Specifications

Router#
Note: This is showing a downstream receive SNR of 33.6 dB at the cable modem. Acceptance levels are >30
dB for 64 QAM and > 35 dB for 256 QAM.
Annex B is the DOCSIS MPEG framing format standard for North America. Annex A is the European
standard supported only when using the Cisco MC16E cable modem card and Cisco CMTS images that
support EuroDOCSIS Annex A operation. Annex A or B framing format is automatically set when
configuring Cisco cable modem cards. The cable modem card's downstream ports and the connected CPEs on
the network must be set to the same MPEG framing format and support either DOCSIS or EuroDOCSIS
operations as appropriate. For more information about the MC16E card, read Cisco uBR−MC16E 8−MHz
Modem Card.
Setting a downstream modulation format of 256−QAM requires approximately a 6 dB higher carrier−to−noise
ratio (CNR) than 64−QAM at the subscriber's cable modem. If your network is marginal or unreliable at
256−QAM, use the 64−QAM format instead.
If a Cable modem is offline one of the first thing to investigate is the RF plant. For more information read the
offline and ranging troubleshooting sections.
Checking the Upstream
On the upstream side many RF problems are indicated by a low SNR level. Note that upstream impulse noise
is the major source of degraded bit error rate performance. The Broadcom SNR estimate generally will not
show the presence of impulse noise.
Later in this section you will be shown how to check the upstream SNR levels.
Begin by checking the upstream interface, ensuring that the configuration is correct. In most cases when
configuring the upstream cable interface on the CMTS the default values are sufficient. You do not need to
specify individual parameters unless you want to deviate from system defaults. Use the diagram below to
match the upstream configuration parameters with the matching values seen in the show commands at the
CMTS:
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 upstream 0
Cable6/1 Upstream 0 is up
Frequency 19.984 MHz, Channel Width 3.200 MHz, QPSK Symbol Rate 2.560 Msps
Spectrum Group is overridden
SNR 35.1180 dB
Nominal Input Power Level 0 dBmV, Tx Timing Offset 2738
Cisco − Determining RF or Configuration Issues On the CMTS