The main goal in the development of telecommunication systems is to meet an always growing data demand. Research is focused on finding new solutions for more flexible networks able to sustain high data rates maintaining a good quality of the transmitted signal. In the area of optical networks, the development passed from the introduction of WDM transmission to the current use of Reconfigurable Add-Drop Multiplexers (ROADMs) in network nodes, enabling the possibility to implement dynamic wavelength routing. As a drawback, the introduction this flexibility also introduced distortions due to the filtering effect caused by cascading several ROADMs along the optical path of channel: this is becoming more and more relevant so a detailed study is necessary in order to find a good solution for reliable transmissions.