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Cisco uBR10012 Universal Broadband Router Software Configuration Guide
OL-1520-08
Chapter1 Overview of CiscouBR10012 Universal Broadband Router Software Supported Software Features for the CiscouBR10012 Router
•Maximum downstream rate in bps
•Maximum transmit burst length
•ToS overwrite byte
Using these service class profiles, you can define a guaranteed-rate service queue to store bandwidth requests
from CMs subscribing to a class with minimum upstream rate on the upstream channel, and a best-effort
service queue for CMs subscribing to a class with no minimum upstream rate on the upstream channel.
The CiscouBR10012router also supports multiple service classes per CM and dynamic service identifiers.
This allows the CiscouBR10012router to dynamically allocate and delete service flows.
The CMTS also supports QoS profile enforcement to override interference from cablemodems that might be
improperly rate limited. The CMTS system administrator can assign a default DOCSIS 1.0 service class that
overrides a pre-existing service class on the modem. The CMTS can do traffic shaping based on the QoS
profile the administrator enforces.

DOCSIS 1.0 Traffic Shaping and Rate Limiting Features

Traffic shaping reduces the chance that information is retransmitted to hosts on the HFC network and,
therefore, conserves bandwidth. Without traffic shaping, the Cisco IOS Release 12.2XF software drops
bandwidth requests from CMs found to be exceeding their configured peak upstream transmission rate.
Dropping bandwidth requests (and eventually upstream packets) from rate-exceeding cableinterface
causes TCP-related timeouts, which cause the host sending the information to retransmit its information.
The Cisco IOS Release 12.2XF supports the following traffic shaping features:
•Downstream rate limiting—Allows downstream grants to rate-exceeding CMs to be buffered
without incurring TCP-related timeouts and retransmits. Downstream rate shaping enables you to
partition downstream traffic for a CM into multiple classes of service and multiple data rates by
using the three precedence bits in the ToS byte in the IP header to specify a class of service
assignment for each packet. Those packets with the precedence bit set in the ToS field are given
higher priority.
Using the ToS byte, you can calculate the data rate for a specified flow, in addition to the data rate
configured on a per-CM basis. By specifying a maximum data rate for a particular ToS, you can
override the common maximum downstream data rate.
Note Packets that contain ToS bytes that have not been configured for downstream data rates continue to use
the common data rate limits.
•Upstream rate limiting—Allows upstream bandwidth requests from rate-exceeding CMs to be
buffered without incurring TCP-related timeouts and retransmits. This enables the CMTS to enforce
the peak upstream rate for each CM without degrading overall TCP performance for the subscriber
CPEs. Upstream grant shaping is per cableinterface (SID).
Token-bucket policing with shaping is the per-upstream default rate-limiting setting at the CMTS.
Shaping can be enabled or disabled for the token-bucket algorithm.
Upstream traffic shaping delays the scheduling of an upstream packet, which causes the packet to
be buffered on the cable CPE device instead of being dropped. This allows the TCP/IP stack to pace
the application traffic appropriately and approach throughput commensurate with the subscriber’s
defined QoS levels.
DOCSIS 1.0 Payload Header Suppression
Payload Header Suppression (PHS) conserves link-layer bandwidth by suppressing unnecessary packet
headers on both upstream and downstream traffic flows. For configuration information, refer to the
“Configuring Payload Header Suppression and Restoration” section on page27.