The SLM 1630 is used to monitor the working state of wet plant systems and to locate faults. In a system with repeaters, the SLM 1630 can monitor the gain of the repeater and locate fault positions by analyzing back-scattered signals. The SLM 1630 reports performance events and fault locating information to the network management system. These can then can be queried on the NMS. If there is a power switch BU (PSBU) in the submarine network, the SLM 1630 can also control the polarity of the switch.