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 Upstream Cable Interface
Setting the Upstream Input Power Level
The Cisco uBR7100 series CMTS controls the output power levels of CMs to meet the desired upstream
input power level. The nominal input power level for the upstream RF carrier is specified in decibels per
millivolt (dBmV). The default setting of 0 dBmV is the optimal setting for the upstream power level.
The valid range for the input power level depends on the data rate. At 1.6 MHz, the valid range is
–10 to 25 dBmV. If your power levels operate at greater than the maximum valid level, use an inline
attenuator to bring the power level to within the valid range.
Caution If you increase the input power level, CMs on your HFC network will increase their transmit power level.
This increases the carrier-to-noise ratio (C/N) on the network, but also increases distortion products.
Composite Second Order Beat (CSO) and Composite Triple Beat (CTB) values worsen by 2 dB for every
1 dB-increased C/N. The return path laser immediately enters a nonlinear mode called clipping and all
communication is no longer reliable. Many return lasers send “short” bursts above the clipping
thresholds and fail on longer or successive bursts.
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 interface service on your network is
disrupted. If you decrease the power level by more than 5 dB within 30 seconds, cable interfaces on your
network are forced offline.
Note When you run the cable upstream 0 power-level command, Cisco recommends that the adjacent
channel not have a large variation. The recommended maximum input power variance is 5 to 6 dBmV.
To set the upstream input power level in dBmV, use the following command in cable interface
configuration mode. Default = 0 dBmV.
CMTS01(config-if)# cable upstream usport power-level dbmv
Verifying the Upstream Input Power Level
To verify the current value of the upstream input power level, enter the show controllers cable command
for the upstream port you have just configured:
CMTS01# show controllers cable1/0 u0
Cable1/0 Upstream 0 is up
Frequency 24.016 MHz, Channel Width 0.800 MHz, QPSK Symbol Rate 0.640 Msps
Spectrum Group is overridden
SNR 33.2560 dB
Nominal Input Power Level 0 dBmV, Tx Timing Offset 2288
Ranging Backoff automatic (Start 0, End 3)
Ranging Insertion Interval automatic (60 ms)
Tx Backoff Start 0, Tx Backoff End 4
Modulation Profile Group 1
Perform these steps if you are having difficulty with verification:
1. Verify that the upstream amplitude of an optimal RF carrier injected at the fiber node reference input
point reaches the cable modem card input point at a consistent level (node-to-node and port-to-port).
2. Verify that this absolute level, as installed, matches both the design and software settings on the
Cisco uBR7100 series CMTS.