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Configure the Downstream Cable Interface
VC-140
Voice, Video, and Home Applications Configuration Guide
Troubleshooting Tips
If you are having trouble:
• Make sure the cable connections are not loose or disconnected.
• Make sure the cable modem card is firmly seated in its Cisco uBR7246 chassis slot.
• Make sure that you have entered the correct slot and port numbers when you typed the command.
• Make sure you have calculated and entered the center frequency for your headend accurately.
Set the Downstream Symbol Rate
You need to set the megasymbols per second (Msps) rate for a downstream port on a cable modem
card. A symbol is the basic unit of modulation. QPSK encodes 2 bits per symbol, 16-QAM encodes
4 bits per symbol, 64-QAM encodes 6 bits per symbol, and 256-QAM encodes 8 bits per symbol.
The valid range for the downstream symbol rate is 0 to 6,000,000 Msps.
Caution The default downstream symbol rate is set to comply with MCNS specifications for Annex B
cable modem support at 5.056941 Msps with 64-QAM modulation and 5.36037 Msps with 256-QAM
modulation. This command should only be used to change the symbol rate to support Annex A cable modems
that are used outside North America.
To set the downstream symbol rate, use the following command in cable interface configuration
mode:
Validation Tips
To verify the downstream symbol rate, enter the show controllers cable command for the
downstream port that you have just configured:
router# show controllers cable 6/0 downstream
Cable6/0 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
Troubleshooting Tips
If you are having trouble:
• Make sure the cable connections are not loose or disconnected.
• Make sure the cable modem card is firmly seated in its Cisco uBR7246 chassis slot.
• Make sure that you have entered the correct slot and port numbers when you typed the command.
• Make sure you have selected the default if you are using Annex B cable modems.
• Make sure that you have entered the correct symbol rate for Annex A specifications if you are
using Annex A cable modems. Do not enter the decimal point when entering the symbol rate.
• Make sure the downstream carrier is active by entering the cable downstream if-output
command.
Command Purpose
cable downstream symbol-rate
number Set the downstream symbol rate for Annex A
(5.056944
Msps). Do not enter the decimal point in the
symbol rate.