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Comparisons of FEC and Codec Robustness on VoIP Quality and Bandwidth Efficiency

This study analyzes the impact of Forward Error Correction (FEC) and robust codecs on the quality and bandwidth efficiency of Voice over IP (VoIP) systems. It compares various codecs and their performance under different loss conditions and bandwidth constraints.

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Comparisons of FEC and Codec Robustness on VoIP Quality and Bandwidth Efficiency

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  1. Comparisons of FEC and Codec Robustness on VoIP Quality and Bandwidth Efficiency Wenyu Jiang Henning Schulzrinne Columbia University ICN 2002, Atlanta, GA Aug 29, 2002

  2. Introduction to VoIP • The Internet is still best-effort • Subject to packet loss and delay jitter • Options for repairing packet loss • Forward error correction (FEC) • Low complexity; bit-exact recovery • Packet loss concealment (PLC) • Receiver-only; no extra BW overhead • More robust (error resilient) codec  better PLC quality, and higher bit-rate • Question: use FEC or a more robust codec?

  3. Metric of VoIP Quality • Mean Opinion Score (MOS) [ITU P.830] • Obtained via human-based listening tests • Listening (MOS) vs. conversational (MOSc)

  4. FEC and IP Header Overhead • An (n,k) FEC code has (n-k)/k overhead • Typical IP/UDP/RTP header is 40 bytes

  5. Predicting MOS in VoIP • The E-model: an alternative to human-based MOS estimation • Do need a first-time calibration from an existing human MOS-loss curve • In VoIP, the E-model simplifies to two main factors: loss (Ie) and delay (Id) • A gross score R is computed and translated to MOS. • Loss-to-Ie mapping is codec-dependent and calibrated

  6. Predicting MOS in VoIP, contd • Example mappings • From loss and delay to their impairment scores and to MOS

  7. Predicting MOS under FEC • Compute final loss probability pf after FEC [Frossard 2001] • Bursty loss reduces FEC performance • Increasing the packet interval T makes FEC more efficient under bursty loss [Jiang 2002] • Plug pf into the calibrated loss-to-Ie mapping • FEC delay is n*T for an (n,k) code • Compute R value and translate to MOS

  8. Quality Evaluation of FEC vs. Codec Robustness • Codecs under evaluation • iLBC: a recent loss-robust codec proposed at IETF; frame-independent coding • G.729: a near toll quality ITU codec • G.723.1: an ITU codec with even lower bit-rate, but also slightly lower quality. • Utilize MOS curves from IETF presentations for FEC MOS estimation • Assume some loss burstiness (conditional loss probability of 30%) • Default packet interval T = 30ms

  9. G.729+(5,3) FEC vs. iLBC • Ignoring delay effect, a larger T improves FEC efficiency and its quality • When considering delay, however, using a 60ms interval is overkill, due to higher FEC delay (5*60 = 300ms)

  10. G.729+(5,2) vs. iLBC+(3,2) • When iLBC also uses FEC, and still keeping similar gross bit-rate • G.729 still prevails, except for low loss conditions when considering delay

  11. G.729+(7,2) vs. iLBC+(4,2) • Too much FEC redundancy (e.g., for G.729)  very long FEC block and delay  not always a good idea • iLBC wins in this case, when considering delay

  12. G.729+(3,1) vs. iLBC+(4,2) • Using less FEC redundancy may actually help, if the FEC block is shorter • Now G.729 performs similar to iLBC

  13. Comparison with G.723.1 • MOS(G.723.1) < MOS(iLBC) at zero loss  iLBC dominates more low loss areas compared with G.729, whether delay is considered or not

  14. G.723.1+(3,1) vs. iLBC+(3,2) • iLBC is still better for low loss • G.723.1 wins for higher loss

  15. G.723.1+(4,1) vs. iLBC+(4,2) • iLBC dominates in this case whether delay is considered or not, • (4,2) code already suffices for iLBC • (4,1) code’s performance essentially “saturates”

  16. The Best of Both Worlds • Observations, when considering delay: • iLBC is usually preferred in low loss conditions • G.729 or G.723.1 + FEC better for high loss • Example: max bandwidth 14 kb/s • Consider delay impairment (use MOSc)

  17. Max Bandwidth: 21-28 kb/s

  18. Effect of Max Bandwidth on Achievable Quality • 14 to 21 kb/s: significant improvement in MOSc • From 21 to 28 kb/s: marginal change due to increasing delay impairment by FEC

  19. Conclusions • Compared listening and conversational MOS achieved by conventional vs. robust codecs, with same BW constraint • iLBC is better under low loss conditions • Conventional codec + FEC is better under high loss, but • Usefulness of FEC redundancy saturates beyond a certain point considering delay • At roughly a max BW of 21 kb/s • Reveals max achievable quality with current FEC mechanism

  20. Future Work • Implement the MOS prediction and optimization procedure in software • Investigate the effect of jitter on conventional vs. robust codecs • FEC cannot reduce jitter unless there are many out-of-order packets • PLC in a robust codec like iLBC incurs a much lower delay, thus may be preferable to FEC

  21. References • W. Jiang and H. Schulzrinne, Comparison and optimization of packet loss repair methods on VoIP perceived quality under bursty loss, NOSSDAV 2002 • P. Frossard, FEC performance in multimedia streaming, IEEE Comm Letter 3/2001 • ITU-T, Subjective performance assessment of telephone-band and wideband digital codecs, Recommendation P.830 2/1996

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