A Wakeup Scheme for Sensor Networks: Achieving Balance between Energy Saving and End-to-end Delay

1. A Wakeup Scheme for Sensor Networks: Achieving Balance between Energy Saving and End-to-end Delay Xue Yang and Nitin H. Vaidya University of Illinois at Urbana-Champaign IEEE RTAS 2004 May 26, 2004

2. Problem Statement • Sensor networks have limited energy supply • Two different ways to exploit the energy saving • Spatial redundancy • Temporal energy saving • Wakeup schemes

3. Problem Statement • Wakeup schemes • have great potential in energy saving • incur wakeup delay, which can lead to long message delay. • Focus of this paper: • Achieve the balance between energy saving and end-to-end delay

4. Related Work • STEM (Sparse Topology and Energy Management) [appeared in mobihoc’02 by Schurgers et al ] • Two radios, wakeup radio and data radio,have the same power consumption characteristics and channel bit rate • A node periodically wakes up to listen to the wakeup radio, and keeps awake upon receiving a wakeup packet intended for itself. • The wakeup packets are sent by some nodes that have packets to send.

5. awakened Twack Twack Twack Sender Tw Tw Tw Tw T Duty Time (Td) Duty Time (Td) Receiver sleep STEM • Wakeup delay largely depends on T • Energy saving is determined by the duty ratio of T/Td • Td, which equals to 2Tw + Twack, depends on the wakeup radio bit rate.

6. Wakeup Delay of STEM • To achieve a ten-fold decrease of energy consumption, the wakeup latency is about 1.3 s per hop, using a wakeup radio with bit rate of 2.4 Kbps. • The end-to-end delay only due to wakeup delay is as large as 13 s if the message needs to be forwarded 10 hops.

7. Network stack of wakeup schemes Access delay Packet delay Transmission delay

8. Wakeup delay Wakeup next hop node Wakeup next hop node Wakeup next hop node Packet delay Transmit packet Transmit packet Proposed solution to reduce end-to-end delay • Pipelined Wakeup: • Pipeline the wakeup procedure with the packet transmission to hide the wakeup delay.

9. Problems need to be addressed • Which node is on the packet forwarding path and should be awakened in advance? • Wakeup delay should be less than packet delay so that it can be effectively hidden. Under such a constraint, how to retain sufficient amount of energy saving?

10. Pipelined Tone Wakeup (PTW) • Which node is on the packet forwarding path and should be awakened in advance? • Wakeup all one-hop neighbors. • How to retain sufficient amount of energy saving while hiding most of the wakeup delay?

11. PTW (cont.) • Which node is on the packet forwarding path and should be awakened in advance? • Wakeup all one-hop neighbors. • How to retain sufficient amount of energy saving while hiding most of the wakeup delay? • Duty Ratio T/Td determines the energy saving. • Reduce the duty time Td of each node using wakeup tones instead of wakeup packets

12. Wakeup Tone Detection Time Probability of correct detection Wakeup tone detection time (us) (Comply to the design guidelines of RF Monolithics TR1000)

13. Duty time 1 Wakeup period T Duty time 2 Wakeup period T PTW (cont.) • Advantages of using wakeup tone in PTW: • All one-hop neighbors can be awakened by sending a wakeup tone • A shorter duty time is needed for each node, which can help to retain the energy saving under the constraint for wakeup delay.

14. PTW (cont.) • Disadvantage of using wakeup tone: • Not only the intended receiver but all the one-hop neighbors around the sender will be awakened • Energy wastage caused by unnecessary wakeup should be reduced as much as possible. • Transmit short notification packets before data packets

15. Example of PTW

16. Sender: Tevent Initiate wakeup procedure Turn off radio duty time Td Receiver: React to wakeup T T T T Duty Ratio: Duty = T / Td Turn off radio Analysis for Energy Consumption • Objective: Identify the condition under which the proposed PTW scheme can show improvements on energy saving, while using pipelining to reduce the end-to-end delay. “monitoring state”

17. STEM awakened Twack Twack Twack Sender Tw Tw Tw Tw T Duty Time (Td) Duty Time (Td) sleep Receiver PTW Sender Send wakeup tone T awakened Duty Time (Td) Duty Time (Td) Receiver sleep Wakeup Procedures of STEM and PTW

18. Result of Analysis • A loose bound: the proposed PTW scheme can save more energy when the following constraint is satisfied:

19. Result of Analysis • When Dutystem = 10and Dutytone = 100, Tevent > 21 T is sufficient. • Constrained by the wakeup delay, T cannot be too large. Assume T = 1s, then Tevent > 21 s can be easily satisfied by many sensor network applications.

20. Performance Evaluation Radio Power Consumption (TR1000)

21. Simulation Results

22. Results for random networks Dutystem=5.33 NPM Dutystem=13.33 PTW (S)

23. Results for random networks Dutystem=13.33 Dutystem=5.33 PTW NPM

24. Conclusion • A pipelined tone wakeup scheme is proposed for sensor networks • An asynchronous wakeup pipeline is constructed to overlap the wakeup procedures with packet transmissions. • An end-to-end delay close to the case without power management can be achieved while a major energy saving can be obtained.

25. Thank you!

26. Backup Slides

27. Traffic prediction • Chunyu Hu and Jennifer C. Hou, LISP: a link-indexed statistical traffic prediction approach to improving IEEE 802.11 PSM, in Proc. of IEEE Int'l Conf. on Distributed Computing Systems (ICDCS'04), March, 2004.