  We study the properties of nonlinear interference noise (NLIN) in fiber-optic
communications systems with large accumulated dispersion. Our focus is on
settling the discrepancy between the results of the Gaussian noise (GN) model
(according to which NLIN is additive Gaussian) and a recently published
time-domain analysis, which attributes drastically different properties to the
NLIN. Upon reviewing the two approaches we identify several unjustified
assumptions that are key in the derivation of the GN model, and that are
responsible for the discrepancy. We derive the true NLIN power and verify that
the NLIN is not additive Gaussian, but rather it depends strongly on the data
transmitted in the channel of interest. In addition we validate the time-domain
model numerically and demonstrate the strong dependence of the NLIN on the
interfering channels' modulation format.
