Communication System and Its Device

Abstract

A transmission device uses transmission delay time of reciprocation with the terminal and time information of the transmission device to create a correction value of time information of the terminal and transmits it to the terminal. The terminal includes expected arrival time information based on the time information of the terminal and the correction value received from the transmission device in a frame transmitted to the transmission device. The transmission device compares reception time of the frame with the expected arrival time information in the frame. If they match, the time information of the terminal synchronizes with the time information of the transmission device. If they do not match, the transmission device transmits a new correction value to the terminal. The terminal transmits a frame including expected arrival time information using the new correction value to the transmission device.

1 . A communication system comprising:
 a terminal including: a first storage device that stores first time information; a receiving device that receives a correction value to correct the first time information; and a transmitter that transmits a frame including expected arrival time information based on the first time information and the correctness value; and   a transmission device including: a receiving device that is connected with the terminal through a communication line, and receives the frame from the terminal through the communication line; a second storage device that stores second time information and transmission delay time (RTD) of reciprocation with the terminal; and a transmitter that transmits the correction value based on the second time information and the transmission delay time (RTD) of the reciprocation, and the correction value modified based on the difference between reception time relative to the second time information of the frame and the expected arrival time information in the received frame to the terminal through the communication line.   
     
     
         2 . The communication system according to  claim 1 ,
 wherein the terminal corrects the first time information within the first storage device in response to the reception of the modified correction value, and the transmitter of the terminal transmits a frame including new expected arrival time information based on the corrected first time information and the modified correction value.   
     
     
         3 . The communication system according to  claim 1 ,
 wherein when the difference between reception time relative to the second time information of the frame and the expected arrival time information in the received frame exceeds a specific value determined correspondingly to time accuracy demanded from the terminal, the transmitter of the transmission device transmits the modified correction value to the terminal through the communication line.   
     
     
         4 . The communication system according to  claim 1 ,
 wherein the second time information stored in the second storage device is distributed from GPS and the like that satisfy time accuracy demanded from the terminal, and updated by a clock within the transmission device, and the first time information stored in the first storage device is updated by a clock within the terminal.   
     
     
         5 . The communication system according to  claim 1 ,
 wherein the terminal is an ONU (Optical Network Unit), and the transmission device is a PON (Passive Optical Network) system that is configured with an OLT (Optical Line Terminal) that is connected with the ONU through an optical network as the communication line and has ranging means that determines transmission delay time (RTD) of the reciprocation.   
     
     
         6 . A transmission device comprising:
 a receiving device that is connected with a terminal through a communication line, and receives a frame including expected arrival time information of the frame from terminal through the communication line;   a storage device that stores time information and transmission delay time (RTD) of reciprocation with the terminal; and   a transmitter that transmits a time correction value of the terminal based on a difference between reception time relative to the time information of the frame and the expected arrival time information in the received frame to the terminal through the communication line.   
     
     
         7 . The transmission device according to  claim 6 ,
 wherein when the difference between reception time relative to the time information of the frame and the expected arrival time information in the received frame exceeds a specific value determined correspondingly to time accuracy demanded from the terminal, the transmitter of the transmission device transmits the time correction value to the terminal through the communication line.   
     
     
         8 . The transmission device according to  claim 6 ,
 wherein the time information is distributed from a GPS and the like satisfying time accuracy demanded from the terminal, and updated by a clock within the transmission device.   
     
     
         9 . The transmission device according to  claim 6 ,
 wherein the terminal is an ONU (Optical Network Unit) in a PON (Passive Optical Network) system, and the transmission device is an OLT (Optical Line Terminal) in the PON (Passive Optical Network) system that is connected with the ONU through an optical network as the communication line and has ranging means that determines the transmission delay time (RTD) of the reciprocation.   
     
     
         10 . A terminal comprising:
 a storage device that stores time information;   a transmitter that is connected with a transmission device through a communication line, and transmits a frame including expected arrival time information of the frame to the transmission device; a receiving device that is used along with the time information to create expected arrival time information of the frame, and receives a time correction value created by the transmission device based on a difference between the expected arrival time information and time information owned by the transmission device from the transmission device through the communication line; and   correction means that correct time information stored in the storage device based on the received time correction value.