Patent ID: 11936423
Assignee: NEC CORPORATION
Field: Telecommunications (Electrical engineering)
Classification: CPC H | IPC H

Claim 13:
14. A fault detection method comprising:
transmitting a first optical signal to an optical transmission path;
receiving a second optical signal from the optical transmission path in response to transmission of the first optical signal;
measuring a reception level of the second optical signal;
specifying a section where the second optical signal according to the first optical signal has been generated;
calculating, based on the reception level, an optical level associated with a loss in the section;
determining that a first fault has occurred in the section when the optical level of the section has changed from a first reference level by a first threshold value or more;
setting a second reference level and a second threshold value after occurrence of the first fault; and
determining that a second fault has occurred in the section when the optical level has changed from the second reference level by the second threshold value or more; wherein the optical transmission path is a submarine cable through which a plurality of optical repeaters are connected in cascade, and the second optical signal is the first optical signal looped-back in each of the plurality of optical repeaters; said method further comprising specifying the section, based on a transmission time of the first optical signal, a reception time of the second optical signal, and a distance to each of the plurality of repeaters.