The description of the nonlinear noise generation as FWM is not unique. Similar results are obtained through several different approaches (e.g., Volterra series) which are independent of FWM. The traditional classification of nonlinear effects selfphase modulation (SPM), XPM, cross pol modulation (XPolM), etc., give rise to very specific signal features are not readily accessed using the GN methodology where all nonlinearities add up to the total “nonlinear noise” regardless of the source. For example, it may be tempting to call the nonlinear noise generated with in a channel upon itself SPM, however, it does not create the same phase-only effect as per its traditional definition. The same argument can be made for XPM and XPolM. Nevertheless,
FWM does provide a general understanding the overall derivation of the GN model—three slices of spectrum beat to form a fourth frequency, with some deterministic efficiency.