Pon system and the ranging method

【Task】 While effectively utilizing the bandwidth, and provides OLT, ONU, the PON system capable of activation in a short time 100 units or more ONU. A The OLT in the first interval of the ranging window region, to receive a plurality of distance measurement signals transmitted from the plurality of ONU, immediately after receiving the distance measurement signal, to activate the delimiter detection circuit, and the automatic to reset the threshold circuit. .FIELD 7