Performance of two-level forward error correction for lost cell recovery in ATM networks

Abstract

The major source of errors in Asynchronous Transfer Mode (ATM.) networks is expected to be buffer overflow during congestion, resulting in cell losses. The large ratio of the end-to-end propagation time for a typical connection to the cell transmission time makes lost cell recovery by means of retransmissionbased error control techniques impractical especially for delay-sensitive highspeed applications. As has been shown by many authors, forward error correction is a promising alternative since it can improve end-to-end reliability without requiring retransmissions. This thesis discusses the use of a two-level forward error correction scheme for virtual channel and virtual path connections in ATM networks. The performance of the scheme, which exploits erasure correcting simple and interleaved block codes simultaneously, is studied via both analyses and simulations. For a single-node virtual channel connection, a novel and accurate discrete-time analytical cell loss model is developed first. Based on this model, the reduction in the cell loss rate achieved by two-level coding is then investigated extensively via iterative computational methods. For the case of a four-node, long-distance virtual channel connection that cannot tolerate any loss, the use of the two-level coding scheme in conjunction with an automatic repeat request mechanism is considered, and detailed simulations are made to quantify the improvement achieved in the delay-throughput performance. The results obtained indicate substantial performance improvements even for very high network loads provided that an appropriate coding technique is chosen according to the traffic characteristics. Typically, bursty traffic requires code interleaving be used for effective loss recovery whereas small-latency simple block codes suffice for random traffic. Two-level coding, which is shown to effectively combine the fast and burst loss recovery capabilities of the individual coding techniques, is attractive for traffic streams of unpredictable or time-varying characteristics.