Specifications

interface Cable6/1
ip address 192.168.161.1 255.255.255.0 secondary
ip address 10.1.61.1 255.255.255.0
no keepalive
cable insertion−interval 100
cable downstream annex B
cable downstream modulation 64qam
cable downstream interleave−depth 32
cable downstream frequency 405000000
cable upstream 0 frequency 20000000
cable upstream 0 power−level 0
cable upstream 0 channel−width 3200000
no cable upstream 0 shutdown
cable upstream 1 shutdown
cable upstream 2 shutdown
cable upstream 3 shutdown
VXR#show controller cable 6/1 downstream
Cable6/1 Downstream is up
Frequency 405.0000 MHz, 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
Downstream channel ID: 3
VXR#
Make sure that the physical CMTS cable connections are not loose or disconnected, and that the cable modem
card is firmly seated in its chassis slot with the installation screws tight. Also check that you have entered the
correct slot and port numbers for the downstream interface that you are checking.
Remember that entering the downstream center frequency on the CMTS is cosmetic only for uBR7200 and
uBR10000. The uBR7100 has an integrated Upconverter. To learn how to set it up read Setting the Integrated
Upconverter.
Entering a shut/no shut command on the downstream interface that you are checking may resolve problems
where the cable modems find a downstream signal but not an upstream signal.
Important: Using a shut/no shut on the downstream interface in a production environment with several
hundred cable modems may cause them to take a long time before coming back online. In non−production
environments such as new cable installations it is safe to do this.
The downstream SNR value needs to be checked at the cable modem where it is received and not at the
CMTS where it is input into the upconverter that is responsible for the signal sent to the cable modem. This
measurement at the cable modem can pose the following problems:
Most cable installations do not have Cisco cable modems. Even if they do, the console port on the
Cable Modem is by default locked.
•
You have to Telnet to the cable modem to measure the received SNR value. If you do not have IP
connectivity to Telnet you will have to manually go to the customer site where the Cisco Cable
Modem is installed in order to connect using the console port. Ensure that the cable modem has a
configuration that allows you access to the console port.
•
At the cable modem, use the show controllers cable−modem 0 | include snr command below to check the
downstream SNR value received at the cable modem. Verify that the received SNR level is within the
permitted limits of >30 dB for 64 QAM and >35 for 256 QAM.
Router#show controller cable−modem 0 | include snr
snr_estimate 336(TenthdB), ber_estimate 0, lock_threshold 23000
Cisco − Determining RF or Configuration Issues On the CMTS