Abstract : The throughput performance of several ARQ schemes was evaluated. The purpose is to provide data-communication design engineers with information to help them choose error-control techniques or to assess the performance of their proposed ARQ or hybrid designs. The schemes considered included the stop-and- wait, the go-back-N, and the selective-repeat schemes and some variations of these. In addition, the performance of a hybrid scheme that uses the best properties of both the ARQ scheme and the forward-error-correction (FEC) scheme was assessed. Most of the equations derived for computing throughput efficiency (eta) or optimal blocklength were obtained as direct modifications or extensions or equations found in the published literature. The equations were derived primarily for random-error channel models, although some burst-error conditions were considered. Among items yet to be studied are the optimal transmission rate and blocklength to maximize the throughput (in bits per second).