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)
Grade
Quality 5
Excellent 4
Good 3
Fair 2
Poor 1
Bad

4. FEC and IP Header Overhead
An (n,k) FEC code has (n-k)/k overhead
Typical IP/UDP/RTP header is 40 bytes
codec
media pkt size (T=30ms)
rmedia
rIP
iLBC
(4,2) FEC
54 bytes
14.4 kb/s
25.1 kb/s
108 bytes
28.8 kb/s
39.5 kb/s
G.729
30 bytes
8 kb/s
18.7 kb/s
60 bytes
16 kb/s
26.7 kb/s
G.723.1
24 bytes
6.4 kb/s
17.1 kb/s
48 bytes
12.8 kb/s
23.5 kb/s

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 wins for higher loss

15. G (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 FEC better for high loss
Example: max bandwidth 14 kb/s
Consider delay impairment (use MOSc)

17. Max Bandwidth: 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