THE INFLUENCE OF CELL LOSS PATTERNS AND OVERHEADS ON RETRANSMISSION CHOICES IN BROAD-BAND ISDN

In this paper, we consider retransmission-based error recovery schemes for broadband ISDN. Retransmission performance is affected by patterns of cell loss behaviour and by the amount of overhead required. In B-ISDN, data are transmitted in frames, each consisting of a number of ATM cells. When cell losses are detected within a frame, we may either request the retransmission of the cells that were lost (selective cell-based retransmission) or the retransmission of the frame in its entirety (selective frame-based retransmission). The former policy appears to result in the retransmission of fewer cells, but incurs a higher overhead per unit of user data transmitted. The latter policy may incur far less processing overhead per retransmission request, but also incurs the risk that cells may be lost during one or more reattempts at transmission also. We shall show that the bandwidth efficiency of selective cell-based retransmission is worse than that of selective frame-based retransmission when the cell loss rate is less than a reasonable threshold. This indicates that selective frame-based retransmission is the better policy in reasonable operating ranges. When the cell loss rate exceeds that threshold, the network service is so poor that there is little to choose between the two policies. However, a Markovian model of a space-division switch shows that cell losses tend to be bunched within frames, especially when the cell loss rate is high, suggesting that a selective frame-based retransmission policy is appropriate under heavy congestion also.