Cisco ONS 15501 User Manual page 18

Chapter 1 Product Overview
Cisco ONS 15501 Applications
Figure 1-1 Power Equalization in an Amplified Ring Network
In general, EDFAs in a ring topology should be placed so they maintain the power level at the receiver,
as well as the OSNR, of each channel. In this case, EDFAs serving as postamplifiers are located at nodes
2 and 4.
At node 2, the input power level of the EDFA is much higher than the input power level of the
pass-through band (bands C and D), due to the added power from bands A and B. If trunk attenuation is
employed directly before the EDFA at node 2 to keep the unit's total input power within the required
range, the power levels of both the add band (bands A and B) and the pass-through band (bands C and
D) are attenuated equally. As a result, the power level of the pass-through band is much lower than that
of the add band. This significantly degrades the OSNR of the pass-through band, and in cases where
there are more than two EDFAs in the ring, some of the channels in the ring will not meet OSNR
requirements.
To solve this problem, optical power attenuation should be applied on a per channel or per band basis.
More attenuation is typically required for the add band than for the pass-through band. At the EDFA
input, the individual channel or band power levels should be equalized as close as possible to the
maximum per channel input power level, (for example, -15 dBm in a 32-channel system). This process
of optical power equalization is necessary to obtain better OSNR.
Cisco ONS 15501 User Guide
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