OSNR and Dispersion-Based Design
For a given network, it is important to calculate the OSNR and make a design based on both OSNR and dispersion limitations. It is possible to compensate for dispersion to a great extent. However, OSNR compensation needs 3R (O-E-O) regeneration, which is expensive. In other words, OSNR compensation is almost impossible for multichannel WDM systems. Therefore, when we are designing a WDM link, it is imperative to first consider OSNR's limitations. OSNR-based design essentially means whether the OSNR at the final stage (at the receiver) is in conformity with the OSNR that is desired to achieve the required BER. This also guarantees the BER requirement that is essential for generating revenue.
Following OSNR-based design, dispersion is the next issue to compensate from a design perspective. Dispersion-compensating units are readily available, but an important issue is where to place them. Various algorithms have been suggested depending on the network topology, the transmission length, and the bit rates. For most designs, optimization placements have to be done on a span (per length) basis.
Shown in Figure 4-11 is an OSNR map that carefully disseminates the optical signal level and the noise level as the signal passes through each amplification stage.
Figure 4-11 OSNR Levels in Terms of Signal and Noise Power Levels for Multistage WDM Transmission