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This paper introduces an in-service line monitoring technique that can locate a fault in branched optical fibers in passive optical networks (PONs) from a central office. This technique can upgrade the conventional optical fiber line testing and monitoring system. To distinguish a backscattered signal from each branched PON fiber, we employ the fibers with individually assigned Brillouin frequency shifts (BFSs) fibers as drop cables and a 1650-nm Brillouin optical time-domain reflectometer (B-OTDR) instead of the OTDR used in the conventional testing system. This paper describes the design of the BFS taking the outside plant environment into consideration, and discusses the performance of the proposed testing system. We also demonstrate an experimental measurement that locates a fault in a branching fiber after a 1 ?? 8 optical splitter, and in-service line monitoring for gigabit Ethernet PON (GE-PON) transmission systems.