Diagnosing Wireless Packet Losses in 802.11: Separating Collision from Weak Signal

1. Diagnosing Wireless Packet Losses in 802.11: Separating Collision from Weak Signal Presented By: Jacob H. Cox Jr For ECE 256: Wireless Networking and Mobile Computing February 10, 2009

2. Acknowledgments • Authors ~ ShravanRayanchu, AruneshMishra, DheerajAgrawal, SharadSaha, SumanBanerjee • Kuo-Chung Wang (Slide Presentation) • http://lion.cs.uiuc.edu/group_seminar_past/fall06/group_seminar_slides/kim-rateadaptation06.ppt+RRAA

3. Presentation Outline • Packet Loss Problem • Current Rate Adaption Schemes • COLLIE Overview • COLLIE Metrics • COLLIE Analysis • Conclusion

4. Motivation • Packet Loss • 2 Causes: Weak Signal and Collision • 802.11 Solution Inadequate • defaults to BEB for a substantial number of packet losses • Question: • Does the type of packet loss matter? • What if we could determine its cause?

5. Problem Defined • Collision or Weak Signal, why does knowing matter? Beamforming?

6. Fixing packet loss • Appropriate actions • For collision • BEB CW Max Retries REF: http://pages.cs.wisc.edu/~shravan/coll-infocom.pdf

7. Rate Adaptation • 802.11 a/b/g standards allow for the use of multiple transmission rates • 802.11a, 8 rate options (6,9,12,18,24,36,48,54 Mbps) • 802.11b, 4 rate options (1,2,5.5,11Mbps) • 802.11g, 12 rate options (11a set + 11b set) • Some papers report that rate adaptation isimportant yetunspecified in 802.11 standards Reference: Robust Rate Adaptation in 802.11 Networks Presentation by Kuo-Chung Wang

8. 54Mbps Signal is good Signal becomes weaker Receiver Rate Adaptation Example 12Mbps Sender • Rate adaptation affects throughput performance and should be adjusted by channel condition Reference: Robust Rate Adaptation in 802.11 Networks Presentation by Kuo-Chung Wang

9. Related Work Rate Adaptation Algorithms –Differentiate between loss behaviors –Adapt to realistic scenarios –Handle hidden stations ARF ~ Auto-rate Fallback CARA ~ Collision-Aware Rate Adaptation MRD ~ Multi-Radio Diversity RBAR ~ Receiver Based Auto Rate RRAA ~ Robust Rate Adaptation Algorithm

10. 54Mbps Signal is good 54Mbps Signal is good 54Mbps Signal is good 12Mbps Signal is still good 12Mbps Signal is still good 12 Mbps Sender Sender Sender Sender Sender RAA Problem Receiver With hidden terminals, reducing the rate prolongs transmission time for each packet and results in more collisions

11. Introduction to COLLIE • 802.11, CARA, and RRAA use multiple attempts to deduce cause of packet loss • COLLIE uses a direct approach • Error packet kickback • Client analysis

12. COLLIE • Collision InferencingEngine • Utilizes receiver feedback • Analyzes: • Bit and symbol level error patterns • Received signal strength • Design: • Signal analysis algorithms • Link layer protocol which adjusts link layer parameters

13. Link Adaptation Mechanism Enhancements • Auto Rate Fallback (ARF) • Used in conjunction w/COLLIE for this paper • Rate adaption mechanism enhanced with inferencing component • Using COLLIE, observed throughput gains of 20-60%

14. COLLIE Continued ? Client AP Data X Feedback Received Signal Strength Adjust Data Rate/Power Or Contention Window Collision Inference Algorithm Symbol error patterns Bit error distribution and patterns Note: assumes Feedback is successfully received and sender’s MAC address is decoded correctly by the AP

15. Metrics for Analysis http://pages.cs.wisc.edu/~shravan/coll-infocom.pdf • Received Signal Strength (RSS) = S + I • S ~ Signal Strength • I ~ Interference • Bit Error Rate (BER) = total % incorrect bits • Symbol level errors: errors within transmission frame • Multiple tools used to analyze symbol-level errors

16. Symbol-level Errors Symbol Error Rate (SER)- % symbols received in error Errors Per Symbol (EPS)- average # errors within each symbol Symbol Error Score (S-score): , where Bi is a burst of n bits

17. S-Score Collision • 0011 0011 0011  0111 1011 0010 S-Score = S-Score = Channel Fluctuation http://pages.cs.wisc.edu/~shravan/coll-infocom.pdf 0011 0011 0011  0011 1101 0011

18. Experimental Design • Three possibilities at R: • Packet received without error • Packet received in error • No packet received

19. Experimental Design Two transmitters, T1 and T2 Two receivers, R1 and R2 Receiver R hears all signals

20. Analysis of Results

21. Analysis of Results

22. Begs the Question • Is it worth it? Successful almost 60%, false positive rate of 2.4% Check out this accuracy?

23. Design Components Client Module

24. Multi-AP COLLIE • Error packet sent to a central COLLIE server • Most important where the capture effect is dominant

25. Multi-AP Results • Static situation averaged 30% gains in throughput • For multiple collision sources and high mobility, throughput gains reached 15-60%

26. Collision Analysis

27. Some Problems • Capture Effect • Packet size • Packet Kickback

28. Conclusions • COLLIE implementation achieves increased throughput (20-60%) while optimizing channel use • 40% reduction in retransmission costs • Implementation can be done over standard 802.11, resulting in much lower startup costs than other protocols

29. Questions?