language: en

AFEC: An Adaptive Forward Error-Correction Protocol and Its Analysis

Kihong Park
Department of Computer Sciences Purdue University
West Lafayette, IN 47907
park@cs.purdue.edu CSD-TR 97-038

Abstract This paper presents an adaptive protocol for packet-level forward error-correction in dynamic networks. The objective is to facilitate best-effort real-time applications whose timing constraints rule out the use of retransmission-based ARQ schemes. The degree of redundancy is adjusted as a function of network state, decreasing when the network is well-behaved and increasing when it is not. The control problem is nontrivial due to the fact that increased redundancy, beyond a certain level, backfires resulting in self-induced congestion which impedes the timely recovery of information at the receiver. In the first part of the paper, we present a comprehensive analysis of the control problem associated with dynamic forward error-correction, concentrating on a particular protocol called Adaptive Forward Error-Correction (AFEC). We show that instabilities can arise from two distinct sources|desired operating point location and network delay|and we give solutions to handle them. The first causal factor is intimately tied to optimality, making its achievement potentially perilous in the context of QoS-greedy applications. The second part of the paper presents simulation results that confirm the qualitative dynamics predicted by the analysis. We quantitatively estimate the redundancy-recovery rate function which relates redundancy to the quality of service rendered at the receiver. We show under what conditions the curve's shape is unimodal and to what degree. We compare the performance of AFEC against a static FEC protocol in which the redundancy factor is fixed. We show that AFEC exhibits superior performance when the network is subject to structural changes that persist for nonnegligible durations. Under short-range dependent traffic conditions, AFEC is able to closely match the performance of optimum static FEC but not exceed it.