Communication system and time synchronization method

A time synchronization method in a communication system, the communication system comprising:

• a first communication network including a time dissemination device;
• a second communication network including a time receiving device;
• a relay network that is connected to the first communication network and the second communication network, and relays communication signals of the first communication network and the second communication network;
• a first delay control device that is disposed at a first interconnection point that connects the first communication network and the relay network;
• a second delay control device that is disposed at a second interconnection point that connects the relay network and the second communication network;
• and
• a high-precision time source that sets the same time information for the first delay control device and the second delay control device,
• wherein,
• when the first delay control device receives a first communication signal which is a delay time measurement signal in a forward path extending from the time dissemination device toward the time receiving device through the relay network, the first delay control device inserts a reception time into the first communication signal as a first reception time, and transfers the first communication signal,
• the second delay control device, according to a second reception time at which the first communication signal is received, the first reception time extracted from the first communication signal, and a predetermined first delay setting value, obtains a first wait time from a difference between the first delay setting value, and a time difference between the second reception time and the first reception time,
• when the second delay control device receives a second communication signal which is a delay time measurement signal in a return path extending from the time receiving device toward the time dissemination device, the second delay control device inserts a reception time into the second communication signal as a third reception time, and transfers the second communication signal,
• the first delay control device, according to a fourth reception time at which the second communication signal is received, the third reception time extracted from the second communication signal, and the same first delay setting value as that of the first communication signal, obtains a second wait time from a difference between the first delay setting value, and a time difference between the fourth reception time and the third reception time, and
• the first delay control device and the second delay control device conduct a delay control of packet transfer by the second wait time and the first wait time, respectively, to transmit the communication signals in the same delay time as the first delay setting value in the forward path and the return path.