A system for transmitting data between a first device and a second device, the first device being arranged to transmit data to the second device and the second device being arranged to transmit an acknowledgement to the first device if the data transmitted by the first device is properly received by the second device; wherein the system comprises means to measure the quality of reception of acknowledgements by the first device and to modify one or more acknowledgement transmission parameters depending on the measurement. The parameters may include: transmission rate, degree of forward error correction coding, and output power. The parameters may be modified to improve the quality of reception of acknowledgements if the measured quality falls below a threshold. The means to measure the quality of reception may comprise means to determine a measure of the number, or relative number, of acknowledgements that were not properly received by the first device. In an embodiment the first device is arranged to retransmit the data if an acknowledgement is not properly received, and the means to determine a measure of the number of acknowledgements that were not properly received by the first device comprises means to determine the number of data packets that have been received by the second device which were previously properly received by the second device.