The objective of this paper is to compare the performance of the different operational modes of the standard data link-control procedure HDLC: The Normal Response Mode (NRM), an unbalanced procedure with primary and secondary stations which have different functional capabilities, and the Asynchronous Balanced Mode (ABM), a balanced procedure with functionally equivalent, so-called combined stations. Performance is measured in terms of: (1) the maximum throughput to be achieved over a point-to-point link, and (2) the mean transfer time of messages as a function of the link utilization. To obtain an accurate account of the performance of these procedures, the approach taken is to implement the two protocols in a simulation model of a data link, i.e., including all details of the sequencing mechanism and of the error-recovery procedures. The general conclusion from the study is that throughput and delay performance of ABM is better than, or at least equal to NRM. However, there exists a broad range of parameters where the performance of both modes is equivalent. © 1982.