Transmission system, sending device, receiving device, and transmission method

A sending device comprising:

• a first sending unit that sends data at a first speed;
• and
• a second sending unit that sends, at a second speed that is lower than the first speed, some of the data sent from the first sending unit.

A transmission method executed by a transmission system that includes a sending device that sends data at a first speed and a receiving device that receives, by using a plurality of clocks having different phases, the data sent from the sending device at a first speed, the transmission method comprising:

• sending, performed by the sending device, at a second speed that is lower than the first speed, some of the data sent at the first speed;
• and
• changing, performed by the receiving device, in accordance with determination as to whether the content of the received data sent at the second speed matches a corresponding portion of the data that is received by using the plurality of clocks, the timing at which the data sent at the first speed is received.