1. Versatile low power media access for wireless sensor networks Joseph PolastreJason HillDavid Culler Computer Science Department University of California,Berkeley JLH Labs Camino Capistrano Capistrano Beach Computer Science Department University of California,Berkeley Speaker: Yung-Lin Yu ACM SenSys’04

2. Outline Introduction Design and Implementation –Clear Channel Assessment (CCA) –Low Power Listening (LPL) Evaluation Experiment Conclusion

3. Introduction What is BMAC? –A configurable MAC protocol for WSNs –Small core Factors out higher-level functionality –Energy efficient Goals –Low Power operation –Effective collision avoidance –Simple and predictable –Small code size and RAM usage –Scalable to large numbers of nodes

4. Introduction (cont.) Reconfigure –Bidirectional interface for WSN application –Extend network lifetime by 50%

5. Design and Implementation Traditional –SMAC design Users pre-configure duty cycle Applications rely on S-MAC to adjust its operation as things change BMAC –Small core functionality: media access control –RTS/CTS, ACKs, etc are considered higher layer functionality (services) Applications can turn them on and off –More flexible

6. Design and Implementation(cont.)

7. MAC must accurately determine if channel is clear –Need to tell what is noise and what is a signal –Ambient noise changes depending on the environment BMAC ’ s solution –Use Clear Channel Assessment (CCA) CCA is used to determine the state of the medium

8. Design and Implementation (cont.) 0=busy, 1=clear Packet arrives between 22 and 54 ms Single-sample thresholding produces several false ‘ busy ’ signals

9. Design and Implementation (cont.) Low Power Listening –Goal: minimize listen cost –Principles Node periodically wakes up, turns radio on and checks channel –Check interval variable If signal is detected, node powers up in order to receive the packet Node goes back to sleep –If a packet is received –After a timeout Preamble length matches channel checking period –No explicit synchronization required Noise floor estimation used to detect channel activity during LPL

10. Design and Implementation (cont.) LPL 125 ms Receiver Sender preamble data

11. Evaluation LPL check interval vs Lifetime

12. Evaluation (cont.) LPL check interval vs neighborhood size 50ms 25ms

13. Experiment Wireless sensor node –Mica2 Software –TinyOS Environment –Unobstructed Deployment –Place the nodes with 1 meter spacing Experiment Three subject –Throughput –power consumption –Energy vs Latency

14. Experiment (cont.) Throughput (Channel Utilization) –2.5 times than S-MAC broadcast,4.5 time than S-MAC unicast Because CCA and lower sync. overhead –As the Nodes Increase Channel contention cause performance converge to S-MAC

15. Experiment (cont.) power consumption –Duty cycle increase In S-MAC, havemore SYNC overhead In B-MAC 1.no sync. requirements. 2.reconfigure check interval to adept network bandwidth Because SYNC overhead

16. Experiment (cont.) Energy vs Latency –10-hop network –Source sends 100 byte packet every 10 seconds S-MAC Default Configuration B-MAC Default Configuration

17. Conclusions BMAC appears to be better than SMAC –Easier to tune –Has better channel assessment –Doesn ’ t use explicit sync packets –Doesn ’ t use RTS/CTS/ACK if it doesn ’ t have to –Is smaller and less complex