Configuring Headend Broadband Access Router Features
Traffic Shaping Features
MC-530
Cisco IOS Multiservice Applications Configuration Guide
Traffic Shaping Features
Traffic shaping is a Cisco patent-pending feature that conserves bandwidth by reducing the chances that
information will be retransmitted to hosts on the HFC network. Traffic shaping in the upstream direction
delays the scheduling of the upstream packet, causing the packet to be buffered on the cable CPE device
instead of being dropped. Traffic shaping allows the TCP/IP stack to pace the application traffic
appropriately and approach throughput commensurate with the QoS levels defined for the subscriber.
Without traffic shaping, the Cisco uBR7200 series software drops bandwidth requests from cable
modems that are found to have exceeded their configured peak upstream transmission rate. Dropping
bandwidth requests (and eventually upstream packets) from a rate-exceeding cable modem causes
TCP-related timeouts which cause the host sending the information to resend its information. Resent
information wastes bandwidth on the network.
The Cisco uBR7200 series supports the following traffic shaping features:
• Downstream rate shaping—Allows downstream grants to rate-exceeding cable modems to be
buffered without incurring TCP-related timeouts and retransmits.
Downstream rate shaping enables you to partition downstream traffic for a cable modem 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 also calculate the data rate for a specified flow in addition to the data
rate configured on a per-cable modem 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 shaping—Allows upstream bandwidth requests from rate-exceeding cable modems to
be buffered without incurring TCP-related timeouts and resends. This enables the CMTS to enforce
the peak upstream rate for each cable modem without degrading overall TCP performance for the
subscriber CPE devices. Upstream grant shaping is per cable modem (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.
Operations and Provisioning Features
The following sections describe operational and provisioning features and enhancements of the
Cisco uBR7200 series cable access routers.
Dynamic Ranging
The dynamic ranging feature is a Cisco patent-pending feature that supports quick restoration of service
following a catastrophic plant failure. With dynamic ranging, hundreds of cable modems can come back
online quickly. The time that the cable modems spend deferring contention ranging slots is minimized,
significantly reducing cable modem reinitialization time.
