  Through a series of extensive system simulations we show that all of the
previously not understood discrepancies between the Gaussian noise (GN) model
and simulations can be attributed to the omission of an important, recently
reported, fourth-order noise (FON) term, that accounts for the statistical
dependencies within the spectrum of the interfering channel. We examine the
importance of the FON term as well as the dependence of NLIN on modulation
format with respect to link-length and number of spans. A computationally
efficient method for evaluating the FON contribution, as well as the overall
NLIN power is provided.
