Cisco ONS 15454 DWDM - Automatic Power Control

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Cisco ONS 15454 DWDM Installation and Operations Guide, R6.0
August 2005
Chapter 18 Network Reference
18.4 Automatic Power Control
18.4 Automatic Power Control
The ONS 15454 automatic power control (APC) feature performs the following functions:
• Maintains constant per-channel power when changes to the number of channels occur.
• Compensates for optical network degradation (aging effects).
• Simplifies the installation and upgrade of DWDM optical networks by automatically calculating the
amplifier setpoints.
Note APC functions are performed by software algorithms on the OPT-BST, OPT-PRE, and TCC2/TCC2P
cards.
Amplifier software uses a control gain loop with fast transient suppression to keep the channel power
constant regardless of any changes in the number of channels. Amplifiers monitor the changes to the
input power and change the output power according to the calculated gain setpoint. The shelf controller
software emulates the control output power loop to adjust for fiber degradation. To perform this function,
the TCC2/TCC2P needs to know the channel distribution, which is provided by a signaling protocol, and
the expected per-channel power, which you can provision. The TCC2/TCC2P compares the actual
amplifier output power with the expected amplifier output power and modifies the setpoints if any
discrepancies occur.
18.4.1 APC at the Amplifier Card Level
In constant gain mode, the amplifier power out control loop performs the following input and output
power calculations, where G represents the gain and t represents time.
Pout (t) = G * Pin (t) (mW)
Pout (t) = G + Pin (t) (dB)
In a power-equalized optical system, the total input power is proportional to the number of channels. The
amplifier software compensates for any variation of the input power due to changes in the number of
channels carried by the incoming signal.
Amplifier software identifies changes in the read input power in two different instances, t1 and t2 as a
change in the carried traffic. The letters m and n in the following formula represent two different channel
numbers. Pin/ch represents the per-channel input power:
Pin (t1)= nPin/ch
Pin (t2) = mPin/ch
Amplifier software applies the variation in the input power to the output power with a reaction time that
is a fraction of a millisecond. This keeps the power constant on each channel at the output amplifier, even
during a channel upgrade or a fiber cut.
Amplifier parameters are configured using east and west conventions for ease of use. Selecting west
provisions parameters for the preamplifier receiving from the west and the booster amplifier transmitting
to the west. Selecting east provisions parameters for the preamplifiers receiving from the east and the
booster amplifier transmitting to the east.
Starting from the expected per-channel power, the amplifiers automatically calculate the gain setpoint
after the first channel is provisioned. An amplifier gain setpoint is calculated in order to make it equal
to the loss of the span preceding the amplifier itself. After the gain is calculated, the setpoint is no longer