Abstract: A data communications system employing sliding window protocol where the size of the window of the sending or receiving station can be made selectable by the user according to the speed, length or error rate of the communication link or the frame size used to maximize the utilization of the communication link. The negative acknowledgements sent by the receiving station specifies the upper and lower limit of a range of identification numbers of frames unsuccessfully received to increase efficiency. Before data is transmitted, the sending and receiving stations exchange the preferred sets of link parameters and generate a modified set of link parameters to resolve potential conflicts. Either the sending or the receiving station stores a table defining the frame sizes for use with different bit error rates of the communication link. The station then evaluates the current bit error rate to select the optimum frame size from the table and adjust the frame size accordingly.