Understanding the Real-World Performance of Carrier Sense

1. Understanding the Real-World Performance of Carrier Sense Kyle Jamieson, Bret Hull, Allen Miu, Hari Balakrishnan MIT Computer Science and Artificial Intelligence Laboratory Networks and Mobile Systems http://nms.csail.mit.edu

2. Introduction • Carrier sense is a crucial building block for many radio networks • Wireless sensor networks • Wireless local area networks • Performance depends on carrier sense Application layer MAC layer Physical layer Carrier sense

3. A research direction • Let’s quantify how well carrier sense performs in real-world radio networks • Let’s study diverse radio networks and draw high-level conclusions • Modulation type • Network size (number of nodes) • Data rates

4. Experimental setup

5. Sensor network testbed • 60-node Mica2 sensor network • Six radio hops in diameter • Ethernet backchannel to log packet receptions 16,076 sq. ft. http://mistlab.csail.mit.edu 100 ft.

6. Outline • Introduction • Implementing carrier sense • Benefits of carrier sense • Drawbacks of carrier sense • Conclusion

7. Instantaneous signal strength Squelch (“noise floor”) Energy detect clear Energy detect busy How carrier sense works:energy detection Signal strength (dBm) Time

8. × Transmit data × Received data Spreading code Spreading code How carrier sense works: other mechanisms • Preamble detection • Decorrelation amplitude • Unique to spread-spectrum radios • AGC unlock • True when AGC adjusts rapidly Preamble Packet

9. Outline • Introduction • Implementing carrier sense • Benefits of carrier sense • Drawbacks of carrier sense • Conclusion

10. Aggregate load lowers link delivery rate ~360 links > 70% at 4 pps WSN experiment with all nodes sending, carrier sense on

11. Carrier sense improves link delivery rates Only 80 links in the network are > 70% without CS Carrier sense avoids collisions under high load

12. Carrier sense improves throughput Large-scale experiment with an offered load of 1 pps/node

13. Outline • Introduction • Implementing carrier sense • Benefits of carrier sense • Drawbacks of carrier sense • Conclusion

14. Sender-side decision;receiver-side collision Will any transmissions collide with mine? R S Carrier sense is at best a heuristic for predicting transmissions’ success

15. Exposed terminals fool carrier sense Missed transmission opportunity R S S΄ R΄ Carrier sense indicates busy, yet the transmission would have succeeded (S, S’ are exposed terminals)

16. Carrier sense misses transmit opportunities Large-scale experiment with CS energy detect, 0.25 pps per node

17. Carrier sense misses transmit opportunities Large-scale experiment with carrier sense off, 0.25 pps per node

18. Capture fools carrier sense Missed transmission opportunity A R B R captures B’s transmission despite A’s concurrent transmission

19. Capture prevalent at low bit rates Capture Collision At some low 802.11 bit rates, node B should disable carrier sense

20. Hidden terminals fool carrier sense Carrier sense is free! S R S’ Carrier sense indicates free, yet both transmissions fail (S, S’ are hidden terminals)

21. Related work • Capture-aware MAC • Whitehouse et al., Em-Nets ’05 • Priyantha, PhD thesis ‘05 • Channel sampling to infer congestion • CODA, Wan et al., SenSys ’04 • Models to pick carrier sense sensitivity • Yang and Vaidya, INFOCOM ’05

22. Conclusion and future research • An experimental evaluation of the benefits and drawbacks of carrier sense • Algorithm to track correlation between signal strengths and packet reception • Use a congestion control algorithm: CODA or Fusion [SenSys] and turn off or reduce carrier sense