Specifications

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Cisco Cable Modem Termination System Feature Guide
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Chapter 3 Spectrum Management for the Cisco Cable Modem Termination System
Feature Overview
Downstream Traffic Shaping
The CMTS supports basic downstream traffic shaping by effecting data rate limiting on a per-modem
basis. A downstream traffic shaping feature called downstream rate limiting with type-of-service (ToS)
bits extends that capability by allowing the CMTS administrator to configure the ToS byte to calculate
the data rate for a specified flow.
Downstream rate limiting with ToS bits 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.
The administrator can override the maximum common downstream data rate limits by configuring the
ToS byte.
Note Packets that contain ToS bytes that have not been configured for downstream data rates continue to use
the common data rate limits.
Frequency Hopping Capabilities
Noise in the upstream transmission line, that is from the consumer to the service provider, can degrade
data transmission from the subscriber’s home. If the noise impairment is of substantial duration, it may
cause the cable modem to temporarily lose communication with the headend facility. As a contingency
plan, the multiple service operators (MSOs) can reserve multiple channels or upstream frequencies for
their subscribers. If one channel suffers too much interference, the CMTS requests that the cable
modems “hop” to another channel.
To provide frequency hopping capability, Cisco routers contain a spectrum manager that continuously
monitors the noise in unused upstream channels. If the CNR reaches an unacceptable level on a particular
channel, the spectrum manager automatically assigns a new upstream channel to the cable modem using
that channel.
Cisco routers using Cisco uBR-MC11C, Cisco uBR-MC12C, Cisco uBR-MC14C, and
Cisco uBR-MC16C cable interface line cards support the following techniques for upstream frequency
hopping when the frequency band in use is not clean:
• Guided frequency hopping—In guided frequency hopping, the spectrum manager automatically
assigns a new upstream channel frequency when a configurable threshold of station maintenance
(keepalive) messages fails. Failed station maintenance messages represent an impairment of the
upstream channel due to noise, plant, or equipment failure. Explicit frequency subbands and
associated input power levels are assigned in a spectrum group in guided frequency hopping.
Note Guided frequency hopping used to be called blind frequency hopping in previous
documents.
• Time scheduled frequency hopping—Frequency reassignment is based on the time of every day or
a specific day of the week.
• Combined guided and time-scheduled frequency hopping.
Note Frequency hopping is not effective against broadband noise phenomena such as impulse noise.