In this project we will examine the limitations of Wave Division Multiplexing (WDM) and Dense Wave Division Multiplexing (DWDM) exerted by the nonlinear effect. We shall examine existing networks utilising the existing data we managed to collect for these networks. We will utilise (OptSim) software to simulate the network under consideration to investigate the fiber nonlinearities. We will concentrate mainly on Single Phase Modulation (SPM), Cross Phase Modulation (XPM) and Four-Wave Mixing (FWM). We prove under this project that Large Effective Area Fibers (LEAF) is the best candidates to reduce fiber nonlinearities.