An accurate modeling of very high bit-rate transmission systems over standard-fiber using mid-span spectral inversion (MSSI) is reported. As for the pulsewidth, comparison of simulations with experimental data in an 80-Gb/s transmission system over 106 km shows that second-order group velocity dispersion (GVD) is the limiting factor in this transmission scheme. Additionally, it is shown that even though first-order GVD can be compensated easily second-order GVD, is still remaining. It is pointed out that the performance of the system is very sensitive to the length of the different fiber spans.