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

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

3. 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) 컴퓨터 구조 특강

4. 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 컴퓨터 구조 특강

5. 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 컴퓨터 구조 특강

6. 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 컴퓨터 구조 특강

7. System overview • In-network sensor queryprocessing 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 <A typical WSN setup> <Architecture of scheduling module> 컴퓨터 구조 특강

8. Schedule construction(1/4) • Schedule construction module • Constraints for schedule construction <Example of constraint ⅲ> 컴퓨터 구조 특강

9. 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 <An example of the collision> 컴퓨터 구조 특강

10. 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 컴퓨터 구조 특강

11. Schedule construction(4/4) • Complete schedule • ConstructSchedule arrangesthe time slots for PL/R, Q/M <Example of complete schedules> 컴퓨터 구조 특강

12. 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 컴퓨터 구조 특강

13. 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 컴퓨터 구조 특강

14. 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) <Scheduling overhead> <Average frequency of switching> <Number of dead nodes> 컴퓨터 구조 특강

15. 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 <The VMN topology with schedules> 컴퓨터 구조 특강

16. 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 <Power consumption improvement> 컴퓨터 구조 특강

17. 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 컴퓨터 구조 특강