to provide a system in which a configuration is simple and a switching of an optical line can be performed at a high speed . a protection unit for performing optical line protection by using k1 - k2 information inserted into a payload is provided for each physical interface . the protection unit includes a filter for separating a payload of user data and a payload including k1 - k2 information according to a control signal in a received payload , a selector for switching user data to be transmitted or k1 - k2 information to be transmitted after adding a control signal of a value corresponding thereto , and a control unit for controlling an output buffer to a switch block and creating k1 - k2 information to be transmitted and communicating with a physical interface card of another system on the basis of k1 - k2 information .