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
Advanced Spectrum Management Features
The Advanced spectrum management features for the Cisco uBR-MC16S cable interface line card are
available in 12.2(8)BC2 and later 12.2 BC releases as a software-only upgrade. These enhancements
provide the following additional features.
• Supports proactive channel management, to avoid the impacts of ingress and keep subscribers online
and connected.
• Offers flexible configuration choices, allowing users to determine the priority of the actions to be
taken when ingress noise on the upstream exceeds the allowable thresholds. The configurable
actions are frequency hopping, switching the modulation profile, and reducing the channel width.
• Performs carrier-noise ratio (CNR) calculations using Digital Signal Processor (DSP) algorithms in
real-time on a per-interface and a per-modem basis.
Note In Cisco IOS Release 12.2(8)BC2 and later 12.2 BC releases, the CNR value is typically
more accurate than the SNR value because the CNR value is an estimate calculated on a per
modem basis, while the SNR value is a general estimate for the entire upstream. For this
reason, the CNR and SNR values might not exactly match for any particular period.
• Intelligently determines when to modify the frequency, channel width, or modulation profile, based
on CNR calculations in the active channel and the number of correctable FEC errors and
uncorrectable FEC errors. Frequency hopping, channel width change, or profile change occurs in the
following circumstances:
– The CNR value falls below the user-defined threshold value for the primary modulation profile;
AND
– Either the correctable FEC error value exceeds its user-defined threshold,
OR
– The uncorrectable FEC error value exceeds its user-defined threshold.
This approach helps avoid unneeded channel changes due to transient noise problems that do not actually
cause any errors in the data stream. The channel changes only when noise both affects the CNR of the
upstream and generates an unacceptable number of FEC errors in the data. If you want channel changes
to occur only in response to the CNR, you can set the FEC error threshold values to zero.
Separate CNR threshold values are configured for the primary and secondary modulation profiles.
When the upstream has moved to the secondary modulation profile, further frequency hopping or
channel width changes occur only when the CNR value falls below the user-defined threshold value
for the secondary profile.
Note Previously, channel hopping occurred when the number of missed station maintenance polls
exceeded a user-defined threshold or the SNR reported from the Broadcom chip exceeded a
certain threshold.
• Enhances the Dynamic Upstream Modulation feature for the Cisco uBR-MC16S line card. This
feature supports dynamic modulation using two upstream profiles. The primary profile (typically
using QAM-16 “mix” modulation) remains in effect at low noise conditions, but if upstream
conditions worsen, the cable modems switch to the secondary profile (typically using QPSK
modulation) to avoid going offline. When the noise conditions improve, the modems are moved back
to the primary profile.