Patent ID: 8712244

Claim:
A communication system including: a passive optical network (PON) having an optical line terminal (OLT), an optical fiber, an optical splitter, and a plurality of optical network units (ONUs); a master device that is connected to the OLT and transmits standard time information; and a slave device that serves under one of the ONUs, and receives the time information, the communication system comprising: a first ranging process in which the OLT transmits a first delay measurement signal to an ONU, receives a first return signal responsive to the first delay measurement signal from the ONU, and determines a one-way propagation delay time between the OLT and the ONU according to a value of half of a difference between a transmit time of the first delay measurement signal and a receive time of the first return signal; a first stage time synchronization process in which the master device transmits a time synchronization message having a clock timer value of the master device inserted as a time stamp value to the OLT, and the OLT receives the time synchronization message and sets a clock timer of the OLT to the time stamp value; a second stage time synchronization process in which the OLT transmits a second delay time measurement signal to the master device, receives a second return signal responsive to the second delay measurement signal from the master device, and determines a one-way propagation delay time between the master device and the OLT according to a value of half of a difference between a transmit time of the second delay time measurement signal and a receive time of the second return signal, and the OLT updates a clock timer value of the OLT to a value obtained by adding the one-way propagation delay time between the master device and the OLT to the clock timer value of the OLT; and a third stage time synchronization process in which the master device transmits the time synchronization message having the clock timer value inserted as the time stamp value to the OLT, the OLT rewrites the time stamp value of the time synchronization message to a value obtained by adding, to the time stamp value, the one-way propagation delay time between the OLT and the ONU which is obtained by the first ranging process, and the one-way propagation delay time between the master device and the OLT which is obtained by the second stage time synchronization process, and transmits the time synchronization message having the rewritten time stamp value to the slave device, and the slave device sets a clock timer of the slave device to the time stamp value.