Communication method, communication apparatus, and communication system

A communication apparatus for sharing a transmission channel with a first communication device that transmits a first synchronous signal for synchronizing a plurality of communication devices including at least a second communication device, said communication apparatus comprising:

• a receiving unit that receives, from the second communication device, information based on a transmission time of the first synchronous signal in the event that the first communication device transmits the first synchronous signal to the second communication device;
• a transmission control unit that estimates, on the basis of both the information and a cycle of the first synchronous signal, a period in which there is no synchronous signal on the transmission channel;
• and
• a transmission unit that transmits data to the transmission channel within the period, wherein:
• the receiving unit receives, through the transmission channel, the information based on the transmission time at a first given cycle, the communication apparatus and the first communication device being included in a first power line network, and receives second data including second time information based on a second transmission time when a second synchronous signal is transmitted to the transmission channel from the second communication device at a second given cycle, the second communication device and another communication device except for the communication apparatus being included in a second power line network, and
• the transmission control unit estimates the period on the basis of the first time information, the second time information, the first given cycle, and the second given cycle, which are received by the receiving unit, and controls the transmitting unit to transmit data within the estimated period.