PON system, optical network unit, optical line terminal, method of registering optical network unit, and data structure

A PON system according to one manner of the present invention includes an optical line terminal (OLT), at least one optical network unit (ONU), and an optical fiber that connects the optical line terminal and the optical network unit to each other. A reception level category for categorizing a reception level in the optical network unit, of an optical signal sent from the optical line terminal through the optical fiber is set. In discovery processing for searching for and registering an unregistered optical network unit, the optical line terminal registers an optical network unit corresponding to the reception level category.