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Versatile Low Power Media Access for Wireless Sensor Networks

Versatile Low Power Media Access for Wireless Sensor Networks. Sarat Chandra Subramaniam. Goals. Low Power operation Effective collision avoidance Simple and predictable Small code size and RAM usage Tolerable to changing RF/networking conditions Scalable to large numbers of nodes.

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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 Sarat Chandra Subramaniam

  2. Goals • Low Power operation • Effective collision avoidance • Simple and predictable • Small code size and RAM usage • Tolerable to changing RF/networking conditions • Scalable to large numbers of nodes

  3. In a nutshell (1) • Low power operation achieved by: • Clear Channel Assessment (reducing idle listening) • Low Power Listening • Adaptive preamble sampling • Effective collision avoidance • Factoring of MAC functionalities • MAC reconfigurability

  4. In a nutshell (2) • Tolerant to changing RF conditions • Scalable to large number of nodes

  5. Significant Contributions • More flexible and more tunable as small core and factored functionality • RTS/CTS, ACKs, etc are considered higher layer functionality (services) • Has bidirectional (set and get) interfaces to MAC functionalities • Applications can turn them on and off – therefore adaptable to radio environment • Clear channel assessment with outlier detection

  6. Core MAC functionalities

  7. Reconfigurability • All core functionalities can be configured (either modifiable or modifiable and removable) • Use? • Adaptability to traffic conditions • Scalability to include larger/smaller number of nodes • Adaptability to radio environment

  8. CCA (1) • BMAC solution: ‘software automatic gain control’ • Signal strength samples taken when channel is assumed to be free • Samples go in a FIFO queue (sliding window) • Median added to an EWMA filter • Once noise floor is established, a TX requests starts monitoring RSSI from the radio

  9. CCA (2) • Comparing signal strength with noise floor causes false negatives (noise amplitude fluctuates). • Detect outliers: • Samples whose energy is significantly below noise floor. • This can’t happen if packet is being sent.

  10. CCA Results • 0=busy, 1=clear • Packet arrives between 22 and 54 ms

  11. LPL (1) • Familiar Wake-up – Active –Sleep Mechanism • Has CCA – potentially reducing idle listening • Preamble length matches channel checking period • No explicit synchronization required (unlike S-MAC) • Packet checking period and Preamble length - configurable

  12. Single-hop application doing periodic data sampling Sampling rate (traffic pattern) defines optimal check interval Check interval Too small: energy wasted on idle listening Too large: energy wasted on transmissions (long preambles) In general, it’s better to have larger preambles than to check more often! LPL (2)

  13. Lifetime Modeling (1) • Lifetime of node determined by energy consumption • Various components are: • Energy for receiving • Energy for transmitting • Energy for listening • Energy for sensing • Sleep energy • Key: Energy depends on time taken to achieve all of the above

  14. Lifetime Modeling (2) • All the times are known – eg for listening, time depends on preamble length and channel check interval • Lifetime estimated at compile-time or run-time • Provides feedback to network services to configure MAC

  15. Beauty of reconfigurability • Example of achieving RTC-CTS channel acquisition (all this is implemented by services above the MAC): • Send RTS using LPL cycle • Listen for CTS using LPL cycle • Once CTS is heard, disable LPL, CCA at both ends • Send data as burst • Send link layer ACK • Re-enable LPL, CCA • RTS – CTS/ ACK etc used depending on the situation.

  16. Adaptive Preamble Sampling • Mentioned, but not explained. • WiseMAC implements adaptive preamble sampling. • Preamble sampling = process of listening for activity on the radio. • It is done during LPL. • Adaptive preamble sampling indicates the adaptability of LPL?

  17. Experimental results: throughput

  18. Throughput vs power consumption

  19. S-MAC Default Configuration B-MAC Default Configuration Energy vs Latency

  20. Summary • B-MAC is small, extensible and flexible. • CCA increases channel utilization. • LPL results in decreased power listening. • B-MAC may be better or equal S-MAC performance in almost all scenarios.

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