Institute for computer Communication and Applications

1. Adaptive Delay Aware Error Control for Internet telephonyCatherine BoutremansJean-Yves Le BoudecIP Telephony Workshop’2001 Institute for computer Communication and Applications phone: + 41 21 693.5258; fax: +41 21 693.6610 Catherine.Boutremans@epfl.ch http://icawww.epfl.ch

2. Framework • Real-time audio over Best Effort networks suffers from varying packet loss rates, delays and available bandwidth. • Forward Error Correction (FEC) is an efficient way to recover from packet losses but : •  bandwidth requirement • end-to-end delay • Adaptive rate/error control was proposed but it does not try to optimize the end-to-end delay

3. Motivation • Above a certain threshold (around 150ms): the end-to-end delay is annoying • New differentiated services, such as Alternative Best Effort offer applications the trade-off between receiving lower end-to-end delay (and higher loss rates) or more overall throughput. important to take the end-to-end delay into account in the adaptive control scheme

4. Adaptive Delay aware error control • Problem definition:develop an error control scheme for audio which is delay aware (namely, which chooses the FEC according to its impact on the end-to-end delay) for Best Effort Internet services such as: 1) Flat 2) ABE • Out of scope:validation of perceptual models

5. Outline • A.Our Joint rate/error/delay control scheme • 1. Error recovery taken from media specific FEC • 2. An RTCP-based Rate control scheme • 3. A new Delay control scheme • 4. Our Global optimization problem • B.Simulation examples • 1. Flat Network: what do we improve? • 2. ABE: Is it worth to be green?

6. Start with high rate audio encoding • copy audio, compress, and delay • if packet lost, recover from redundancy ... ... packet stream 5 4 4 3 3 2 3 reconstruction secondary/redundancy encoding 5 4 1.How media specific FEC Works Audio Frame 4 Audio Frame 3 Qmax Qmax Dp Dp Dr Dr Quality Rp Rr Rr Rp Rmax Rmax rate primary/source encoding

7. p 1-p 0 1 1-q q 1.Efficiency of FEC mainly depends on packet loss process • Loss process of audio packets in the Internet can be modeled as low order Markov chain • In this work: Gilbert Model

8. 2.Our RTCP-based Rate Control scheme • Audio streams have to share bandwidth fairly with TCP connections  TCP-Friendly • Equation-Based TCP Friendly rate control (Padhye): • Rate controlled via packet size (constant packet intervals) • Use RTCP for Feedback • EWMA filter of PLR in order to smooth the rate

9. 2. Our Rate control achieves fairness Our scheme shares bandwidth fairly withTCP connections

10. 3.Quality is function of rate and delay • Audio quality is function of end-to-end delay: decreases rapidly above 150ms • Audio quality is function of reconstructed rate (SNR,MOS,...)

11. 3.Quality is function of rate and delay • Consider the user perceived quality (utility) as a function of the reconstructed rate (R) and the overall end-to-end delay (D) • We use several utility curves as we don’t know which one is the best.

12. 4.Our Joint rate/error/delay control Consider: • A source with flexibility to encode audio at rate • Unreliable network characterized by • a loss distribution  Gilbert Model : r.v. • a delay distribution • an available bandwidth TCP-Friendly rate constraint • A utility function ofdelay and reconstructed rate

13. 4.Our Joint rate/error/delay control Define: • K =  of copies of audio segment sent over the network • ,the delay spent on FEC • the r.v. the set of copies that make it across the network Constrained optimization problem: Under

14. 4.Our Joint rate/error/delay control Solution: • General solution is derived using Lagrange Multipliers for small values of K and via numerical method SQP for K4 • The solution has the following properties: • if (p+q)1  it pays to offset and to put more quality into the end packets • if (p+q)1  better not to offset and

15. B.Simulation examples • Single bottleneck • Topology with small and long flows • Bottleneck BW = 15Mbps, 5Mbps • variable number of connections d2 d1 d1 d2

16. Example 1: when delay is important

17. Example 2: when delay is not important

18. Example 3: tradeoff between delay and audio distortion

19. 1. FLAT: Conclusion • The Delay aware scheme increases utility by avoiding that the source waste delay on the FEC when it is not really needed.

20. 2.ABE: Alternative Best Effort • ABE is a novel service for IP networks which offers the choice between receiving a lower end-to-end delay or more overall throughput. • Packets are marked either green or blue. • Green packets receive a low, bounded queuing delay but they receive more losses during bouts of congestion. • Blue packets receive more throughput (and less losses) but also more delay jitter.

21. Is it worth being green? • Green packets receive a lower delay but they experience more losses (and hence, receive less throughput). • Losses are repaired using FEC but FEC increases the e2e delayand the BW requirement.

22. With small RTT, difference is minor.

23. Higher RTT (2 bottlenecks): green is better if load not too high.

24. Higher RTT (1 bottleneck): green always better

25. 2. ABE: Conclusions • It is worth accepting to receive less bandwidth (and more losses) except in trivial cases where • the RTT is small anyway • the network is badly congested • Need for adaptive Color choosing algorithms

26. Conclusions • We proposed an adaptive Delay aware error control scheme. • We showed it could prevent a source from wasting delay on FEC when not necessary. • It helped us to figure out that it was worth trading throughput for delay.