Simulation study of two-level forward error correction for lost packet recovery in B-ISDN/ATM

The major source of errors in B-ISDN/ATM systems is expected to be buffer overflow during congested conditions, resulting in lost packets. A single lost or errored ATM cell will cause retransmission of the entire packet data unit (PDU) that it belongs to. The performance of the end-to-end system can be made much less sensitive to cell loss by means of forward error correction. In this paper, we present the results of a simulation study for an ATM network where forward error correction is performed at both the cell level and the PDU level. The results indicate that (i) cell losses are highly correlated in time, and analytical models ignoring this fact will not yield accurate results, (ii) the correlation of cell losses is similar to burst errors in digital communication, and similar code interleaving techniques should be used, (iii) coding cells and PDUs separately provides this interleaving effect, and this joint code outperforms coding only at the cell level or only at the PDU level in almost all cases simulated

Soft ARQ for layered streaming media

A growing and important class of traffic in the Internet is so-called `streaming media,' in which a server transmits a packetized multimedia signal to a receiver that buffers the packets for playback. This playback buffer, if adequately sized, counteracts the adverse impact of delay jitter and reordering suffered by packets as they traverse the network, and if large enough also allows lost packets to be retransmitted before their playback deadline expires. We call this framework for retransmitting lost streaming-media packets `soft ARQ' since it represents a relaxed form of Automatic Repeat reQuest (ARQ). While state-of-the-art media servers employ such strategies, no work to date has proposed an optimal strategy for delay-constrained retransmissions of streaming media-specifically, one which determines what is the optimal packet to transmit at any given point in time. In this paper, we address this issue and present a framework for streaming media retransmission based on layered media representations, in which a signal is decomposed into a discrete number of layers and each successive layer provides enhanced quality. In our approach, the source chooses between transmitting (1) newer but critical coarse information (e.g., a first approximation of the media signal) and (2) older but less important refinement reformation (e.g., added details) using a decision process that minimizes the expected signal distortion at the receiver. To arrive at the proper mix of these two extreme strategies, we derive an optimal strategy for transmitting layered data over a binary erasure channel with instantaneous feedback. To provide a quantitative performance comparison of different transmission policies, we conduct a Markov-chain analysis, which shows that the best transmission policy is time-invariant and thus does not change as the frames' layers approach their expiration times

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

Ankara : Department of Electrical and Electronics Engineering and the Institute of Engineering and Science of Bilkent University, 1995.Thesis (Ph.D.) -- Bilkent University, 1995.Includes bibliographical references leaves 81-83.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.Oğuz, Nihat CemPh.D