Abstract: A diagnostic monitoring system for a WDM network, the system comprising an electrical non-blocking cross-connect switch having a plurality of demultiplexer ports, a plurality of multiplexer ports, at least one tributary receiver port, at least one tributary transmitter port, and at least one diagnostic port; and at least one multi-protocol performance monitoring unit connected to one of said at least one diagnostic ports such that said at least one multi-protocol performance monitoring unit, in use, can selectively monitor one of said demultiplexer or tributary receiver ports whereby said system, in use, supports multi-protocol intrusive monitoring of a test signal and multi-protocol non-intrusive monitoring of live traffic at said at least one demultiplexer or tributary receiver port