Summary form only given. Transparent optical networks (TONs) based on Wavelength Division Multiplexing (WDM) are the only viable future-proof solution for supporting rapidly growing traffic demands. In TONs, optical signals are propagated transparently between edge nodes without any O/E/O conversion. In this way, it is possible to reduce cost and energy consumption in the network, as well as obtain transparency to signal bit rates, modulation formats and protocols. However, the absence of O/E/O conversion makes it easier for erroneous signals to propagate through the network and affect existing traffic.