Abstract: An optical communication system is provided that includes first and second optical transmitters/receivers remotely located with respect to one another and which are coupled together by first and second optical transmission paths for bidirectionally transmitting optical information therebetween. First and second optical amplifiers are respectively disposed in the first and second optical transmission paths. At least one loop-back path optically couples a portion of a WDM optical signal from the first to the second transmission path. The loop-back path includes a filter for transmitting a monitoring channel but not a data channel included in the optical signal portion traversing the loop-back path.

Abstract: A supervisory apparatus able to prevent recirculation of a supervisory frame when sending a supervisor frame for supervision, which supervises an optical communications apparatuses connected to transmission lines in cooperation with the same by adding a sequence number to a supervisory frame when sending the supervisory frame.