A system processes data units in a network. The corresponding optical subscriber unit then notifies, based on the data amount accumulated in a buffer, the optical line terminal of a data amount, as a transmission requirement, up to a data separation that is less or equal to the notified requirement threshold and in which a maximum transmission amount can be transmitted. The generated phase differences are fed to another differential circuit, which in turn generates, for each symbol, a difference between both received phase differences to produce phase-difference difference information. The third wall may include an inner surface with a first opening and a magnet disposed in the first opening.