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Versatile Low Power Media Access for Wireless Sensor Networks

Versatile Low Power Media Access for Wireless Sensor Networks. Joseph Polastre † , Jason Hill †† , and David Culler † † Computer Science Department, University of California, Berkeley †† JHL Labs. ACM Conference on Embedded Networked Sensor Systems (SenSys 2004). Wang, Sheng-Shih

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Versatile Low Power Media Access for Wireless Sensor Networks

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  1. Versatile Low Power Media Access for Wireless Sensor Networks Joseph Polastre†, Jason Hill††, and David Culler† †Computer Science Department, University of California, Berkeley ††JHL Labs ACM Conference on Embedded Networked Sensor Systems (SenSys 2004) Wang, Sheng-Shih Oct. 27, 2005

  2. Outline • Introduction • Design and Implementation • Experimental Results • Conclusion

  3. Introduction --- B-MAC • Berkeley Media Access Control • A configurable MAC protocol • Additional interfaces • Small core • Higher-level functionality • Energy efficient

  4. Introduction --- S-MAC vs. B-MAC • S-MAC • A link, network and organization protocol • Duty cycle should be pre-configured • Inflexible • Applications and services must rely on the duty cycle to adjust its operations as node and network conditions change • B-MAC • A link protocol • Small core of media access functionality • More flexible • CCA, acknowledgement, backoff, and LPL interfaces • Allow services to adjust its operation

  5. Introduction --- B-MAC Interfaces CCA ACK Backoff LPL

  6. Design and Implementation • Clear channel assessment (CCA) and packet backoffs • For channel arbitration • Link layer acknowledgement • Reliability • Optional • Low power listening (LPL) • Low power communication

  7. CCA --- Single-sample Thresholding vs. Outlier Detection • Determine the channel status (busy/clear?) • Noise should be distinguished from signal • Single-sample thresholding • Take single sample, and then compare to noise floor • Noise floor determination • Exponentially Weighted Moving Average (EWMA) filter • E(t)=E(t-1)+(1-)O(t) • Best parameter value:=0.06, FIFO queue size=10

  8. CCA --- Single-sample Thresholding vs. Outlier Detection • Outlier detection • B-MAC searches for outliers in RSSI • If an outlier exists during the channel sampling period, the channel is clear • If five samples are taken and no outlier is found, the channel is busy

  9. CCA --- CCA Result Packet arrives between 22ms and 54 ms Single-sample thresholding produces several false ‘busy’ signals

  10. Packet Backoff • B-MAC uses an initial channel backoff • B-MAC does not set the backoff time • CCA outlier algorithm is run after the initial backoff • Channel is busy  congestion backoff time service • Initial backoff time  CCA is applied • Ignore  small random backoff event TinyOS (B-MAC)

  11. Low Power Listening (LPL) • Goal: minimize listen cost • Principles • Node periodically wakes up, turns radio on and checks channel • Variable check time • Activity is detected • Node stays awake to receive the packet, and then returns to sleep after reception • Activity is not detected • Node goes to sleep after a timeout • Preamble length must be larger than channel checking interval

  12. LPL --- LPL and Neighborhood Size • More neighbors • More transmissions • More time spent receiving packets

  13. LPL --- Check Interval • Single-hop application • Periodic data sampling • Samplerate (traffic pattern) defines optimal check interval

  14. Experimental Results --- Parameters Parameters of Mica2 motes B-MAC parameters on Mica2 Application semantics

  15. Experimental Results --- Throughput • Nodes are placed equidistant from a receiver • No duty cycle in S-MAC • B-MAC outperforms S-MAC-broadcast and S-MAC-unicast • Less obvious differences as number of nodes increases • B-MAC vs. S-MAC-broadcast • CCA and lower preamble overhead • B-MAC vs. S-MAC-unicast • RTS-CTSoverhead

  16. Experimental Results --- Energy vs. Throughput • 10 nodes in a neighborhood • Low data rates: S-MAC is better • Very low duty cycle • High data rate • S-MAC’s duty cycle must increase because more active periods (i.e., more SYNC periods) • B-MAC • Larger preambles at low throughput,progressively becoming smaller

  17. Experimental Results --- Energy vs. Latency • Source sends a 100-byte packet every 10 seconds • Latency < 6 sec • B-MAC is better • Latency  6 sec • S-MAC is better • All nodes are synchronized, data is transmitted in an active period • Latency > 3 sec • Energy consumption of B-MAC is bounded (idle listening cost) 10% duty cyclew/ adaptive listening Mica2 mote parameter

  18. Conclusion • B-MAC uses higher-layer interfaces • CCA & backoff, acknowledgement, LPL • B-MAC’s advantages • Simple implementation • Flexible (configurable) • Better channel assessment • RTS/CTS/ACK is optional • Without explicit sync packets

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