1. Network and Systems Laboratory nslab.ee.ntu.edu.tw Te-Yuan Huang, Kuan-Ta Chen, Polly Huang Network and Systems Laboratory National Taiwan University Institute of Information Science Academia Sinica, Taiwan INFOCOM, 2009

2. Network and Systems Laboratory nslab.ee.ntu.edu.tw Motivation Voice traffic is sensitive to network impairment Why VoIP sending rate is important? Most important factors on user satisfaction Sending Rate and its Variation High and Stable voice quality Why adapting sending rate is difficult? Aggressively? Conservatively?

3. Network and Systems Laboratory nslab.ee.ntu.edu.tw Motivation – Cont. Skype – one of the most popular VoIP software Q1: How Skype adapts its voice traffic? Q2: Is their mechanism good enough? Q3: How can Skype’s policy be improved?

4. Network and Systems Laboratory nslab.ee.ntu.edu.tw Related Work Skype’s voice traffic is governed by: [Bonfiglio et al.] Bit Rate Framing Time Redundant Data PC-PSTN calls G.729 PC-PC calls iSAC Only Redundant Data is controlled by Skype

5. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Motivation Related Work How does Skype adapt the redundancy ratio? Is Skype’s mechanism good enough? How can we do better? Conclusion

6. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Motivation Related Work How does Skype adapt its redundant data? Is Skype’s mechanism good enough? How can we do better? Conclusion

7. Network and Systems Laboratory nslab.ee.ntu.edu.tw Experiment Setup PC-PC(iSAC) PC-PSTN(G.729) Only observing UDP sessionsVoice Input : American English Speech Samples from Open Speech Repository

8. Network and Systems Laboratory nslab.ee.ntu.edu.tw Observation G.729 (PC-PSTN) Constant bit rate Constant framing time 2% 0% 1% 0% 3% 4% 5% 6% 7% 8% 9% 10% As network loss rate , number of packets with doubled payload size also 

9. Network and Systems Laboratory nslab.ee.ntu.edu.tw Redundancy Ratio Definition The percentage of packets that carry redundant voice data 1234 Redundancy Ratio = 2/4 = 0.5

10. Network and Systems Laboratory nslab.ee.ntu.edu.tw Estimate Redundancy Ratio of G.729 2% 0% 1% 0% 3% 4% 5% 6% 7% 8% 9% 10% the latter is used in Taiwan and Hong Kong, and the former is used more often in Mainland Set Threshold at 40 bytes G.729 (PC-PSTN) Constant bit rate Constant framing time

11. Network and Systems Laboratory nslab.ee.ntu.edu.tw Identify Redundancy Ratio Redundancy Ratio of G.729 (PC-PSTN Calls)

12. Network and Systems Laboratory nslab.ee.ntu.edu.tw Identify Redundancy Ratio Redundancy Ratio of iSAC (PC-PC Calls)

13. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Motivation Related Work How does Skype adapt its redundancy ratio? Is Skype’s mechanism good enough? How can we do better? Conclusion

14. Network and Systems Laboratory nslab.ee.ntu.edu.tw Skype’s Redundancy Control Algorithm Adapt to network loss rate Adapt to other factors? Codec Network Loss Burstiness

15. Network and Systems Laboratory nslab.ee.ntu.edu.tw Effect of Codec iSAC G.729 Skype’s mechanism doesn’t take the difference of codec into consideration

16. Network and Systems Laboratory nslab.ee.ntu.edu.tw Effect of Network Loss Burstiness G.729 (PC-PSTN) BR=1 BR=1.5 BR=2 Skype’s mechanism doesn’t take the level of burstiness into consideration

17. Network and Systems Laboratory nslab.ee.ntu.edu.tw Outline Motivation Related Work How does Skype adapt its redundancy ratio? Is Skype’s mechanism good enough? How can we do better? Conclusion

18. Network and Systems Laboratory nslab.ee.ntu.edu.tw Optimal Redundancy Control Policy What’s the Optimal Policy? Minimum amount of redundancy data we need to sustain the same audio quality under different network conditions

19. Network and Systems Laboratory nslab.ee.ntu.edu.tw Emulation Flow Voice Input : American English Speech Samples from Open Speech Repository G.729 Optimal Redundancy Ratio

20. Network and Systems Laboratory nslab.ee.ntu.edu.tw Skype vs. Optimal – G.729 Burst Ratio = 1 Skype

21. Network and Systems Laboratory nslab.ee.ntu.edu.tw Optimal Redundancy for the Burst Ratio G.729, MOS=3.5 BR=1 BR=1.5 BR=2 Skype

22. Network and Systems Laboratory nslab.ee.ntu.edu.tw Modeling Optimal Redundancy Ratio Based on the targeted voice quality Take codec and burstiness into consideration Optimal Policy for G.729, targeted MOS=3.5

23. Network and Systems Laboratory nslab.ee.ntu.edu.tw Conclusion Explore how Skype adapts its voice traffic Redundancy Ratio Skype’s policy does not factor in the individual codec and loss patterns in to consideration Propose a general model for optimal policy Consistent user satisfaction Extensible to general VoIP software

24. Network and Systems Laboratory nslab.ee.ntu.edu.tw