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

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

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

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

10. Diversity Scenarios Conventional single path case Sender Mobile Receiver Wired 100Mbps Ethernet 802.11b 11Mbps WLAN AP1 AP2 AP1 only 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)

11. Path Diversity Reduces Packet Loss 20406080100Infinite 0 5 10 15 20 25 30 Avg Packet Loss Rate (%) Delay cutoff (ms) AP1 AP2 Balanced Site Selection Oracle

12. Path Diversity Reduces Burst Loss 20406080100Infinite 0 500 1000 1500 2000 Number of Burst Events Burst event = 2 or more consecutive losses Delay cutoff (ms) AP1 AP2 Balanced Site Selection Oracle

13. H.263 Video Performance 20406080100Infinite 24 26 28 30 32 34 Mother and Daughter Sequence Delay Cutoff (ms) PSNR (dB) AP1 AP2 Balanced Site Selection PSNR gain = 1.6 – 3.0 dB 1/3 delay reduction

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 0011223344556677…

16. Single Stream from AP1 01234567… 01234567 Discarded

17. Single Stream from AP2 01234567…

18. Split stream from AP1 & AP2 0011223344556677…

19. 00 11 22 33 44 55 66 77 … Discarded

20. Split stream from AP1 & AP2 00 11 22 33 44 55 66 77 … Discarded

21. Split stream from AP1 & AP2 01 11 23 33 45 55 76 77 … Discarded Re-numbered

22. Split stream from AP1 & AP2 01234576…

23. Fine-grained Site Selection 0011223344556677…

24. 01234567… Selected site transmits 95% packet Other site transmits 5% packet for probing Site selection based on error rate of last 300 packets

25. Oracle 0011223344556677… Equivalent to repetition coding

26. Oracle 0011223344556677… Equivalent to repetition coding 01234567

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 N thresh (# 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