In this paper, tandem links with burst errors are analyzed using the Gilbert model for individual links. The results, apart from being of general interest, are of particular relevance to the Integrated Services Digital Networks (ISDN) for which the International Telegraph and Telephone Consultative Committee (CCITT) is currently developing performance specifications based on hypothetical reference connections (HRX's), i.e., composite channels consisting of several tandem reference circuits. The average error rate, the error gap distribution, its mean, and a tight upper bound on its standard deviation are analytically derived for the composite channel (equivalent to an HRX) withNtandem links (circuits). These statistics are then related to the performance measures proposed for ISDN. Some numerical results are presented to highlight the large differences in the estimated performance with a burst-error model approach and with an independent-error approach (having identical average error probabilities).

Use of code division multiple access (CDMA) in conjunction with slotted ALOHA improves throughput, since some of the "collided" packets can be retrieved, although at the expense of enlarged bandwidth. In view of the feasibility of recovery of packets, such schemes are attractive to carry receiver-to-transmitter acknowledgment (ACK) traffic also. The presence of ACK traffic is known to significantly reduce the throughput of slotted ALOHA channels. In this correspondence, the impact of such ACK traffic on the performance of slotted ALOHACDMA schemes is examined in detail for both finite and infinite terminal population.

On most real channels hybrid error control schemes are expected to provide a throughput higher than that of automatic repeatrequest (ARQ) systems and a reliability better than forward error correction (FEC) systems. On compound channels, channels with a mixture of random and burst errors, generalized burst-trapping (GBT) codes seem to be quite effective for FEC. In this paper, a hybrid scheme with Go BackNARQ as the retransmission component and GBT code as the FEC component, is described. Its performance is analyzed in terms of throughput efficiency and undetected error probability and is compared with that of a forward-acting GBT code. Numerical calculations of the parameters are presented to illustrate the performance.

Large round trip delay associated with satellite channels reduces the throughput for automatic repeat-request (ARQ) system of error control rather drastically under high error rate conditians. Ground segments that usually accompany satellite circuits at both ends introduce bursts of errors, during which block error rates tend to be quite high, bringing down the throughput to very low values. In this paper, some schemes that modify the currently known variations of ARQ (Stop-and-Wait and continuous systems) are suggested with a view to obtaining higher throughput under high block error rate conditions. Specifically, the modified Go-Back-Nsystem appears to be quite attractive, as it gives substantial improvement with little additional complexity in system implementation.

The effectiveness of hybrid error control schemes involving forward error correction (FEC) and automatic repeat request (ARQ) is examined for satellite channels. The principal features of the channel are: large round-trip transmission delay due to the satellite link, and burst errors introduced by the terrestrial links that connect the users to the satellite link. The performance is estimated for two channels described by Fritchman's simple partitioned finite-state Markov model, and is compared to that obtainable if the channel is considered as a binary symmetric channel of the same bit error probability. Results show that the hybrid schemes offer substantial improvement over ARQ and FEC, and that an optimum exists for the number of errors corrected to obtain maximum throughput efficiency.