Queueing analysis of simple FEC schemes for IP telephony

In interactive voice applications, FEC schemes are necessary or the recovery from packet losses. These schemes need to be simple with a light coding and decoding overhead in order to not impact the interactivity. The objective of this paper is to study a well known simple FEC scheme that has been proposed and implemented [1], [2], in which for every packet _ , some redundant information is added in some subsequent packet ___ If packet _ is lost, it will be reconstructed in case packet ___ is well received. The quality of the reconstructed copy of packet _ will depend on the amount of information on packet _ we add to packet ___ . We propose a detailed queueing analysis based on a ballot theorem and obtain simple expressions for the audio quality as a function of the amount of redundancy and its relative position to the original information. The analysis shows that this FEC scheme does not scale well and that the quality will finish by deteriorating for any amount of FEC and for any offset delay is large so that a retransmission deteriorates the overall quality.