a method for monitoring a passive optical network , pon , having a tree - like structure with a main line and at least two branches , comprising : transmitting a wake - up signal from the main line to at least two monitoring units arranged in the at least two branches , in each of the monitoring units , detecting the wake - up signal and transmitting a response signal back to the main line , each of the monitoring units generating a pre - defined time delay between the detection of the wake - up signal and the start of the transmission of the response signal , and receiving the response signals at the main line , the receiving times of the response signals being different from each other , the difference between the receiving times being adjusted by the pre - defined time delays of the monitoring units .