Installation guide

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Cisco uBR7100 Series Universal Broadband Router Software Configuration Guide
OL-2238-03
Chapter 3 Configuring the Cisco Cable Interface
Configuring the Downstream Cable Interface
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. A symbol is the basic unit of modulation. QPSK encodes 2 bits per
symbol, 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 6 dB higher
signal-to-noise ratio (SNR) than QAM-64 at the subscriber’s cable interface. If your network is marginal
or unreliable at QAM-256, use the QAM-64 format instead. Also, consider the significance of your data.
Use the following command in cable interface configuration mode to set the downstream modulation.
The standard DOCSIS modulation rate (and the Cisco default) is QAM-64.
CMTS01(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 cable1/0 downstream
Cable1/0 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:
1. Ensure the cable connections are not loose or disconnected.
2. Ensure the cable modem card is firmly seated in its chassis slot.
3. Ensure the captive installation screws are tight.
4. Verify that you have entered the correct slot and port numbers
5. Verify the downstream carrier is active using the cable downstream if-output command
6. Verify that you have selected the default if you are not certain about the modulation rate needed.
Setting the Downstream Interleave Depth
Set the interleave depth for the downstream port on the Cisco cable modem card. A higher interleave
depth provides more protection from bursts of noise on the HFC network; however, it will increase
downstream latency.
Note The valid values are 8, 16, 32 (default), 64, and 128.
To set the downstream interleave depth in milliseconds, use the following command in cable interface
configuration mode:
CMTS01(config-if)# cable downstream interleave-depth {8|16|32 |64|128}