Abstract: Data communications terminals efficiently transfer image and audio data across a data circuit without being affected by the degree of congestion of the data circuit by altering, in accordance with the degree of congestion, the image and sound qualities and/or the transfer rate of each frame while maintaining minimum preferred qualities of the image and audio data. To do this, a transmitting terminal (2) and a receiving terminal (4) have the same data transfer capabilities. The data transfer capability include a transmission capability that enables regulation of the image and sound qualities and/or frame rate of each data frame to be transmitted from the transmitting terminal (2) to the receiving terminal (4) based on whether the measured data transmission time required to transmit a respective data frame has increased or not.