Cisco Systems uBR7200 Setting the Downstream Modulation, Verifying the Downstream Modulation

Configuring Headend Broadband Access Router Features

Configuring the Downstream Cable Interface

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-outputcommand.

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:

Command	Purpose
CMTS01(config-if)#cable downstream modulation 64qam	Sets the downstream modulation. The standard DOCSIS
	modulation rate (and the Cisco default) is 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:

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

Cisco IOS Multiservice Applications Configuration Guide

MC-539