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Cisco uBR7200 Series Universal Broadband Router Software Configuration Guide
OL-2239-03
Chapter3 Configuring Cable Modem Interface Features
Configuring the Downstream Cable Modem Interface
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 downstream
Cable5/0 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:
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
Step6 Verify that you have selected the default if you are not certain about the modulation rate needed.
Setting the Downstream MPEG Framing Format
The MPEG framing format must be compatible with DOCSIS specifications at
http://www.cablemodem.com/specifications.html and your local cable plant operations.
Tip AnnexB is the DOCSIS MPEG framing format standard for North America.
Note Annex B framing format is automatically set when configuring Cisco cable interface line cards. The
cable interface line card’s downstream ports and the connected CMs on the network must be set to the
same MPEG framing format and must support DOCSIS operations as appropriate.
The following command appears in the CiscouBR7200series router configuration file to designate
AnnexB operation. This command sets the downstream MPEG framing format.
Router(config-if)# cable downstream annex {B}