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 FUNCTIONMETHOD Channel ArbitrationCCA (sense if channel is busy) and packet backoffs (if busy, then backoff – congestion backoff).Start by backing off – initial backoff ReliabilityLink Layer Acks Low power commsLow Power Listening (LPL)

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. LPL (2)  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!

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

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.