1. 1 MobiQuery: A Spatiotemporal Query Service in Sensor Networks Chenyang Lu, Guoliang Xing, Octav Chipara, Chien-Liang Fok, Sangeeta Bhattacharya Department of Computer Science and Engineering Washington University in Saint Louis ICDCS 2005

2. 2 Outline Introduction Problem formulation MobiQuery – Motion prediction – Prefetching – Query dissemination – Data collection Simulation Conclusion

3. 3 Introduction ~motivation~ Supporting query from users is one of the most important function of sensor networks Query from mobile users in mission-critical applications has not been addressed – Mobile users and moving query areas – Stringent real-time requirement

4. 4 Mission-Critical Applications

5. 5 Problem Formulation Example: “ Update a temperature map within 100m every 2s. Data can be at most 1s old. ” Spatial constraint – Query area: range of 100m all and only the sensors within the query area should respond to the query – Query area moves with the user Temporal constraints – Query period: 2s results must be delivered before end of current period – Data freshness: 1s

6. 6 Assumption Network runs a power management protocol – Maintain a backbone of active nodes – Examples: CCP, SPAN, GAF Every node knows its location and neighbor information Nodes have synchronized clocks

7. 7 MobiQuery User Sensor nodes

8. 8 MobiQuery ~Motion prediction~ Motion profile User plan his path in advance Motion profile available before actual movement – Ta: advance time of prediction – Tv: how long the prediction is valid – tg: motion profile generation time – ts: the start time in which the prediction active tgtg tsts TaTa TvTv time Motion profile

9. 9 MobiQuery ~Prefetching~ MobiQuery uses prefetching to notify future query areas of an impending query – Send prefetch msgs to future pickup points – pickup point: the user expects to receive the next query result Greedy prefetching Just-in-time prefeching

10. MobiQuery ~Greedy Prefetching~ : Pickup point : Collector

11. MobiQuery ~Just-in-time Prefetching~ : Pickup point : Collector

12. 12 MobiQuery ~prefetching forwarding time~ prefetch forwarding time – The time at which the prefetch message should be forwarded to the next pickup point such that the temporal constraints are met the (k −1)th collector node should send the prefetching message to the kth collector node – – T period : the time that the user expects to receive query results – T sleep : Max delay due to duty cycle – T fresh : Max data age

13. MobiQuery ~example~ 1 2 3 T period :2s T sleep :0sT fresh :1s (2-1)*2-0-2*1 = 0 (3-1)*2-0-2*1 = 2 (4-1)*2-0-2*1 = 4 4 (5-1)*2-0-2*1 = 6 5 : collector : user

14. 14 MobiQuery ~query dissemination~ The node receiving a prefetch msg distributes the query to all nodes in query area A tree is set up during query dissemination Sleeping nodes are restricted to be leaves – Wake up when user arrives – Resume sleeping after collecting & sending data

15. MobiQuery ~query dissemination~ : Pickup point : Collector

16. 16 MobiQuery ~data collection~ Must finish within T fresh due to data freshness constraint Parent nodes wait for results from children to enable data aggregation – May miss query deadline due to child failures – Solution based on timeouts Each node sends results received so far when timeout Leaf nodes send results at T fresh before query deadline Nodes closer to the root have later timeouts Query results always meet deadline due to the timeouts, possibly with incomplete results

17. MobiQuery ~query dissemination~ : Pickup point : Collector

18. 18 Simulation ns2 simulator CCP (Coverage Configuration Protocol) 802.11 PSM 200 nodes randomly distributed 450×450 m 2 rectangle area Sensing range: 50m Communication range: 105m Query tree 2~4 levels Query period 2s Query freshness 1s

19. 19 Simulation

20. 20 Simulation

21. 21 Simulation

22. 22 Simulation

23. 23 Conclusion Presented MobiQuery, a novel sptiotemporal query service – Meet stringent spatiotemporal constraints through just- in-time prefetching