Cisco Broadband Cable Command Reference Guide

cable upstream power-adjust
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Cisco Broadband Cable Command Reference Guide
Chapter 2 Cisco Cable Modem Termination System Commands
Determining if the CM has Reached Its Maximum Power Level
Per the DOCSIS specification, the CMTS does not adjust the CM power level by commanding the CM
to set itself to an absolute power level. Instead, the CMTS changes the CM power levels by commanding
the CM to make relative power adjustments, using increments of +/–0.25 dB (or multiples thereof).
If the CMTS commands a CM to adjust its power level by a particular amount, but the measured power
level afterwards shows that the CM has made a smaller adjustment than requested, the show cable
flap-list and show cable modem commands show an exclamation point (!) for that particular CM. This
typically indicates that the CM has reached its maximum power level.
If the CM has not reached its maximum power level, but the show cable flap-list and show cable modem
commands still show an exclamation point (!) for that CM, the problem could be a fault in the upstream
path of the cable network. This fault is typically found as amplifier misalignment, where one upstream
amplifier is adjusted with too much input attenuation, and the next has too little. The amplifier with too
little attenuation enters non-linear operation and begins serious intermodulation.
Another possible cause is a faulty CM or a faulty connection between the CM and cable network. For
example, a faulty coaxial cable or bad F-connector could also create noise problems.
Note A CM that shows an exclamation point (!) in the show cable flap-list and show cable modem displays
is typically more prone to packet loss than other CMs.
Setting the noise Option
The noise option specifies that when the number of power adjustments beyond the threshold amount
exceeds a certain percentage of the total Ranging Requests for a CM, the CMTS switches into its noise
power-adjustment method. The noise power-adjustment method uses an averaging algorithm to make
power adjustments for a particular CM, so that if a modem is experiencing wide swings in its power
levels (known as “bouncing’), the CMTS averages the power levels before sending a power adjustment
to the CM. The CMTS returns to the normal power adjust method after receiving ten Ranging Request
messages with power adjustments below the threshold value.
For example, the cable upstream power-adjust noise 30 command instructs the CMTS that when
30 percent (3 out of the last 10) of the Ranging Responses for a particular CM resulted in a power
adjustment beyond the threshold level, the CMTS begins using power averaging for that CM to adjust
its power level.
Tip The cable upstream power-adjust noise command is typically not needed during normal operations,
but it can help in certain situations where the cable plant is experiencing ingress noise and other
interference that is forcing many CMs to frequently re-range. If the show cable flap-list and show cable
modem commands show a large number of CMs with asterisks (*), experiment with the noise option, so
that the CMTS will average its CM power readings before sending any power adjustments to the CMs.
Note When a CM is in power averaging mode, the show cable flap-list and show cable modem commands
shows an asterisk (*) for that particular CM.
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