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The performance capabilities of several retransmission systems are compared with each other and with conventional errorcorrecting codes in hypothetical channels exhibiting independent or dependent errors. These evaluations are based on the analysis in the companion paper which admits errors in the feedback channel and undetectable errors in either channel. The basic problem of deciding whether to use retransmission or forward error-correction for a particular application is investigated. The areas of relative superiority based on throughput rate R and uncorrected error rate ρ for each type of error control are determined for a wide range of codes and channels. It is shown that in almost all burst error cases and in independent error channels with low error probabilities, retransmission sytsems are superior to forward error-correcting systems in both throughput rate R and undetected error rate ρ. In independent error channels with high error rates forward error correction shows substantial improvement over retransmission provided that extremely high reliability is not required. It is shown that in designing a retransmission logic, the decision to interpret garbled messages bearing service information as retransmission requests lowers the throughput rate slightly but improves the reliability considerably. The problem of selecting one of the basic retransmission logics for a particular application is also considered. The systems characteristics of the logics were seen to be the prime consideration in many cases since the differences in R and ρ are generally not substantial.
Date of Publication: December 1964