An optical transmission system with a mechanism to locate a fault on a transmission line effectively and efficiently to ensure the quality of communication between end stations and repeaters. Repeaters respond to a monitoring control command sent from an end station, returning a monitoring report signal that indicates their current operating status and input/output signal conditions. In the end station, a monitoring report processor identifies a faulty link section of the optical transmission line, if the monitoring report signal indicates a fiber fault. The end station sends a troubleshooting control command to cause a relevant repeater to transmit a probing light pulse signal and a complementary light pulse signal simultaneously in opposite directions. Some of the probing light pulse is reflected back as a result of Rayleigh scattering. The end station locates the fiber fault by analyzing the backscatter, using the complementary light pulse signal for synchronization.