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Cisco uBR10012 Universal Broadband Router Software Configuration Guide
OL-1520-05
Chapter 1 Overview of Cisco uBR10012 Universal Broadband Router Software
Supported Software Features for the Cisco uBR10012 Router
DOCSIS 1.1 Rate Limiting and Traffic Shaping
Cisco IOS Release 12.2XF software supports rate limiting per DOCSIS-1.0-99, which limits the data rate to
and from a CM; the MAC scheduler supports traffic-shaping capabilities for downstream and upstream traffic.
Rate limiting ensures that no single CM consumes all of the channel bandwidth and allows a CMTS
administrator to configure different maximum data rates for different subscribers. Subscribers requiring
higher peak rates and willing to pay for this can be configured with higher peak rate limits in their CM
DOCSIS configuration file over regular subscribers who pay less and get lower rate limits.
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.
When rate-limiting CMs are implemented on the network, the Cisco IOS Release 12.2XF software
typically drops packets to enforce the rate limit. Dropping packets from the requesting CM causes the
host sending the information to retransmit its information. Retransmitted information wastes bandwidth
on the network. If both hosts sending and requesting information are on the cable plant, the upstream
bandwidth is wasted as well.
The traffic shaping feature delays the scheduling of the upstream packet, which in turn causes the packet
to be buffered on the cable CPE 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 QoS levels.
The Cisco uBR10012 router supports the following traffic shaping feature:
•
Downstream rate shaping to include ToS
—Allows traffic shaping from the CMTS on a DOCSIS
downstream channel. The feature allows administrators to configure the ToS byte to calculate the
data rate for a specified flow. You can override the common maximum downstream data rate.
For additional information about downstream rate limiting (shaping), refer to the
“Setting
Downstream Rate Limiting and Traffic Shaping” section on page 3-9
.
Tip
Packets that contain ToS bytes that have not been configured for downstream data rates continue to
use the common data rate limits.
•
Upstream rate shaping
—Allows upstream rate shaping from the CMTS on a DOCSIS upstream
channel. Upstream grant shaping is per CM (SID). The grant shaping feature is a configurable option
for the current upstream token-bucket rate-limiting algorithm.
For configuration information, refer to the
“Setting Upstream Rate Limiting and Traffic Shaping”
section on page 3-24
.
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.
