are found to have exceeded their
configured-peak-upstream rate (using different local drop policies). The effect of bandwidth requests
(eventually upstream packets) being dropped causes degraded throughput performance of window-based
protocols (like TCP) for these rate-exceeded modems because of the timeouts and retransmits that
follow.
Upstream grant shaping is on a per-CM (SID) basis. The grant shaping feature is a configurable option
for the current upstream token-bucket rate-limiting algorithm.
A traffic shaping feature is restricted QoS class assignment, which allows a CMTS administrator to
override the class of service provisioned for a CM. When this feature is enabled, the user-defined QoS
profile is enforced on the CM attempting to register with the CMTS, regardless of the CM’s provisioned
class of service. Use the cable qos profile command to configure a QoS profile.
Note The restricted QoS class assignment feature is added to address instances where a cable operator
implemented rate limiting incorrectly. The feature allows an administrator to override the statically
provisioned QoS parameters of the CM and force the CM to use a specific QoS profile defined at the CMTS.
For configuration task information on upstream traffic shaping, refer to the “Setting Upstream Rate
Limiting” section on page 3-26.