Cisco Systems uBR7200 Traffic Shaping Features, Operations and Provisioning Features, MC-530

Configuring Headend Broadband Access Router Features

Traffic Shaping Features

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.

Cisco IOS Multiservice Applications Configuration Guide

MC-530