A communication network including a master switch and one or more local switches is provided with a loop-back test device for in line loop-back testing. The local switches convey communication traffic between one another using one or more channels of a transmission medium and configuration information using an out-of-band channel of the transmission medium that is separate from the channels used to convey the communication traffic. The master switch includes an application that generates configuration information including loop-back connection information for configuring the out-of-band channel using at least one loop-back test device, transmits the generated configuration information to the loop-back test device using the out-of-band channel of the transmission medium, and conducts loop-back testing using the out-of-band channel.

There is provided a transmission apparatus, which includes: a filter configured to pass an optical signal having a predetermined wavelength received via an optical fiber, a variable optical attenuator configured to attenuate a power level of the optical signal passed by the filter, a first measuring circuit configured to measure a first power of the optical signal before being passed by the filter, a second measuring circuit configured to measure a second power of the optical signal attenuated by the variable optical attenuator, a detector configured to detect a difference between the first power and the second power, and a controller configured to increase an attenuation factor of the variable optical attenuator when the difference is equal to or larger than a first predetermined value.

In a wavelength division multiplexing transmission system, a controller of a relay node of a post-stage notifies level decrease information to a transmission node, upon detecting a decrease in a level of a wavelength in a wavelength multiplex signal received from the transmission node of a pre-stage by the optical channel monitor of the transmission node. The controller of the transmission node of the pre-stage determines that a failure occurs in the wavelength of the transponder of the transmission node, upon detecting a decrease in the level of the wavelength in the wavelength multiplex signal transmitted to the relay node by the optical channel monitor of the transmission node and upon receiving the level decrease information for the wavelength from the relay node. Therefore, a wavelength channel failure in the transponder can be determined in a preferable manner.