Versatile Low Power Media Access for Wireless Sensor Networks

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.