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
Time-scheduled and guided hopping techniques are independent concepts:
• The spectrum is controlled by a script, not a frequency table.
• The available spectrum is time-scheduled as an option.
• A guided hopping frequency is selected from the available spectrum at the current time.
You can configure and activate frequency hopping by using spectrum groups. You can create up to 32
cable spectrum groups, each containing multiple upstream ports. The configured channel width is used
for each upstream frequency.
After you have created one or more spectrum groups for your cable network, you can add characteristics
to them, providing you with more definitive control over frequency usage and frequency hopping.
You can configure hopping thresholds. For example, the frequency hop threshold percentage method
prevents a single failing cable modem from affecting service to other working cable modems. As long
as a high enough threshold is configured, the system does not hop endlessly due to one cable modem
failing to respond to 90 percent of its station maintenance (keepalive) messages.
You can also configure the minimum period between frequency hops, with a default setting of 300
seconds. If the destination channel is expected to be impaired, the minimum period between frequency
hops can be reduced to a small value such as 10 seconds. This allows the frequency hop to continue more
rapidly until a clear channel is found. If excessive frequency hop is an issue, the minimum period
between hops can be increased.
To configure different techniques of frequency hopping, see the “Configuring Spectrum Group
Characteristics” section on page 3-28.
Note Spectrum management is not supported for one-way (telco return) cable modems, because spectrum
management capabilities focus on the upstream path over an HFC network.
Guided Frequency Hopping
The implementation is known as guided frequency hopping. It is called “guided” because the frequency
chosen when hopping is guided by the configured frequencies specified in the spectrum group, which
can be either a set of discrete frequencies or a band. In guided frequency hopping, when the CMTS
decides to make a frequency hop, it has no “look-ahead” mechanism to tell what the quality of the
frequency or band about to be hopped to will be. The lack of “look-ahead” capability is because the
Cisco uBR-MC1xC cable interface line cards have no built-in spectrum analysis capability. In this case,
the search time is 0 seconds and the switching time is 20 ms.
You can specify some rules the system uses when hopping to another frequency when the frequency band
in use is not clean. You can assign explicit frequency subbands and associated input power levels in a
spectrum group. All cable modems then on the upstream port migrate to the next frequency with an
assigned input power level. The number of lost station management messages exceeding a configured
threshold can initiate an upstream channel frequency reassignment. For example, you can specify a
frequency hop based on lost station management messages that exceed a threshold. The default threshold
may be 10-20 percent depending on the Cisco IOS release. The frequency change occurs rapidly without
data loss and minimal latency.
Take care to reduce the spectrum allocation when it is used with small channel widths. Otherwise, there
will be a large number of upstream channel slots. For example, if the allocation is from 20.0-to-28.0
MHz and an upstream port has its channel width set to 0.2 MHz, there are 40 possible slots for that
channel width. Guided frequency hopping can require a long time to find the clean slot, because it tries
each available slot, one at a time, for several seconds during each try.