Chia-Hung Tsai, Tsu-Wen Hsu, Meng-Shiuan Pan, and Yu-Chee Tseng

1. Cross-Layer, Energy-Efficient Design for Supporting Continuous Queries in Wireless Sensor Networks A Quorum-Based Approach Chia-Hung Tsai, Tsu-Wen Hsu, Meng-Shiuan Pan, and Yu-Chee Tseng Springer Netherlands Wireless Personal Communications 2009

2. Outline • Introduction • System architecture • Quorum layer • Query-processing layer • Simulation results • Conclusions

3. Introduction • Power saving and query processing are two main issues in WSNs • Applying the quorum-based power-saving protocols to the continuous query-processing problem • This paper contributes in proposing a cross-layer approach to integrating the grid-quorum system with continuous queries

4. System architecture

5. Query session information • Each node will maintain a QSI table to keep track of the query paths that currently pass it and the quorums to support these paths

6. Query format • Query is denoted by a 5-tuple (sn, sr, t, p, len) • sn is the sink node • sr is the source node • t is the lifetime of the query • p is the period that sr will generate reports • len is the expected packet length per report

7. Quorum layer • Grid quorum system • Quorum set for continuous queries

8. Grid quorum system

9. Quorum set for continuous queries • The wake-up/sleep schedule of a node will be determined by one or multiple grid quorums, which we call quorum set • Each grid quorum is denoted by a 4-tupleg = (n1, n2, R, C) • We define the duty cycle of a grid quorum g

10. Example dty(g) = (4+3-1) / 12 = 1/2

11. Query-processing layer • As more and more continuous queries pass the node, its quorum set will contain more grid quorums • A DSR-like routing protocol will be applied • An energy cost function will be defined to evaluate the quality of a query path

12. Query-processing • Query-Requesting Process • Query-Replying Process • Query-Removing Process • Local Slot Synchronization

13. Query-Requesting Process • Quorum preparing • QREQ initiating and processing • QREQ rebroadcasting

14. Quorum preparing y = (sn, sr, t, p, len) • When a sink node sn has a query y to a source node sr, it will compute a grid quorum gini to support the query y

15. Example len/r * 1/p 100/10 * 1/50 = 0.2 y = (sn, sr, t, p, len) y= (0, 8, 500, 50, 100) n1=3 n2=3 dty(g) = (3+3-1)/9 = 0.56

16. QREQ initiating and processing • There are two cases involving in producing a QREQ packet: • a node initiates a new query • a node receives a QREQ and rebroadcasts it

17. QREQ • We suppose that node xi receives from node xi−1 a QREQ(gini, y, c, PATH) for possibly supporting a query y initiated by node x0 • gini is the grid quorum computed by x0 • c is the cost calculated by xi−1 • PATH is a list of 2-tuples, where each 2-tuple is of the form (node_id, quorum)

18. Duty cycle • xi will find a quorum to serve query y, which we call gser(y) • Given G(xi), we can estimate xi’s duty cycle as follows:

19. Example 1-0.5=0.5 1-0.625=0.375 0.5*0.375=0.1875 Dty=1-0.1875=0.8125

20. Traffic load • From xi’s QSI, we can measure xi’s current traffic load as follows • len(z) is the length of each sensing report • p(z) is the period per report for query z • xi’s current traffic load is

21. Capacity(1/2) • xi can measure whether its current quorum set can accommodate y or not by checking LD(xi) + ld(y) ≤ DTY (G(xi)) • The capacity of gcan is defined as follows • QS(gcan) means the set of quorum slots of gcan • s-deg(sj) is the share degree of the quorum slot sj in gcan

22. Capacity(2/2) • If there exists one gcan such that then gcan will be assigned to support y and we will set gser(y) = gcan

23. Costs • The average extra energy cost Cact for xi to remain active per slot • The average extra energy cost Ctx for xi to transmit data for y per slot

24. The average extra energy cost to remain active per slot • Eact is the energy to remain active for one full slot

25. The average extra energy cost to transmit data for y per slot • Etx is the energy to transmit one full slot of data

26. QREQ rebroadcasting • Node xi will also maintain the minimum cost cmin for all paths from x0 to xi that xi has learned so far

27. Query-replaying process • Node xi will collect QREQs for a while and choose the QREQ(gini,y, c, PATH) with the lowest cost c • Then xi will unicast QREP(y, PATH) back to x0

28. Quorum set • If G(xj) = {gdef}, xj will directly set G(xj) = {gser(y)} • Otherwise, xj will set G(xj) = G(xj) ∪ {gser(y)} • After a node adjusts its quorum set, it can wake up and sleep according to the quorums in its set

29. Query-removing process • Eachintermediate node when receiving the QREM(y) will remove the corresponding entry from its QSI table

30. Local slot synchronization • At the clock level, two neighboring nodes will try to synchronize their clocks by aligning their slots • At the quorum level, they will try to synchronize this quorum by aligning the first slot of this quorum at each side

31. Assign priority • Along a query path, a node that is closer to the source node has a higher priority • Between two query paths, the path which was established earlier has a higher priority

32. Simulation results • Set up a 400 × 400 m2 sensing field, on which hundreds of sensor nodes are randomly deployed • Transmission range and carrier sensing range of each sensor node are set to 50 and 100 m • The whole simulation time is 7200 seconds

33. Quorum setting • The default quorum gdef is set to (40, 40, {1}, {1}) with each quorum slot fixed to 0.1 second • Initially operate under 5% duty cycle and each quorum group is 160 seconds

34. Path sharing

35. Residual energy

36. Impact of our cross-layer design • The first one lets each query path adjust its quorum on this own [referred to as SP-NC (shortest-path, no-coordination)] • The second one enforces all quorum paths to share the same quorum [referred to SP-GQ (shortest path,global-quorum)]

37. Comparison with the SP-NC Scheme • Each query reporting period is set to 60 seconds

38. Comparison with SP-GQ scheme • The SP-GQ scheme will pick the quorum with the lowest duty cycle that can meet all nodes’ requirement as the global quorum

39. Impact of traffic loads • Impact of Transmission Rate • Impact of Packet Length • Impact of Query Period

40. Impact of Transmission Rate • A smaller transmission rate r will result in slower transmission (and thus a higher traffic load) • Evaluate the energy consumption of our system by varying the transmission rate at 250 kbps, 100 kbps, 50 kbps, and 10kbps

42. Impact of Packet Length • Vary the length len per report to evaluate the energy performance • The transmission rate r is fixed to 250 kbps and len varies from 100, 1000, to 5000 bytes

44. Impact of Query Period • We set r = 250 kbps and len = 100 bytes and vary the reporting period p from 30 to 70 seconds • A higher reporting period will incur less energy consumption

46. Conclusions • Increasing the overlapping of query paths for energy efficiency • We modify the original DSR routing scheme by adding a cost metric to choose quorums along a query path • Simulation results also verify the correctness and performance of the proposed scheme