This paper reports on an investigation of optical crosstalk caused by nonlinear interactions in short- and long-haul wavelength division multiplexion transmission systems called inter-domain interfaces. These investigations are based on numerical simulations of multichannel optical transmission systems, using specialist software: Gigabit Optical Line Designer v.2.0 from Virtual Photonics. The nonlinear phenomena in fibres that have to be taken into account include four-wave mixing, self-phase modulation, cross-phase modulation, stimulated Raman scattering, stimulated Brillouin scattering and nonlinear interactions in erbium-doped fibre amplifiers.