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Low Latency Wireless Video Over 802.11 Networks Using Path Diversity

Low Latency Wireless Video Over 802.11 Networks Using Path Diversity. John Apostolopolous Wai-tian Tan Mitchell Trott Hewlett-Packard Laboratories. Allen Miu MIT Laboratory for Computer Science. Motivation. Prevalent 802.11 infrastructure provides inexpensive connectivity

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Low Latency Wireless Video Over 802.11 Networks Using Path Diversity

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  1. Low Latency Wireless Video Over 802.11 Networks Using Path Diversity John Apostolopolous Wai-tian Tan Mitchell Trott Hewlett-Packard Laboratories Allen Miu MIT Laboratory for Computer Science

  2. Motivation • Prevalent 802.11 infrastructure provides inexpensive connectivity • Emergent mobile devices integrated with cameras and 802.11 interfaces • High quality (low-latency) conversational communication over 802.11 networks (e.g. VoIP, Video conferencing)

  3. Challenges • 802.11 operates in ISM band • Interference from other electronic devices (e.g. BT, microwaves) • Lossy Environment • Access point coverage can be spotty • Quality changes over time • Signal fading due to multi-path • Shadowing due to obstacles and human traffic • Contention among exposed and hidden nodes • Low Latency requirement • 802.11 ARQ error recovery can add large delays

  4. Our Approach • Use error resilient video compression • H.264/MPEG-4 AVC • Best-effort error recovery (standard 802.11 ARQ) • Distributed AP infrastructure to stream video via multiple access points (exploit path diversity) • Use multiple paths simultaneously or switch between them (site selection) as a function of channel characteristics

  5. Preliminary investigation In a 802.11 path diversity network with mobile clients, • How does path diversity affect packet loss characteristics? • What is the resulting performance gain for conversational video communication?

  6. Test-bed Setup ~40m (max) AP1 Wired 100Mbps Ethernet 802.11b 11Mbps WLAN Sender MobileReceiver ~25m AP2 • Two 360kbps cbr streams • 1500 byte UDP packets • time-stamped • Ad hoc mode • ARQ up to 16 retries • Open cubicle area • Receiver moves @1m/s

  7. Diversity Scenarios AP1 only • Conventional single path case AP2 only AP1 Wired 100Mbps Ethernet 802.11b 11Mbps WLAN Sender MobileReceiver AP2

  8. Diversity Scenarios AP1 only • Conventional single path case AP2 only • Balanced split stream (non-adaptive) AP1 Wired 100Mbps Ethernet 802.11b 11Mbps WLAN Sender MobileReceiver AP2

  9. Diversity Scenarios AP1 only • Conventional single path case AP2 only • Balanced split stream (non-adaptive) • Adaptive, fine-grain site selection (based on loss rate) AP1 Wired 100Mbps Ethernet 802.11b 11Mbps WLAN Sender MobileReceiver AP2

  10. Diversity Scenarios AP1 only • Conventional single path case AP2 only • Balanced split stream (non-adaptive) • Adaptive, fine-grain site selection (based on loss rate) • Oracle (optimal adaptive, can be realized by repetition coding) AP1 Wired 100Mbps Ethernet 802.11b 11Mbps WLAN Sender MobileReceiver AP2

  11. Path Diversity Reduces Packet Loss AP1 AP2 Balanced Site Selection Oracle 30 25 20 Avg Packet Loss Rate (%) 15 10 5 0 20 40 60 80 100 Infinite Delay cutoff (ms)

  12. Path Diversity Reduces Burst Loss Burst event = 2 or more consecutive losses 2000 AP1 AP2 Balanced Site Selection 1500 Oracle Number of Burst Events 1000 500 0 20 40 60 80 100 Infinite Delay cutoff (ms)

  13. H.263 Video Performance AP1 AP2 Balanced Site Selection Mother and Daughter Sequence 34 PSNR gain = 1.6 – 3.0 dB 1/3 delay reduction 32 30 PSNR (dB) 28 26 24 20 40 60 80 100 Infinite Delay Cutoff (ms)

  14. Conclusion • All path diversity schemes help reduce burst losses • Optimal path diversity drastically reduce loss rate and improves video quality • A simple site selection algorithm can effectively increase video quality without increasing bandwidth usage

  15. Complete Trace 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 …

  16. Single Stream from AP1 0 1 2 3 4 5 6 7 … 0 1 2 3 4 5 6 7 Discarded

  17. Single Stream from AP2 0 1 2 3 4 5 6 7 …

  18. Split stream from AP1 & AP2 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 …

  19. Split stream from AP1 & AP2 0 0 2 2 4 4 6 6 … 1 1 3 3 5 5 7 7 Discarded

  20. Split stream from AP1 & AP2 0 0 2 2 4 4 6 6 … 1 1 3 3 5 5 7 7 Discarded

  21. Split stream from AP1 & AP2 Re-numbered 0 1 2 3 4 5 6 7 … 1 1 3 3 5 5 7 7 Discarded

  22. Split stream from AP1 & AP2 0 1 2 3 4 5 6 7 …

  23. Fine-grained Site Selection 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 …

  24. Fine-grained Site Selection 0 1 2 3 4 5 6 7 … • Selected site transmits 95% packet • Other site transmits 5% packet for probing • Site selection based on error rate of last 300 packets

  25. Oracle Equivalent to repetition coding 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 …

  26. Oracle Equivalent to repetition coding 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 … 0 1 2 3 4 5 6 7

  27. Existing Solutions • Robust video communication in lossy channel: • Error resilient video compression • ARQ • Delay can be intolerable • Head of line blocking • FEC coding • Can be inefficient • Receiver diversity antenna • Does not exploit path diversity

  28. Analysis • Loss Characteristics • Varied delay threshold • Average Packet Loss Rate • Number of burst events • Burst Packet Loss Rate (# Packets lost in burst ) / (Total # packets) • H.264/MPEG4 Video Performance for 4 different sequences • PSNR • Nthresh (# of times when PSNR drops below 30dB)

  29. Methodology • Walking with receiver in open cubicle area • Analyzed packet loss from a 15 minute trace • Experiment repeated once after 3 months, showing similar performance results • Analyzed the performance of 5 different diversity scenarios from sampling the same trace • Difficulty in finding meaningful comparisons between different scenarios

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