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Cisco uBR10012 Universal Broadband Router Software Configuration Guide
OL-1520-05
Chapter3 Configuring Cable Interface Features for the CiscouBR10012Router
Configuring the Downstream Cable Interface

Verifying the Downstream Interleave Depth

To verify the downstream interleave depth setting, enter the show controllers cable command for the
downstream port you have just configured:
Router# show controllers cable5/0/0 downstream
Cable5/0/0 Downstream is up
Frequency=96000000, Channel Width 6 MHz, 64-QAM, Symbol Rate 5.056941 Msps
FEC ITU-T J.83 Annex B, R/S Interleave I=32, J=
Perform these steps if you are having difficulty with verification:
Step1 Ensure that the cable connections are not loose or disconnected.
Step2 Ensure that the cable interface line card is firmly seated in its chassis slot.
Step3 Ensure that the captive installation screws are tight.
Step4 Verify that you have entered the correct slot and port numbers.
Step5 Verify that the downstream carrier is active, using the cable downstream if-output command.
Setting the Downstream Modulation
To set the downstream modulation, define the speed in symbols per second at which data travels
downstream to the subscriber’s CM. Asymbol is the basic unit of modulation. Quadrature Phase Shift
Key (QPSK) encodes 2 bits per symbol, Quadrature Amplitude Modulation (QAM) -16 encodes 4 bits
per symbol, QAM-64 encodes 6 bits per symbol, and QAM-256 encodes 8 bits per symbol.
Note Setting a downstream modulation rate of QAM-256 requires approximately a 6dB higher
signal-to-noise ratio (SNR) than QAM-64 at the subscriber’s cableinterface. If your network is
marginal or unreliable at QAM-256, use the QAM-64 format instead. Also, consider the significance
of your data.
To set the downstream modulation, use the following command in cable interface configuration mode.
The standard DOCSIS modulation rate (and the Cisco default) is QAM-64.
Router(config-if)# cable downstream modulation 64qam

Verifying the Downstream Modulation

To verify the downstream modulation setting, enter the show controllers cable command for the
downstream port you have just configured. See the following example:
Router# show controllers cable5/0/0 downstream
Cable5/0/0 Downstream is up
Frequency=96000000, Channel Width 6 MHz, 64-QAM, Symbol Rate 5.056941 Msps
FEC ITU-T J.83 Annex B, R/S Interleave I=32, J=4
Perform these steps if you are having difficulty with verification: