Abstract: The present invention relates to an arrangement and a method for detecting faults in an optical fiber network, comprising at least two nodes arranged with at least two optical fibers to a bus with the end nodes connected via two spare fibers. Every node comprising a central module (50), at least two protection switches (60,61) and optical amplifiers (21,22,23,24) and said central module comprising at least one central processor (51), at least one logical unit (52), at least one protection signal transmitter (53) and at least one protection signal monitor (54). The end nodes of the bus transmit a protection signal in at least one direction of the bus and said end nodes can detect the own protection signal and the protection signal transmitted from the other end node. All nodes in the bus can detect Optical Power Loss, OPL. The bus will reshape into new end nodes in case of a fault.