  The GN-model has been proposed as an approximate but sufficiently accurate
tool for predicting uncompensated optical coherent transmission system
performance, in realistic scenarios. For this specific use, the GN-model has
enjoyed substantial validation, both simulative and experimental. Recently,
however, it has been pointed out that its predictions, when used to obtain a
detailed picture of non-linear interference (NLI) noise accumulation along a
link, may be affected by a substantial NLI overestimation error, especially in
the first spans of the link. In this paper we analyze in detail the GN-model
errors. We discuss recently proposed formulas for correcting such errors and
show that they neglect several contributions to NLI, so that they may
substantially underestimate NLI in specific situations, especially over
low-dispersion fibers. We derive a complete set of formulas accounting for all
single, cross, and multi-channel effects, this set constitutes what we have
called the enhanced GN-model (EGN-model). We extensively validate the EGN model
by comparison with accurate simulations in several different system scenarios.
The overall EGN model accuracy is found to be very good when assessing detailed
span-by-span NLI accumulation and excellent when estimating realistic system
maximum reach. The computational complexity vs. accuracy trade-offs of the
various versions of the GN and EGN models are extensively discussed.
