Specifications

Waterfall curves. More complex modulation formats require a higher CNR (or C/N ratio) in order to maintain
the same BER.
The upstream input power level at the CMTS is normally expected to be 0 dBmV. This power level can be
increased to overcome noise in the RF plant. If the upstream input power level is increased then cable
modems on your HFC network will increase their upstream transmit power level. This increases the
carrier−to−noise ratio (CNR) overcoming the noise on the RF plant. See the explanation of the cable
upstream x power−level y command for this. You should not adjust your input power level by more than 5
dB in a 30−second interval. If you increase the power level by more than 5 dB within 30 seconds cable
modem service on your network will be disrupted. If you decrease the power level by more than 5 dB within
30 seconds cable modems on your network will be forced offline.
Software adjustments of 1 to 3 dB can be used to adjust for minor variations in measurement, or setup and
port−to−port calibration differences. These adjustments can significantly improve cable modem performance,
especially in marginal situations. Larger adjustments should be made in conjunction with spectrum analyzer
support at the headend or distribution hub.
As mentioned earlier, many RF problems are indicated by a low upstream SNR level. If your upstream SNR
level is low try to use a narrower channel width (cable upstream 0 channel−width xxx) for the upstream, for
example, instead of 3.2 Mhz use 200 khz. If the upstream SNR level increases then you have a noise problem.
Use the show controller cable x/x upstream y command to check the upstream SNR level for a particular
cable interface:
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
!−− Note: Check the Upstream SNR level for an interface here
Nominal Input Power Level 0 dBmV, TX Timing Offset 2738
Ranging Backoff automatic (Start 0, End 3)
Ranging Insertion Interval 100 ms
TX Backoff Start 0, TX Backoff End 4
Modulation Profile Group 1
Concatenation is enabled
part_id=0x3137, rev_id=0x03, rev2_id=0xFF
NB_agc_thr=0x0000, NB_agc_nom=0x0000
Cisco − Determining RF or Configuration Issues On the CMTS