Abstract: When a failure occurs in operational transmission multiplexing transponders 6-1 to 6-N, a wavelength multiplexing optical transmission system carries out selection switching control on N×1 optical switches 5-1 to 5-m and (N+1)×N optical switches 11-1 to 11-m to transmit a wavelength-multiplexed light signal by using reserve transmission and reception multiplexing transponders 6-R and 10-R, thereby sidestepping the failure.

Abstract: When a failure occurs in operational transmission multiplexing transponders 6-1 to 6-N, a wavelength multiplexing optical transmission system carries out selection switching control on N×1 optical switches 5-1 to 5-m and (N+1)×1 optical switches 11-1 to 11-m to transmit a wavelength-multiplexed light signal by using reserve transmission and reception multiplexing transponders 6-R and 10-R, thereby sidestepping the failure.

Abstract: The invention provides a test system of a fault detecting means, in which, during a normal operation, a fault forcibly generating circuit lets a transmission path signal inputted from the outside pass through as it is, generates a test signal which is the same as that of the transmission path signal, and outputs it to a fault detecting circuit. During a time of testing, the fault forcibly generating circuit outputs "Low" level of the test signal forcibly to the fault detecting circuit, according to a test control signal outputted from a control portion. When the fault detecting circuit detects this test signal, an step-out alarm is generated and transmitted to the control portion. The control portion monitors the step-out alarm from the fault detecting circuit. The control portion judges the operation to be normal if the step-out alarm is outputted, and to be abnormal if the step-out alarm is not outputted under the condition that the step-out alarm is not outputted at the time of non-testing.