Comparisons of FEC and Codec Robustness on VoIP Quality and Bandwidth Efficiency - PowerPoint PPT Presentation

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

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


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 (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
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 nT 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
    (560 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) lt 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) codes 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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