Adaptive Playout

1. Adaptive Playout

2. You are Here Encoder Decoder Middlebox Receiver Sender Network

3. How to recv and play? open socket while not done if socket is readable read packet from socket remove RTP header decode play back

4. What’s Wrong? • packet ordering • packet loss • next packet arrive in-time? Especially bad for audio applications

5. Overview Decode RTP Classifier Network

6. Implementation • Single Thread • using select() • Multi-Threads

7. Packet Buffer • Sorted by sequence number • When ADU is complete, send to decoder RTP Classifier

8. Playout Buffer • Stored decoded data in playout order • Post-processing/Mixing may happens Decode

9. Why Buffer?

10. Sending Packets Packet Time

11. Receiving Packets Packet Time

12. With Jitter Packet Time

13. With Jitter Packet Time

14. What causes Jitter? • Network delay = Transmission Delay (fixed) + Propagation Delay (fixed) + Queuing Delay (variable) • Delay jitter is caused by variable queuing delay

15. Delay Jitter Transit Time small jitter large jitter Time

16. Spike Transit Time Time

17. Today’s Question • How big is the playout buffer? • When to playback?

18. Types of Applications • Non-interactive • Buffer can be large • Interactive • As small as possible

19. Types of Applications • Video • Frames are discrete (easier problem) • Audio • Samples are “continuous”

20. Naive Answer • How big is a buffer? • Fixed at a small value, to reduce latency • When to playback? • Playback as soon as possible, to reduce latency

21. A Brief Introduction to Audio Conferencing

22. Audio Conferencing • Live, interactive application • Latency is important • Each packet 20-30ms of audio

23. Silence Suppression • Silence Detection • if no sound, no need to send • Talk spurt • consecutive audio packets (between silence) • hundreds of ms

24. Demo

25. Recall: RTP Header • marker bit: • depends on payload • e.g. beginning of frame

26. RTP and Talkspurt • First packet of a talkspurt will have marker bit set to 1

27. RTP and Talkspurt • Deduce talkspurt from sequence number and timestamp SeqNo TimeStamp 1 20 2 40 3 60 5 190

28. Consequences of Talkspurt • Opportunity to adjust playout delay • if jitter is large, increase delay • if jitter is small, decrease delay

29. Fixed Playout Delay SEND RECV PLAY

30. Adaptive Playout Delay SEND RECV PLAY

31. Adaptive Playout Delay SEND RECV PLAY

32. Trade-Off Latency vs. Packet Loss

33. Latency vs Loss-Rate Loss Rate Latency

34. Adaptive Playout Mechanisms for Packetized Audio Applications in WAN R. Ramjee, J. Kurose, D. Towsley, H. Schulzrinne INFOCOM 1995

35. Variables and Notations

36. Variables and Notations Tnet(i) Tbuf(i) Tarrive(i) Tplay(i) Tdelay(i) Tsend(i)

37. 1st Packet in Talkspurt We can estimate as

38. How to estimate Vnet(i)

39. How to estimate Enet(i) • Method 1: Jacobson’s Method

40. Spike Tnet Time

41. Problems • Does not react to spike fast enough

42. How to estimate Enet(i) • Ramjee’s Method SPIKE NORMAL

43. Ramjee’s Idea if Tnet(i) suddenly increase SPIKE NORMAL if “slope” is small enough

44. In Spike Mode SPIKE Tnet i

45. In Normal Mode NORMAL Tnet i

46. Evaluations Loss Rate Delay

47. Problems with Ramjee’s Method Transit Time Time

48. Packet Audio Playout Delay Adjustment: Performance Bounds and Algorithms S. Moon, J Kurose, D. Towsley Multimedia Systems 1998

49. Recall Previous Methods

50. How to Set Tdelay(i) • Moon’s Method • Collect statistics on packets that have arrived. • Find t such that q% of last w packets have Tnet(i) < t.