Energy-Efficient and Reliable Medium Access in Sensor Networks

1. Energy-Efficient and Reliable Medium Access in Sensor Networks Presenter: Dr. Younghwan Yoo Authors: Vivek Jain, Ratnabali Biswas and Dharma P. Agrawal Department of Computer Science University of Cincinnati ymomo@ececs.uc.edu Department of Computer Science University of Cincinnati {jainvk, biswasr, dpa}@ececs.uc.edu

2. Outline • Wireless Sensor Network • Reliable Sensor MAC • Hidden Node Problem • Energy Efficient Sensor MAC • Protocol Design • Performance Evaluation • Summary • Future Work

3. Wireless Sensor Network (WSN) • Usually a set of small immobile nodes referred as motes • Generally static topology • Cheap alternative to monitor inaccessible or inhospitable terrains • Applications • Medical Applications – wireless bio-sensors • Nuclear and chemical plants • Environmental monitoring • Industrial Automation • Ocean monitoring • Battlefields

4. Reliable Sensor MAC Node receives more than one packet at same time Leads to packet loss due to buffer overflow Wastes energy listening to idle channel Collision Congestion Transmission Rate Control Idle Listening Good Reliable MAC Hidden Node Overhearing Latency Wastes energy receiving packets for other nodes Control Overhead Error Recovery Carrier-sense, backoff, transmission, propagation, processing, queuing Recover packets corrupted at physical layer Wastes energy transmitting control packets

5. Hidden Node Problem A B C D Data • Hidden node problem exists between every other pair nodes along a route • RTS/CTS packets constitute large overhead • Transmission rate control mechanism employed Random Backoff Data Random Backoff Data Data Collision

6. Efficient-Efficient Sensor MAC Energy-Efficient MAC Adaptive Duty Cycling Wakeup On-Demand Overemitting: Node transmits when receiver not ready for reception Reduces Throughput Increases Latency

7. E2RMAC - Design • Two radio solution • A Main radio for actual data transmission/reception • A low power pico radio to detect and transmit busy tones • CSMA/CA based • Skip Backoff mechanism: Intermediate receiving node skips random backoff after successful reception • Implicit/Explicit Ack • Transmission rate control: After receiving implicit Ack refrain from transmitting for 2communication_duration • Adaptive retransmission attempts Retransmission Attempts = Tx_Attempts + , where pe is packet error rate Protocol for always-on requirement, e.g. automotive, telematics

8. E2RMAC – Basic Operation A B C D Wakeup Random Backoff Filter Processing Delay Data Propagation and Processing Delays Wakeup Backoff Skipped Filter Implicit Ack Data Wakeup Transmission Backoff (2xCommunication_Duration + Random_Duration) Filter Data Wakeup Explicit Ack

9. E2RMAC – Handling False Wakeups Set ReceiveTimer = 2xCommunication_Duration Set ReceiveTimer = 2xCommunication_Duration X Z Y A C B Wakeup Wakeup Filter Data Filter Data Set ReceiveTimer = Communication_Duration Set ReceiveTimer = Communication_Duration

10. E2RMAC – Simulation Parameters

11. Performance Evaluation – Linear Topology • All schemes are equally reliable • Latency of E2RMAC is higher than RMAC due to latency involved in transmitting filter packets and switching on/off the main radio • STEM and E2RMAC are the only energy efficient protocols pe=0.4

12. Performance Evaluation – 8-hop Linear Topology • PDR of RTS-CTS based protocols is higher than E2RMAC as it alleviates the hidden terminal problem pe=0.4

13. Performance Evaluation • Transmission of control messages by STEM leads to better PDR, poor latency and more energy consumption • Due to adaptive retransmissions, E2RMAC tries to deliver old packets first, leading to buffer overflow at source nodes and thus dropping newly generated packet  less PDR when pe=0.4

14. Performance Evaluation • E2RMAC consumes less energy by avoiding control overhead, and false wakeup Energy expended by the common intermediate node Energy Expended by the route nodes

15. Performance Evaluation • E2RMAC and STEM protocol have comparable performances. We can conclude that transmission rate control and other optimizations successfully mitigates the hidden terminal problem

16. Summary – E2RMAC • Best suited for dual radio architecture • Energy savings largely depends on power consumption of low-power pico radio • Minimizing energy consumption • Minimum control messages • Implicit Ack by wakeup radio • Timers to avoid false wakeup • Ensuring reliability • Adaptive retransmission attempts • Implicit/explicit Ack • Transmission rate control • Minimizing latency • Skip backoff mechanism • Minimum control overhead

17. Future Work • Energy Consumption Analysis for the proposed and existing protocols • To be energy-efficient than single radio solution (no sleep cycles), preliminary results suggests that pico-radio should consume less than • 25% of Main radio power for E2RMAC • 17% of Main radio power for STEM-T  8% improvement over STEM-T

18. Thank You!!! For further queries, please contact the authors

19. Backup Slides

20. Energy Consumption Analysis

21. RMAC – Design A B C D • CSMA/CA based • Intermediate receiving node skips random backoff after successful reception • Implicit Ack • After receiving implicit Ack refrain from transmitting for transmission backoff duration = 2communication_duration Data Processing Delay Propagation and Processing Delays Backoff Skipped Data Implicit Ack Transmission Backoff Data Random Backoff Ack Explicit Ack Data

22. RMAC – Performance Evaluation • Linear topology, Packet arrival rate = 5 and 10 pkts/sec respectively • Ack-based schemes have better PDR • MultiPath and MultiPacket schemes have constant latency per hop as no retransmissions are involved at any node pe=0.2

23. RMAC – Performance Evaluation • Even at higher packet error rate, RMAC delivers more than 80% of packets • Also, latency per hop of RMAC is less than its Ack-based counterpart pe=0.4

24. RMAC – Performance Evaluation • RMAC and CSMA-Ack schemes are compared for 6-hop linear topology by varying pe from 0 to 0.6 • RMAC performs better than CSMA-Ack in all scenarios

25. RMAC – Performance Evaluation • Two 6-hop routes intersecting at the center node • At pe=0.6, RMAC uses slightly more retransmission attempts than CSMA-ACK but delivers more packets with same latency