Abstract: A light power control system is used in a network in which a control signal is transmitted to instruct setting of a wavelength path. The light power control system is provided with a light amplifier control section configured to carry out a constant output control to a light amplifier which amplifies a light signal transmitted from a node to another node, when said node in said network receives the control signal; and a variable optical attenuator control section configured to adjust an attenuation quantity of a variable optical attenuator to attenuate a light power of the light signal on any of wavelength paths, when said node receives the control signal and moreover the light signal is transmitted on said any of wavelength paths of said node. It becomes possible to receive the data right in a receiving end by a simple unit when there is a change of the number of wavelength paths.

Abstract: A communication system comprises: failure detection means that detects a failure on a first transmission path over which data transmission is being performed; transmission path setting means that, when the failure exists in a link or a node that is located immediately before its own node, sets a second transmission path as far as a transmission terminal node, the second transmission path that excludes a path from the transmission terminal node to a node that has detected the failure within the first transmission path; first failure notification means that transmits a failure notification signal as far as the transmission terminal node over the second transmission path; new path switching means that, upon receipt of the failure notification signal, switches a switch for data transmission so that a path for new data transmission is the same path as a transmission path for the failure notification signal; and data transmission means that, upon the transmission terminal node receiving the failure notification sig