Interference arising from abnormal uplink optical signals in faulty ONU is a common and hardly entirely avoidable problem in PON networks

which seriously threatens the steady network operation.This paper describes several behaviors of the interferences and their possible causes

and summarizes two typical long-shining alarm detection methods

discusses the location of the faulty ONU

compares their advantages and disadvantages and gives a mechanism for abnormal protection against the interferences at the ONU side.