  Efficient evaluation of nonlinearity in modern coherent optical
telecommunication networks is necessary for design, optimization and
management. The original GN model formula can not be used in real time
applications due to long processing time needed for numerical 2-D integration,
particularly when facing with full C band or C+L band. There are closed form
approximations of the GN model which has been mathematically derived based on
some assumptions which fail in low dispersion regime. In this work, we derive a
GN closed form formula without any restrictive assumption capable of handling
arbitrary WDM combs and arbitrary link structures in closed form which can be
used with a very good accuracy with respect to original GN formula for networks
employing both high and low dispersion fibers.
