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

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 MAC layer Physical layer Application 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 Experimental testbed Sensor network802.11b/g LAN Nodes603 RadioChipcon CC1000Atheros 5212 Data rate38.4 Kbps1 to 54 Mbps ModulationFM narrowbandOFDM/DSSS MACB-MAC (software)802.11 (hardware)

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

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

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

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

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

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

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

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 R S Will any transmissions collide with mine? Carrier sense is at best a heuristic for predicting transmissions’ success

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

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 R captures B’s transmission despite A’s concurrent transmission R A B Missed transmission opportunity

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

20. Hidden terminals fool carrier sense RS’S Carrier sense is free! 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