A four-state probabilistic automaton which serves as a simulation model of the NRM, HDX HDLC procedure with Poll/Final bit error recovery procedure and Primary Reply Timer is introduced. By reducing states to obtain a two-state automatoon, a point-to-point throughput equation is developed for various values of the length of the information part of the HDLC I frame, communication line quality, modern turnaround time, time-out duration of Primary Reply Timer is introduced. By reducing states to obtain a two-state such as one-way, two-way alternate file transfers. A case study based on the IIASA data communication network is used to examine the impact of time-out duration on the throughput.