1. KAIS T Distributed cross-layer scheduling for In-network sensor query processing PERCOM 2006 2006. 11. 23(THU) Lee Cheol-Ki Network & Security Lab.

2. 2 / 17 컴퓨터 구조 특강 Contents Introduction Related work System overview Schedule construction Schedule execution Evaluation Conclusion and future work

3. 3 / 17 컴퓨터 구조 특강 Introduction(1/2) In-network sensor query processing systems Data acquisitional applications of wireless sensor networks Inject queries into the network The networked sensor nodes work together to process the queries Send back the query results Critical issue : To reduce power consumption Major sources of energy waste Idle listening, over hearing, collision and control packet overhead Immediate solution To make the nodes sleep as much as possible Focus on tree networks (similar to the existing schemes)

4. 4 / 17 컴퓨터 구조 특강 Introduction(2/2) Process of systems A node first checks Applicable Transmission Slots Sends it to parent The parent sends the assigned transmission timing The node arranges it about tasks in all layers Execution the schedules

5. 5 / 17 컴퓨터 구조 특강 Related work(1/2) Scheduling has been applied to different layers TinyDB in query layer S-MAC in MAC layer Centralized approach (Florens) Sink allocate transmission timing to all other nodes in WSN Sink know the network topology → difficult, high cost Destributed approach Flexible Power Scheduling (FPS) Distributed on-demand power-management protocol for tree networks Reducing the collision between siblings But, not reducing the collision between neighbors that are not siblings

6. 6 / 17 컴퓨터 구조 특강 Related work(2/2) Sichitiu The source node broadcast a special route setup packet with neighbors Temporary schedule → permanent schedule (after arriving at sink node) Frequent collision → dead nodes → inaccurate query result In this paper Considering all tasks query injecting, computation, aggregation Consecutive sleeping and transmission timing to nodes All layers of a query processing system are involved

7. 7 / 17 컴퓨터 구조 특강 System overview In-network sensor query processing system Construct routing tree Sink inject a query Nodes construct schedules Sink broadcasts synch-signal Bottom-up report to sink Run following the schedules Scheduling module Schedule construction Time synchronization Schedule execution

8. 8 / 17 컴퓨터 구조 특강 Schedule construction(1/4) Schedule construction module Constraints for schedule construction

9. 9 / 17 컴퓨터 구조 특강 Schedule construction(2/4) Constraints Focus on collisions at the parent node of a sender Complete collision-free scheduling → high overhead, impractical In practical D and E sends packet simultaneously ⇒ Collision occurs in B But, don’t affect to the query results

10. 10 / 17 컴퓨터 구조 특강 Schedule construction(3/4) Notation Variables CTS (Children Transmission Slots), ATS (Applicable Transmission Slots), TSI (Transmission Slot Information), CATS Type of time slots PL/R(processing-listening/receiving), Q/M(Query injection/maintenance), Sleeping slots, transmission slots Procedures A node, to determine ATS (DetermineApplicableTransmissionSlots) Sends the packet to the parent Parent node, CTS for children (AllocateChildrenTransmissionSlots) A node receive TSI from the parent node Sends ACK

11. 11 / 17 컴퓨터 구조 특강 Schedule construction(4/4) Complete schedule ConstructSchedule arranges the time slots for PL/R, Q/M

12. 12 / 17 컴퓨터 구조 특강 Schedule execution Execution involves the timing control of all layers MAC layer checks whether it is time for transmission Earlier than allocated transmission time Copy the messages to a memory buffer and wait Timer setup (automatically transmission) Waiting time should be shorter than the interval of 2 transmissions Routing layer Control the timing of transmission for the route maintenance messages

13. 13 / 17 컴퓨터 구조 특강 Evaluation(1/4) Scheme comparison (DCS, FPS, SS) The experimental setup The same experimental setup as that Sichitiu used 100 nodes in an 80m*80m rectangular area, Trans-range : 25m Query processing performance The experimental setup 10 Crossbow MICA2 motes, optimized TinyDB, original TinyDB HP-4156 oscilloscope for measuring the power consumption VMNet simulator

14. 14 / 17 컴퓨터 구조 특강 Evaluation(2/4) Scheme comparison Scheduling overhead Time of constructing a schedule Schedule comparison Number of dead node Non-scheduled node, conflict-scheduled node AFS (Average frequency of switching between an active slot and a sleeping slot)

15. 15 / 17 컴퓨터 구조 특강 Evaluation(3/4) Query processing performance Schedule for in-network aggregation VMN : Emulated network in VMNet Query 1 SELECT avg(light) FROM sensors Sample intervals 2, 10, 60 seconds Comparison optimized TinyDB with original TinyDB 42%, 67%, 75% at sample interval of 2s, 10s and 60s

16. 16 / 17 컴퓨터 구조 특강 Evaluation(4/4) Power consumption improvement Difference between measurement in VMNet and that in the real WSN within ±15% Difference in the topologies Measurement errors of the oscilloscope

17. 17 / 17 컴퓨터 구조 특강 Conclusion and future work Propose scheduling scheme for Power efficiency Reduce the number of dead nodes Reduce Switching frequency In-efficient to reconstruct a schedule when A change in the network topology A new query arrives