Queuing analysis of simple FEC schemes for voice over IP

In interactive voice applications, FEC allows to recover from losses. FEC schemes need to be simple in order to allow interactivity. We study a simple scheme implemented in FreePhone audio tool [High Speed Networking Group, INRIA, Sophia Antipolis, http://www-sop.inria.fr/rodeo/fphone] and RAT: Robust Audio Tool [Multimedia Integrated Conferencing for European Researchers, University College, London], where for every packet n, redundant information is added in packet n+. The quality of a reconstructed copy depends on the amount of redundancy. Using queuing analysis we obtain simple expressions for the audio quality as a function of the amount of redundancy. We show that the expected amount of useful information deteriorates for any amount of FEC and any . We find conditions under which FEC leads to quality improvement.