1. Efficient MAC Protocols for Wireless Sensor Networks Mahendra Kumar

2. Properties of a Well Defined MAC Protocol • Energy Efficient • Scalability • Adaptability to changes in network topology • Latency, throughput, bandwidth • Fairness –not so important

3. Reason of Energy Waste • Collision • Overhearing • Control Packet overhead • Idle Listening • Overremitting

4. Existing MAC Protocols • Sensor-MAC (S-MAC) : Listen-sleep • Timeout-MAC (T-MAC) : Activation event • WiseMAC : Preamble Sampling

5. S-MAC • Main goal –reduce power consumption • Three major components: • Periodic sleep-listen • Collision and overhearing avoidance • Message passing

6. Periodic Sleep-Listen • Each node goes to sleep for some time, and then wakes up and listens to see if any other node wants to talk to it. • During Sleep it turn off its radio.

7. Collision and Overhearing Avoidance • Interfering nodes go to sleep after they hear an RTS or CTS packet. • Duration field in each transmitted packet indicates how long the remaining transmission will be.

8. S-MAC

9. Maintaining Synchronization • The listen/sleep scheme requires synchronization among neighboring nodes. • Updating schedules is accomplished by sending a SYNC packet.

10. Advantages/Disadvantages • Energy waste caused by idle listening is reduced by sleep schedules. • Sleep and listen periods are predefined and constant which decreases the efficiency of the algorithm under variable traffic load.

11. Timeout-MAC (T-MAC) • Proposed to enhance the poor results of S-MAC protocol under variable traffic load. • Listen period ends when no activation event has occurred for a time threshold TA. • Reduce idle listening by transmitting all messages in bursts of variable length, and sleeping between bursts. • times out on hearing nothing.

12. S-MAC Vs T-MAC

13. Advantages/Disadvantages • Gives better result under variable load. • Suffers from early sleeping problem –node goes to sleep when a neighbor still has messages for it.

14. WiseMAC • All nodes defined to have two communication channels. • Data channel uses TDMA • Control channel uses CSMA • Preamble sampling used to decrease idle listening time. • Nodes sample the medium periodically to see if any data is going to arrive.

15. WiseMAC

16. Advantages/Disadvantages • Dynamic preamble length adjustment results in better performance. • Conflict when one node starts to send the preamble to a node that is already receiving another node’s transmission where the preamble sender is not within range. Hidden terminal problem

17. Other MAC Protocols • SIFT :Event Driven • TRAMA : Traffic Adaptive MAC, TDMA Based