1. Diagnosing Wireless Packet Losses in 802
Diagnosing Wireless Packet Losses in : Separating Collision from Weak Signal
Shravan Rayanchu, Arunesh Mishra, Dheeraj Agrawal, Sharad Saha, Suman Banerjee

2. 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

3. COLLIE: An Overview

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

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

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

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

8. Metrics for Analysis Received Signal Strength (RSS) = S + I
High RSS  collision
Low RSS  channel fluctuations
Of all packets lost due to low signal, 95% had an RSS less than -73dB, compared to only 10% for collisions

9. BER Bit Error Rate (BER) = total # incorrect bits
BER is higher for collisions, lower for low signal

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

11. 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

12. S-Score
Collision 
S-Score =
Channel Fluctuation 
S-Score =

13. Analysis of Results Metric Collision Weak Signal RSS
Higher (90% > -73dBm)
Lower (98% < -73dBm)
BER
Higher (24% =< 12% BER)
Lower (98% =< 12% BER)
SER
Unremarkable
EPS
Higher (45% =< 28% EPS)
Lower (98% =< 28% EPS)
S_Score
Higher (28% =< 500)
Lower (98% =< 500)
Plots are a cut and paste from the paper.
By using a cutoff of 500, Paper claims that it achieves the ability to detect 74% of collisions, with 2% being false positives.

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

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

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

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