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

Diagnosing Wireless Packet Losses in 802.11: Separating Collision from Weak Signal. Shravan Rayanchu , Arunesh Mishra , Dheeraj Agrawal , Sharad Saha , Suman Banerjee. Motivation. Packet Loss 2 Causes Solution Inadequate 802.11 Can we determine cause of packet loss?.

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

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  1. Diagnosing Wireless Packet Losses in 802.11:Separating Collision from Weak Signal ShravanRayanchu, AruneshMishra, DheerajAgrawal, SharadSaha, SumanBanerjee

  2. Motivation • Packet Loss • 2 Causes • Solution Inadequate • 802.11 • Can we determine cause of packet loss?

  3. Packet loss in Wireless Networks C A B

  4. Packet loss in Wireless Networks A send RTS to B C A B

  5. Packet loss in Wireless Networks While A is transmitting, C initiates RTS to B C A B

  6. Packet loss in Wireless Networks C A B Since neither A nor B knows the other is transmitting, both RTS’s are sent and collide at B, resulting in packet loss

  7. Packet loss in Wireless Networks C A B Since neither A nor B knows the other is transmitting, both RTS’s are sent and collide at B, resulting in packet loss

  8. Packet loss in Wireless Networks B C A Here A and C are in just barely in range of each other, but both are in range of B

  9. Packet loss in Wireless Networks B C A A send its RTS to C, which is received and B is silenced

  10. Packet loss in Wireless Networks B C A C send its CTS to A, but the packet is not heard due to weak signal caused by interference by noise

  11. Detecting Packet Loss • Recap: 2 causes of packet loss • 802.11 Solution • BEB • Different causes lead to different solutions

  12. Fixing packet loss • Appropriate actions • For collision • BEB

  13. Fixing Packet Loss • For low signal • Increase power • Decrease data rate • How to differentiate? Rate = 10 B C E A D Rate = 20

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

  15. COLLIE: An Overview • Client Module • AP Module • Server Module (optional)

  16. COLLIE: An Overview

  17. COLLIE: Single AP • AP error packet kickback • Client-side analysis • Problem: how can the AP successfully re-transmit packet?

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

  19. Experimental Design • Three possibilities at R: • 1. Packet received without error • 2. Packet received in error • 3. No packet received

  20. Error Metrics • Three error metrics: • Bit Error Rates (BER) • Symbol Error Rates (SER) • Error Per Symbol (EPS)

  21. Metrics for Analysis • Received Signal Strength (RSS) = S + I • High RSS  collision • Low RSS  channel fluctuations • Bit Error Rate (BER) = total # incorrect bits • BER is higher for collisions, lower for low signal

  22. RSS: The Details • Of all packets lost due to low signal, 95% had an RSS less than -73dB, compared to only 10% for collisions

  23. Metrics for Analysis • Symbol level errors: errors within transmission frame • Multiple tools used to analyze symbol-level errors

  24. Framing • 0011 0011 0011  0011 1101 0011 Collision • 0011 0011 0011  0111 1011 0010 Channel Fluctuation

  25. 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): calculated as , where Bi is a burst of n bits • 74% accuracy

  26. S-Score • 0011 0011 0011  0011 1101 0011 Collision • 0011 0011 0011  0111 1011 0010 S-Score = S-Score = Channel Fluctuation

  27. Performance • Metric voting scheme • Successful almost 60%, false positive rate 2.4%

  28. Some Problems • RSS: universal cutoff impossible • Capture Effect • Packet size

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

  30. Results • Static situation  average of 30% gains in throughput • For multiple collision sources and high mobility, throughput gains of 15-60%

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

  32. Questions?

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