user manual

Configuring Headend Broadband Access Router Features
Configuring the Downstream Cable Interface
MC-539
Cisco IOS Multiservice Applications Configuration Guide
Verifying the Downstream MPEG Framing Format
To verify the downstream MPEG framing format setting, enter the show controllers cable command for
the downstream port 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
To troubleshoot the configuration, perform the following tasks:
• Make sure the cable connections are not loose or disconnected.
• Make sure the Cisco cable modem card is firmly seated in its chassis slot and the captive installation
screws are tight.
• Make sure you entered the correct slot and port numbers when you typed the command.
• Make sure the downstream carrier is active by entering the cable downstream if-output command.
Setting the Downstream Modulation
By setting the downstream modulation, you define the speed in symbols per second at which data travels
downstream to the subscriber cable modem. 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 modulation rates for a downstream port on a Cisco cable modem card are 64 QAM (6 bits per
downstream symbol rate) and 256 QAM (8 bits per downstream symbol rate).
Note Setting a downstream modulation rate of 256-QAM requires approximately a 6 dB higher
signal-to-noise ratio (SNR) than 64-QAM at the subscriber cable modem. If your network
is marginal or unreliable at 256-QAM, use the 64-QAM format instead.
To set the downstream modulation, use the following command in cable interface configuration mode:
Verifying the Downstream Modulation
To verify the downstream modulation setting, enter the show controllers cable command for the
downstream port 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
Command Purpose
CMTS01(config-if)# cable downstream modulation 64qam
Sets the downstream modulation. The standard DOCSIS
modulation rate (and the Cisco default) is 64qam.