Abstract: Data messages are transmitted between host data processors and a communications processor. Data messages from a communications processor are transformed for the host processor to which the message is assigned. Data messages from a host processor are transformed for the communications processor. Protocol is established between (a) untransformed data messages from the communications processor and transformed data messages for the communications processor, and (b) untransformed data messages from each host processor and transformed data messages for each host processor. A device interface connects a plurality of host processors to a plurality of buses, each bus being connected to one communications processor. The device interface handles the protocol, transformation and control function in parallel so that messages between a given host processor and terminal connected to a communications processor are processed through a device interface.

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.