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Control Mechanisms for Video Streaming Wireless Links Athina Markopoulou Electrical Engineering Dept. Stanford University 1 Real-Time Multimedia over Packet Networks Characteristics • Continuous Stream Playout, Real-Time t src ideally t rcv t src network t rcv loss jitter delay Requirements • low loss, delay, delay jitter 2 Multimedia - Networking Application Network 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) 3 The Bigger Picture 2 backbone 1 wireless 4 Media Streaming over Wireless Last-Hop Wireline Server Mobile Terminal Streaming to • Access Point, Server/Proxy Wireless laptops, cellphones, PDAs, wireless TV displays Challenges • • limited resources, time variation strict application requirements 5 Example Ideally Tx Tx Rx Rx Play Play Tx Control at the Tx Tx Rx Rx Play Play Over wireless Control at the Rx 6 Problem Statement N p i Tx Scenario Objective • • • • • r Rx pre-stored media content at Tx. interference i, according to a Markov chain with deliver and play entire content maximize the playout quality minimize the power cost 7 System State and Controls n p Tx i b r s(p,i) Rx (p,r) = system controls in current time slot (n,i,b, r’) = system state in current time slot n = remaining packets at Tx i = channel interference b = available packets at Rx r’ = playout rate in previous slot 8 System Controls at Tx n p i s(p,i) Rx Tx Control p: transmission power in current slot • s(p,i) : probability of successful reception 1 0.9 0.8 0.7 0.6 s(p,i) r b 0.5 0.4 • Power Cost: Φ = p – – battery lifetime interference stress 0.3 0.2 0.1 0 0 2 4 6 8 10 SIR p/i 9 Dynamic Programming Formulation System Evolution Define to be the minimum expected cost-to-go from n=N…. Power ·W + Quality (n,i,b;r’) ·W (p,r) s(p,i)qij (n-1,j,b+1-r;r) (1-s(p,i))qij (n,j,b-r;r) … until n=0 10 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 11 Special Cases: Individual Controls Scenario Control at Tx Control at Rx No control fix p fix r=R Power Control adapt p fix r=R Power Control & Re-buffering adapt p r in {0, R} Playout Control fix p r in {0, r1, r2…R} Re-buffering fix p r in {0, R} Joint Control adapt p r in {0, r1, r2…R} Similar formulations – obtain optimal policies Compare: no control, special cases, joint control 12 Power-Quality Tradeoff (1) No control Better Performance 13 Power-Quality Tradeoff (2) No control Playout Only 14 Power-Quality Tradeoff (3) No control Playout Only Power Only Power+Rebuffering 15 Power-Quality Tradeoff (4) No control Playout Only Joint control Power Only Power+Rebuffering 16 Heuristics Why heuristics? Justified vs. ad-hoc heuristics • mimic properties of optimal control Steps • • • Power-only heuristic Playout-only heuristic Joint power-playout heuristic 17 Power Heuristic n p i r Fix playout r=R s(p,i) Rx Tx Optimal power: Backlog pressure X(n,i,b) • b n=N has structural properties: X ^ Heuristic: approximate X • mimicking those properties n=1 b 18 Playout - today n p i fix p Tx B2 s(p,i) s(i) Rx B1 r=R/2 r=R b r r=0 Purpose: choose r(b) Fixed Threshold Heuristic • • L thresholds for buffer occupancy b r r=rl r=0 Bl r=R Bl+1 BL b 19 Playout Heuristic B2 i s(i) Rx r=R B1 r=R/2 b r r=0 Observation: channel not taken into account yet Adaptive Threshold Heuristic • • adapt rate and adapt thresholds 20 Joint Power-Playout Heuristic n p Tx • • • • • i s(p,i) b r 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 21 Joint heuristic performs well Optimal Playout Optimal Power+Rebuffering Joint Heuristic Joint Optimal 22 Demo: no-control vs. joint heuristic For the same interference scenario For the same power consumption Compare the playout quality original no control Joint heuristic 23 Comparison Details Controls off Joint Heuristic 24 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 25 Extension: adding more controls control scheduling n R(t) Tx b Rx Additional Controls: • Tx: control scheduling – • how many units to transmit and which to drop Rx: motion-aware playout – content-aware playout (r) 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 26 Example of Motion-Aware Playout Motion-aware playout Motion-unaware playout 27 Future Directions Multimedia over IP • • Network Dependability • • Cross-layer optimization Content distribution 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 … 28 Appendix 29 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 30 Responsive Interference- Power 31 Responsive Interference- Quality 32 Adding mode control: (p, m, r) Add a control m: • Packets transmitted in a time slot Add a cost Psi(m) Modify Bellman equations 33 Power-only heuristic Fix playout r=R and find optimal power p* • similarly to [B&R (1997), B&K(2000), B&Li(2004)] • where p* X i aggres sive soft backoff hard backoff Heuristic: approximate X, plug it in p. 34 [Performance evaluation cont’d] Simulated other channels Simulated responsive interference Found low sensitivity to r-parameters 35