Title: Comparisons of FEC and Codec Robustness on VoIP Quality and Bandwidth Efficiency
1Comparisons of FEC and Codec Robustness on VoIP
Quality and Bandwidth Efficiency
- Wenyu Jiang
- Henning Schulzrinne
- Columbia University
- ICN 2002, Atlanta, GA
- Aug 29, 2002
2Introduction 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?
3Metric 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
4FEC 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 (T30ms) rmedia rIP
iLBC (4,2) FEC 54 bytes 14.4 kb/s 25.1 kb/s
iLBC (4,2) FEC 108 bytes 28.8 kb/s 39.5 kb/s
G.729 (4,2) FEC 30 bytes 8 kb/s 18.7 kb/s
G.729 (4,2) FEC 60 bytes 16 kb/s 26.7 kb/s
G.723.1 (4,2) FEC 24 bytes 6.4 kb/s 17.1 kb/s
G.723.1 (4,2) FEC 48 bytes 12.8 kb/s 23.5 kb/s
5Predicting 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
6Predicting MOS in VoIP, contd
- Example mappings
- From loss and delay to their impairment scores
and to MOS
7Predicting 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 nT for an (n,k) code
- Compute R value and translate to MOS
8Quality 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
9G.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
(560 300ms)
10G.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
11G.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
12G.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
13Comparison with G.723.1
- MOS(G.723.1) lt MOS(iLBC) at zero loss
- ? iLBC dominates more low loss areas compared
with G.729, whether delay is considered or not
14G.723.1(3,1) vs. iLBC(3,2)
- iLBC is still better for low loss
- G.723.1 wins for higher loss
15G.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) codes performance essentially saturates
16The 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)
17Max Bandwidth 21-28 kb/s
18Effect 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
19Conclusions
- 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
20Future 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
21References
- 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