Packet audio playout delay adjustment

1. Packet audio playout delay adjustment Performance bounds and algorithms Moon, Kurose, Towsley

2. Overall Idea • Because of packet jitter/delay changes, we need a playout buffer • The bigger, the better • But, a large buffer hinders responsive transmission of audio • 400ms/5% loss for voice conversation • Interactive media/video conferencing needs the smallest buffers possible

3. A solution, and an approximation • In the first part of the paper, they give bounds on the size of playout buffer needed under certain losses • Not an online algorithm, computationally expensive (the idea is to focus on percentages) • Inelastic medium • In the second part of the paper, they present an on-line algorithm that is computationally feasible to adjust talkspurt playout delay

4. Related Work • Playout delay adjustments • Per-packet and per-talkspurt (assumptions… speech, or music?) • Network level observations • Three graphs, probe compression • Baseline doesn’t change much—real advantages in adjusting delay playout occurs in multi-talkburst delay spikes

5. Problem Statement • For a given set of losses at the receiver, we get to set the playout delays of each talkspurt anyway we want • Which assignment is the best? • For 1 packet lost? 2? 3? 134? • First, let’s fix some notation

6. Notation • tki – sender timestamp of ith packet of kth talkspurt • aki – receiver timestamp of ith packet of kth talkspurt • nk – num packets in kth talkspurt (received) • N – total number of packets in trace (Σknk) • pki(A) – playout time under algorithm A • Delay: pki(A) – tki, loss if pki(A) < aki • Indicator if packet is played: • rki(A)

7. Notation, (con’t) • Total # packets played under A • N(A) = ΣkM Σink rki(A) • Average playout delay: • 1/N(A) ΣkM Σink rki(A)(pki(A) – tki) • Loss rate: • l = (N – N(A)) / N * 100

8. Notation (con’t) • d’ki: delay between sending and receiving • d’: min (d’ki) • dki: normalized delay = d’ki – d’ • dk(i): ith smallest normalized delay

9. Off-line solution w/o collisions • To play i packets from the kth talkspurt, the playout delay must be at least (the unknowable) dk(i) • Remember that if algorithm A uses a large playout delay for one talkspurt, it could delay subsequent talkspurts (collisions) • Let’s ignore them for now • Time: O(MN2) Space: O(MN)

10. Off-line solution w/o collisions • We assume percentages of loss, not actual loss patterns (to simplify the complexity) • D(k,i) is min playout delay for i packets lost • D(k,i) = • 0 if i = 0 • dk(i) if k = M and i <= nM • inf if k = M and i > nM • min (((i-j)D(k+1,i-j) + jdk(j))/i) • Proof by contradiction

11. Offline algorithm with collisions • We might have to adjust the playout times of some of the talkspurts due to collisions, so D must now take those into account • We define a vector S (captures length of silence) • We can capture the sum of the increases • Now D includes C as well (C tracks packets played out at every step of the computation) • D now differs from the old D only in the extra delays incurred by the collisions • The new D does not capture the optimal, though (why?) • Time: O(M2N2) Space: O(M2N2)

12. An online algorithm • Algorithm 1: Linear • Slow to catch up, good at maintaining a solid value • Algorithm 2: Depends on spike detection • Quick at catching up, but sometimes overzealous • Algorithm 3: Two Modes • Track spikes when they are detected • Otherwise update delay and delay varience (q) • Switch when you have a multiple of the delay

13. Evaluation / Conclusion • They instrument the senders and the receivers • Plot average playout delay vs packet loss rate • Results seem to show that Algorithm 3 gets very close to the optimal • However, the results are very close much of the time • Sometimes 1 is much worse, sometimes 2, but 3 seems to always be pretty stable

14. Queue Monitoring A Delay Jitter Management Policy Stone, Jeffay

15. Display and e2e Jitter • Recall the steps for transmitting video: • Acquire, digitize, compress, transmit, decompressed, buffer, display • Display Latency is acquire to display • e2e latency is acquire to buffer • What problems can affect this process? • Delay Jitter (variance in e2e latency) • Can we ensure constant e2e latency? • Even with Isochronous service models? • We’re going to adjust the display latency instead

16. Audio vs video • Recall the audio application • Talkspurts vs Silence Periods • Analog for video? • Are gaps ok during the transmission? • Display perception • Network congestion • Video as a datatype • Can we repeat frames, leave black spaces, etc?

17. Late policies • I-policy: • Discard • All frames now have the same display latency • Static • E-policy: • Play at earliest convenience • Increases latency for subsequent frames • Keeps getting higher than observed e2e delay

18. 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 Example 1

19. Example 2 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10

20. I-vs-E • I policy’s advantage • Low jitter and bursts • E policy’s advantage • Good during high latency and low latency, but not good after bursts • Hybrid approach: Queue Monitoring

21. Queue Monitoring • When displaying a frame • Thresholding operation • If qlen is m, then counters 1 through m-1 are incremented • All others are reset • When the counter exceeds a value, the oldest frame is discarded • If the queue has contained more than n frames, then we can reduce the latency (the jitter is stable) • Large variations occur infrequently and smaller variations occur more frequently (still true today)?

22. Evaluation • The inherent difficulty • Gaps vs display latency • Lexocographic ordering for two axes • Average gap rate • Average display latency • Experimental Design • “academic computer science” network • Time of day, workload seen

23. Evaluation Results • Comparison between I2, I3, and E • Usually the same or better • Except for incomparable results • In comparison to the E-policy, it seems to be workload/network dependent • Instantaneous gap rate, delay policy would be better (perhaps) • More adaptive I-policy • More tests, of course • Addressing ad-hoc quality measures