The study considers the effect of error rate, block length, transmission delay, and error-recovery mechanism on the efficiency of the communications channel in computer-computer communications. The systems investigated are full-duplex communications with unlimited and limited buffering, and half-duplex communications. Both random and burst errors are considered. For all cases, over a significant range of parameters, closed-form expressions for the optimum block length, and the corresponding efficiency are obtained. For random errors the optimum data-block length is found to be proportional to √(a/k), and the maximum efficiency a function ofka, whereais the message overhead andkis the error rate. The burst error results are similar.