Monitoring system using an optical side tone as a test signal

A test system for monitoring a WDM transmission system, comprising:

• a line monitoring equipment (LME), comprising:
• a) a tone generator generating a pseudo-random sequence tone;
• b) a laser transmitter coupled to said tone generator and generating a test signal based on the pseudo-random sequence tone, said test signal having a frequency;
• c) an optical filter for passing only the frequency of the test signal;
• d) a delay system coupled to said tone generator;
• and
• e) a correlator coupled to said delay system and said optical filter, said correlator correlating an output of the delay system with an output of the optical filter;
• an optical coupler combining the test signal with a data signal into a combined signal, said data signal having a data channel, the test signal having a frequency outside the frequency bandwidth of the data channel;
• a repeater coupled to said optical coupler and having an optical loopback, said loopback returning a portion of the combined signal to the optical filter of the LME;
• and
• said delay system delaying the test signal based on a location of said repeater.