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Cisco ONS 15454 DWDM Reference Manual, R8.5
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Chapter 10 Network Reference
10.4.2 APC at the Shelf Controller Layer
In Figure 10-10, Node 1 and Node 2 are equipped with booster amplifiers and preamplifiers. The input
power received at the preamplifier on Node 2 (Pin2) depends on the total power launched by the booster
amplifier on Node1, Pout1(n) (where n is the number of channels), and the effect of the span attenuation
(L) between the two nodes. Span loss changes due to aging fiber and components or changes in operating
conditions. The power into Node 2 is given by the following formula:
Pin2 = LPout1(n)
The phase gain of the preamplifier on Node 2 (GPre-2) is set during provisioning in order to compensate
for the span loss so that the Node 2 preamplifier output power (Pout-Pre-2) is equal to the original
transmitted power, as represented in the following formula:
Pout-Pre-2 = L x GPre-2 x Pout1(n)
In cases of system degradation, the power received at Node 2 decreases due to the change of span
insertion loss (from L to L'). As a consequence of the preamplifier gain control working mode, the
Node 2 preamplifier output power (Pout-Pre-2) also decreases. The goal of APC at the shelf controller
layer is simply to detect if an amplifier output change is needed because of changes in the number of
channels or to other factors. If factors other than changes in the number of channels occur, APC
provisions a new gain at the Node 2 preamplifier (GPre-2') to compensate for the new span loss, as shown
in the formula:
GPre-2' = GPre-2 (L/ L') = GPre-2 + [Pout-Pre-2 –Exp(Pout-Pre-2)]
Generalizing on the above relationship, APC is able to compensate for system degradation by adjusting
working amplifier gain or variable optical attenuation (VOA) and to eliminate the difference between the
power value read by the photodiodes and the expected power value. The expected power values are
calculated using:
• Provisioned per-channel power value
• Channel distribution (the number of express, add, and drop channels in the node)
• ASE estimation
Channel distribution is determined by the sum of the provisioned and failed channels. Information about
provisioned wavelengths is sent to APC on the applicable nodes during circuit creation. Information
about failed channels is collected through a signaling protocol that monitors alarms on ports in the
applicable nodes and distributes that information to all the other nodes in the network.
ASE calculations purify the noise from the power level reported from the photodiode. Each amplifier can
compensate for its own noise, but cascaded amplifiers cannot compensate for ASE generated by
preceding nodes. The ASE effect increases when the number of channels decreases; therefore, a
correction factor must be calculated in each amplifier of the ring to compensate for ASE build-up.
APC is a network-level feature that is distributed among different nodes. An APC domain is a set of
nodes that is controlled by the same instance of APC at the network level. An APC domain optically
identifies a portion of the network that can be independently regulated. An optical network can be
divided into several different domains, with the following characteristics:
• Every domain is terminated by two node sides. The node sides terminating domains are:
–
Terminal node (any type)
–
ROADM node
–
Hub node
–
Cross-connect (XC) termination mesh node
–
Line termination mesh node
• APC domains are shown in both Cisco Transport Controller (CTC) and Transaction Language One
(TL1).
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