Specifications

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Cisco Cable Modem Termination System Feature Guide
0L-1467-02
Chapter 3 Spectrum Management for the Cisco Cable Modem Termination System
Restrictions and Limitations
Restrictions and Limitations
Dynamic Upstream Modulation
• The Cisco router has one preconfigured (primary) modulation profile that defines a typical profile
for QPSK modulation. To use the Dynamic Upstream Modulation feature, you must create a second
modulation profile that has a higher modulation scheme than the preconfigured profile.
• Upstream modulation profiles are assigned to upstream ports and affect all cable modems on those
upstream ports.
• Modulation profiles affect the physical layer of the cable network, so only trained technicians who
are familiar with the DOCSIS specifications should create modulation profiles.
• When using the Dynamic Upstream Modulation feature with Voice over IP (VoIP) services, frequent
changes to the upstream modulation or channel width could briefly impact the quality of voice calls
• The Dynamic Upstream Modulation feature operates differently on the Cisco uBR-MC16S cable
interface line card than on other supported cable interface line cards. See the description of this
feature in the “Guided and Scheduled Spectrum Management” section on page 3-7 and the
“Advanced Hardware-Based Spectrum Management” section on page 3-14 for more information.
Cisco uBR10012 Router and Cisco IOS Release 12.2(8)BC2 Support
The Cisco uBR10012 router using the Cisco uBR-LCP2-MC16S cable interface line card and
Cisco IOS Release 12.2(8)BC2 has the following restrictions and limitations:
• Frequency hopping is based on the carrier-to-noise ratio (CNR) and the correctable FEC error and
uncorrectable FEC error values. A channel will frequency hop if the CNR falls below the
configurable threshold AND either the correctable or uncorrectable FEC error values exceed the
configurable threshold values.
Use the cable upstream threshold command to change these values. In Cisco IOS Release
12.2(8)BC2, the CNR threshold for the primary modulation profile defaults to 25 dB. The CNR
threshold for the secondary modulation profile defaults to 15 dB. The correctable FEC error
threshold defaults to 1 percent of total packets received, and the uncorrectable FEC error threshold
defaults to 1 percent of total packets received.
Note In releases previous to Cisco IOS Release 12.2(8)BC2, the channel would hop if either the
CNR fell below the configurable threshold OR either the correctable or uncorrectable FEC
error values exceeded the configurable threshold values. If you are using only one
modulation profile (QPSK), you might need to reduce the CNR threshold value and increase
the correctable FEC error value to prevent undesired frequency hopping.
• Fixed-frequency spectrum groups are not supported.
• Shared spectrum groups are not supported.
• In Cisco IOS Release 12.2(8)BC2, the short and long grant bursts of the pre-configured mix and
qam-16 modulation profiles (see the cable modulation-profile command) default to a unique word
offset of 8 (uw8). These values should be changed to uw16 for optimal performance. This can be
done with the following commands:
cable modulation-profile n short 6 75 6 8 16qam scrambler 152 no-diff 144
fixed uw16
cable modulation-profile n long 8 220 0 8 16qam scrambler 152 no-diff 160
fixed uw16