1. Diagnosing Wireless Packet Losses in 802.11: Separating Collision from Weak Signal Shravan Rayanchu, Arunesh Mishra, Dheeraj Agrawal, Sharad Saha, Suman Banerjee

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

3. Packet loss in Wireless Networks A B C

4. A B C A send RTS to B

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

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

10. Packet loss in Wireless Networks A B C 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? CE A D B Rate = 20 Rate = 10

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 Channel Fluctuation 0011 0011 0011  0111 1011 0010

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 B i is a burst of n bits 74% accuracy

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

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

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?