Data transmission system and data transmission method suitable for detecting problems in image processing

A data transmission system, comprising:

• a user terminal that has a first storage device, and a simulated image processing unit, the simulated image processing unit performing a simulation process on job data;
• an image forming apparatus that has a second storage device and an image processing unit, the image processing unit performing image processing on the job data;
• and
• a relay device that has a third storage device, registers job data that is received from the user terminal in the third storage device, and then transmits the job data to the image forming apparatus;
• wherein
• the user terminal transmits first information for the job data that is stored in the first storage device to the relay device;
• the image forming apparatus registers the job data that is received from the relay device in the second storage device, and transmits second information for the job data to the relay device;
• and
• the relay device
• compares the first information with the second information, and when there is a difference in the data amounts, registers problem information that indicates that there is a problem in image processing of the simulated image processing unit or the image processing unit in the storage device.