Control Mechanisms for Video Streaming Wireless Links

1. Control Mechanisms for Video Streaming Wireless Links Athina Markopoulou Electrical Engineering Dept. Stanford University

2. t src t src ideally t network rcv t rcv loss jitter delay Real-Time Multimedia over Packet Networks • Characteristics • Continuous Stream Playout, Real-Time • Requirements • low loss, delay, delay jitter

3. Multimedia - Networking • Problems at the interface between multimedia applications and underlying network • Control mechanisms • in the network and/or • at the end-systems • Challenges depend on underlying network(s) Application Network

4. 2 1 The Bigger Picture backbone wireless

5. Wireline Wireless Access Point, Server/Proxy Mobile Terminal Server Media Streaming over Wireless Last-Hop • Streaming to • laptops, cellphones, PDAs, wireless TV displays • Challenges • limited resources, time variation • strict application requirements

6. Ideally Over wireless Tx Tx Rx Rx Play Play Control at the Tx Control at the Rx Tx Tx Rx Rx Play Play Example

7. Problem Statement p i r N • Scenario • pre-stored media content at Tx. • interference i, according to a Markov chain with • deliver and play entire content • Objective • maximize the playout quality • minimize the power cost Tx Rx

8. p i r n b s(p,i) Tx Rx System State and Controls (p,r) = systemcontrolsin current time slot (n,i,b, r’) = systemstatein current time slot n = remaining packets at Tx i = channel interference b = available packets at Rx r’ = playout rate in previous slot

9. r b Rx System Controls at Tx p i n s(p,i) Tx • Control p: transmission power in current slot • s(p,i) : probability of successful reception • Power Cost: Φ = p • battery lifetime • interference stress

10. from n=N…. Quality Power ·W + ·W … until n=0 Dynamic Programming Formulation • System Evolution • Define to be the minimum expected cost-to-go (n,i,b;r’) (p,r) (1-s(p,i))qij s(p,i)qij (n-1,j,b+1-r;r) (n,j,b-r;r)

11. Computing the Optimal Control • A stationary optimal solution (p*,r*) exists and can be obtained by value iteration • Optimal policy: • table p*(n,i,b;r’) and r*(n,i,b;r’) • obtain offline & store in lookup table

12. Special Cases: Individual Controls • Similar formulations – obtain optimal policies • Compare: no control, special cases, joint control

13. Better Performance Power-Quality Tradeoff (1) No control

14. Power-Quality Tradeoff (2) No control Playout Only

15. No control Playout Only PowerOnly Power+Rebuffering Power-Quality Tradeoff (3)

16. No control Playout Only Power Only Power+Rebuffering Power-Quality Tradeoff (4) Jointcontrol

17. Heuristics • Why heuristics? • Justified vs. ad-hoc heuristics • mimic properties of optimal control • Steps • Power-only heuristic • Playout-only heuristic • Joint power-playout heuristic

18. r b n Tx Rx n=N X n=1 b Power Heuristic p i Fix playout r=R s(p,i) • Optimal power: • Backlog pressure X(n,i,b) • has structural properties: • Heuristic: approximate • mimicking those properties ^ X

19. B2 B1 p i n fix p r s(p,i) s(i) Tx r=R/2 r=R r=0 r=R r=rl r=0 BL Bl Bl+1 b Playout - today b r Rx • Purpose: choose r(b) • Fixed Threshold Heuristic • L thresholds for buffer occupancy b

20. B2 B1 i r=R/2 r=R r=0 Playout Heuristic b r s(i) Rx • Observation: channel not taken into account yet • Adaptive Threshold Heuristic • adapt rate • and adapt thresholds

21. p n Tx Joint Power-Playout Heuristic i r b s(p,i) Rx • Tx side: • compute X(n,b) • compute power p: • Rx side: • estimate i, compute p and s(p,i) • adjust thresholds, compute playout r • feedback to Tx ^

22. Joint heuristic performs well Optimal Playout Optimal Power+Rebuffering Joint Heuristic Joint Optimal

23. Demo: no-control vs. joint heuristic no control • For the same interference scenario • For the same power consumption • Compare the playout quality Joint heuristic original

24. Controls off Comparison Details • Joint Heuristic

25. Wireless Video - Summary • Contributions • Joint power-playout control • Modeled in a dynamic programming framework • Developed simple, efficient heuristics • Extensions • Additional Channels and Responsive Interference • Additional Controls • Content-Aware Control • Apply to protocols (802.11h) “Joint Power-Playout Control Schemes for Media Streaming over Wireless Links”, in IEEE Packet Video 2004, Markopoulou joint work with Y.Li, N.Bambos, J.Apostolopoulos

26. Extension: adding more controls control scheduling content-aware playout (r) R(t) n b Tx Rx • Additional Controls: • Tx: control scheduling • how many units to transmit and which to drop • Rx: motion-aware playout • slowdown video scenes with low or no motion • Results: • trade-off: playout speed variation vs. distortion • effect of playout variation is less perceived “Joint Packet Scheduling and Content-Aware Playout Control for Media Streaming over Wireless”, invited paper in IEEE MMSP 2005, A. Markopoulou joint work with Y.Li, N.Bambos, J.Apostolopoulos

27. Example of Motion-Aware Playout Motion-aware playout Motion-unaware playout

28. Future Directions • Multimedia over IP • Cross-layer optimization • Content distribution • Network Dependability • From traditional QoS to Reliability & Security • Measurements for diagnosis and control • Network shared by independent selfish entities • Network-adaptive applications • How bad is selfish routing? • Selfishness in other contexts? Interaction …

29. Appendix

30. Responsive Interference - Setup • Primary Media link, background PCMA links • Pairs of Tx-Rx randomly chosen from area (500x500 wrapped in a torus) • Background: geometric durations, Bernoulli arrivals • Free space path loss G~1/d^4, noise 1^(12) • Estimate I using previous timeslot • N=100, initial 5slots, buffer B=10 • Heuristic gains: 60% in power, 66% in QoS

31. Responsive Interference- Power

32. Responsive Interference- Quality

33. Adding mode control: (p, m, r) • Add a control m: • Packets transmitted in a time slot • Add a cost Psi(m) • Modify • Bellman equations

34. X p* hard backoff aggressive soft backoff i Power-only heuristic • Fix playout r=R and find optimal power p* • similarly to [B&R (1997), B&K(2000), B&Li(2004)] • where • Heuristic: approximate X, plug it in p.

35. [Performance evaluation cont’d] • Simulated other channels • Simulated responsive interference • Found low sensitivity to r-parameters