To achieve effective use of bands of a data transmission line in a communication system for transmitting data between sub modems via a main modem.
A communication system transmits data among sub modems 1-1 to 1-3 to which terminal devices 2-1 to 2-3 are connected via a main modem 4. The main modem 4 includes: a means for generating and sending a beacon signal to be broadcasted to the sub modems 1-1 to 1-3; a means for receiving and identifying a beacon ACK signal from the sub modem in response to the beacon signal; and a transmission band assignment control means for identifying the sub modem that transmits the beacon ACK signal to notify assignment of a data transmission band, and each of the sub modems 1-1 to 1-3 includes a means for generating and sending the beacon ACK signal in response to the beacon signal from the main modem 4, and a means for transmitting data in accordance with the assignment of the data transmission band from the main modem 4.
Shoji Kashiwaya
Koichi Watanabe
Toshiro Ito