COMPARISONS OF FEC AND CODEC ROBUSTNESS ON VOIP QUALITY AND BANDWIDTH EFFICIENCY
Wenyu Jiang, Henning G. Schulzrinne · Networks · 2002
Packet loss and delay in the Internet degrades the quality of Voice over IP (VoIP). Forward error correction (FEC) reduces packet loss at the cost of higher delay and bandwidth. An alternative is a loss-robust voice codec, but also at the expense of higher bit-rates. We compare the perceived quality of both schemes under the same bandwidth requirement. We evaluate two conventional low bit-rate codecs, G.729 and G.723.1, and one robust codec, iLBC. G.729 with FEC generally prevails, but when considering delay impairment, iLBC is better under low packet loss. If iLBC also uses FEC, then its quality can sometimes be better than G.729 plus FEC with the same gross bit-rate. The comparison with G.723.1 is similar, except that G.723.1 is less advantageous for low packet loss because its intrinsic quality is slightly lower than iLBC. Finally, given a target bit-rate, we compute the maximum achievable MOS using any of these codecs. It exhibits an upper limit however large the target bit-rate is, due to the inherent delay impairment of FEC.