Abstract: A line protection switching system of a duplexed fiber interface shelf makes easily to reconcile conditions of duplexed fiber interface shelves, which perform line switching by employing APS bytes on an ISDN network. The line protection switching system has a pair of duplexed line switching controllers that send a line switching control signal to switch the transmission lines and a pair of duplexed line interface devices operatively connected to the pair of duplexed line switching controllers. An active interface device of the duplexed interface devices receives the line switching control signal only from an active line switching controller of the duplexed line switching controllers and echoes the received control signal back to the duplexed line switching controllers.