Versatile low power media access for wireless sensor networks

1. Versatile low power media access for wireless sensor networks ACM SenSys’04 Speaker: Yung-Lin Yu

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

11. Evaluation • LPL check interval vs Lifetime

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

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, have more SYNC overhead • In B-MAC 1.no sync. requirements. 2.reconfigure check interval to adept network bandwidth Because SYNC overhead

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

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