1. The Shadowing Phenomenon: Implications of receiving during a collision Kamin Whitehouse Alec Woo Fred Jiang Joe Polastre Joint work with: Presented by: NEST Retreat, January 2004

2. Got A ! Radio Collisions Got B ! Huh ?? A C B

3. Connectivity-interference Assumption Connectivity implies interference  Determine back-off times  Adjust transmission rates  Achieve fairness  Improve spatial reuse  Manage power consumption  Optimize bandwidth

4. The Shadowing Phenomenon 1 0 Collision 1 0

5. Got A ! The Shadowing Phenomenon Got B ! C B A

6. Contribution Shadowing is common Existing protocols do not presume shadowing Many techniques can exploit shadowing

7. Collision Detection and Recovery Delta t Stronger LastStronger First Preamble Sync Data Crc Preamble Sync Data Crc

8. Collision Detection and Recovery Recovery improves bandwidth/energy Detection different from RTS/CTS and Ack  More information  Applies to broadcast messages  Info available at receiver  Only works with stronger-last (maximum 50%) Could be augmented with termination symbol

9. Evaluation of Detection and Recovery B A C Vary Delta t = t B – t A

10. Collision Detection and Recovery Pream Sync Data Crc Pream Sync Data Crc Delta t = -23msDelta t = +23ms Vary Delta t = t B – t A B A

11. Evaluation of Detection and Recovery Detection: 42% success Recovery: 46% success

12. Implications of Shadowing in Protocol MAC  Acks  RTS/CTS Routing  Flooding  Clustering  Multi-hop Routing Scheduling  Localization

13. Acks and Shadowing Got B ! C Ack Got B ! Ack Got B ! Ack B A

14. RTS/CTS and Shadowing C CTS RTS CTS RTS CTS RTS B A

15. Flooding and Shadowing

16. Spatial Reuse and Shadowing ABCD

17. Localization and Shadowing

18. Open Questions How common are collisions in real life? How often does shadowing happen? How well does detection and recovery work? What is the spatial influence on shadowing?

19. Flooding Experiment 36 nodes in 9 x 4 grid, 2 meter spacing Each node transmits once in 25 experiments Newest CSMA for TinyOS 857 transmissions 12687 receptions 2036 collisions 1142 shadowings

20. Main Findings Detection: ~25% success rate Recovery: 30-40% success rate

21. Main Findings Collisions are common  15% of receptions were in a collision  Up to 40-50% transmissions collided somewhere Shadowing is common  40-100% of collisions have shadowing  As low as 10% of neighbors provide the connectivity-interference assumption

22. Main Findings

24. Future Work Fix synch bytes problem Add termination symbol 802.11, 802.15.4, Bluetooth!

25. Conclusions RF engineers and Protocol Designers must talk!