Efficient Scheduling for Sensor Networks

1. Efficient Scheduling for Sensor Networks Andreea Berfield & Daniel Mosse Computer Science Department University of Pittsburgh

2. Overview • Introduction and motivation • Proposed protocols: ETDMA and OTAG • Example • Experimental results • Discussion • Conclusions

3. Introduction and Motivation • Sensors suffer from different crippling limitations • Power consumption • Communication range • Processing capabilities

4. Introduction and Motivation • Self-organizing sensors • Collisions-detection protocols(TAG, Cougar) • Creation of routing trees using broadcast messages • Transmission: all sensors on a level have an interval of time/epoch to send their results parents BS Level 0 Level 1 Level 2

5. Introduction and Motivation • Fixed timeslots protocols (TDMA) • No collisions, no retransmissions • Less time awake • Problems? • Efficient creation and distribution of the transmission schedule: many messages exchanged or global knowledge required, time and energy consumption

6. Proposed Solutions: ETDMA and OTAG Overview • Sensors organize in tree hierarchy • Passes needed to create the routing tree (RT) & disseminate the schedule: • Message from base station (BS) to join the RT • Communicate to parent the time in your sub-tree • Message from BS to help pick a time slot

7. ETDMA and OTAG Overview • The third phase: • Each sensor receives and interval Ti = [starti, endi] • Selects time to sense, receive, compute and send • Sends smaller sub-intervals to its children Ti, child j = [starti,j, endi,j]  Ti

8. ETDMA (Efficient TDMA) • Actions of leaf nodes are serialized • Each node stays awake minimum time • No collisions • All non-leaf nodes wake up when their first child sends its result, at time endi,0 - TRANSMIT and go to sleep after last child transmits at time endi,totalChildren -1

9. ETDMA Variants • ETDMA-Opt1 • Schedules Sense and Compute in parallel for all leaf nodes • ETDMA-Opt2 • Improves ETDMA-Opt1 by allowing BSs to go to sleep between consecutive transmissions from children

10. OTAG (Optimal TAG) • Perfect TAG or Optimal TDMA • TAG with no collisions • All nodes stay awake minimum, only to sense, receive, compute and send • Ideal protocol, lower bound for comparison

11. Example Possible RT configuration for 5*5 grid topology:

12. ETDMA for Sub-tree in Fig. a) S10 R10 C10 T10 S,C5 T5 S,C11 T11 S,C15 T15 Time

13. OTAG for Sub-tree in Fig. a) S10 R10 C10 T10 S,C5 S,C11 S,C15 T5 T11 T15 Time

14. Experimental Setup • CSIM simulator • Grid topology with different sizes (25*25 to 45*45) and different # of BSs • Sense and Compute are 1ms, Receive and Transmit 9ms and collision backoff 44ms

15. Experimental Setup • The results presented are the averages obtained after 10 different runs • Different network configurations lead to different # levels and # nodes/level • Metrics used: • Average Time Awake (ATA) • ATA per Level (ATAL)

16. Experimental Results: ATAL

17. Experimental Results: ATA

18. Discussion: Potential Issues • Failures and mobility • Global reconstruction • Dynamic local reconstruction • Parallelism • TAG has inherit parallelism in transmission • A*, schedule RT and parts of RT in parallel • Clock synchronization • Assume loose clock synchronization

19. Conclusions • We proposed two energy-minimization protocols for both schedule creation and distribution • They require small amount of global information • Experimental results suggest a 2-3 fold reduction in average time awake per node compared to TAG

20. Thank you! Questions?