Configuring Headend Broadband Access Router Features
Configuring the Upstream Cable Interface
MC-547
Cisco IOS Multiservice Applications Configuration Guide
• Use a recommended or previously tested modulation profile. It is not uncommon to create a
modulation profile that does not allow cable modem-to-headend communication. Because each
message type is individually specified, some messages may not work.
• Verify with IP ping packets of varying lengths (64 to 1500 bytes) by pinging from the headend to
the cable modem.
• Verify with your cable modem vendor that the software is either fully qualified as DOCSIS-based
or is verified for this modulation format.
Setting the Upstream Input Power Level
The Cisco uBR7200 series controls the output power levels of the cable modems 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 dBmV to 25 dBmV. If your power levels operate at greater than the maximum valid level, you must
use an inline attenuator to bring the power level to within the valid range.
Caution If you increase the input power level, the cable modems on your HFC network will increase
their transmit power level. A higher power level increases the CNR 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 of increased CNR. The return path laser
immediately enters a nonlinear mode called clipping and all communication is no longer
reliable. Many return lasers send short bursts well 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 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.
Note We recommend that the adjacent channel not have a large variation when you enter the
cable upstream 0 power-level command. The recommended maximum input power
variance is 5 to 6 dBmV.
To set the upstream input power level, use the following command in cable interface configuration mode:
Command Purpose
CMTS01(config-if)# cable upstream usport power-level dbmv
Sets the upstream input power level. Enter the
upstream power level in dBmV. Default = 0 dBmV.
