Patent ID: 8126333

Claim:
An optical transmission system including an optical terminal unit in network side and a plurality of optical terminal units in user side connected to said optical terminal unit in network side via an optical transmission line, said optical terminal unit in network side comprising: a first transmitter and receiver means for transmitting and receiving signals with said plurality of optical terminal units in user side; a first communication control timer; a measurement means for measuring a round trip time between said optical terminal unit in network side and each of said plurality of optical terminal units in user side; an advance notice time generation means for generating an advance notice time signal by adding a predetermined time to a time information that indicates a time in said first communication control timer in response to a first time reference pulse; and means for controlling said first transmitter and receiver means to transmit the generated advance notice time signal to each of said plurality of optical terminal units in user side, and to transmit signals indicating half values of the measured round trip times to the respective ones of said plurality of optical terminal units in user side, each of said plurality of optical terminal units in user side comprising: a second transmitter and receiver means for transmitting and receiving signals with said optical terminal unit in network side and for receiving the advance notice time signal and the signal indicating a half value of the measured round trip time from said optical terminal unit in network side; a second communication control timer indicating a time delayed from that of said first communication control time by a delay time corresponding to a trip time from said optical terminal units in network side to this optical terminal unit in user side; a decision means for calculating a corrected advance notice time by subtracting a time corresponding to the half value of the round trip time from the advance notice time received by said second transmitter and receiver means, and for judging whether the time indicated by said second communication control timer coincides with the corrected advance notice; and a time-reference pulse generation means for generating a second time reference pulse when said decision means judges that the time indicated by said second communication control timer coincides with the corrected advance notice.