1. Scalable Data Aggregation for Dynamic Events in Sensor Networks Kai-Wei Fan, Sha Liu, Prasun Sinha Computer Science and Engineering, Ohio State University ACM SenSys 2006

2. Outline Introduction Structure-Less Aggregation Experiments and Simulation Conclusion

3. Introduction Data Aggregation  Communication cost is often larger than computation cost.  Redundancy in raw data.  Aggregate packets near sources to reduce transmission cost.  Prolong the lifetime. Aggregation Approaches  Static structure  Dynamic structure  Structure-free

4. Static Structure for Aggregation Routing on a pre-computed structure Pros  Low maintenance cost  Good for unchanged traffic pattern Cons  Long stretch problem  Unsuitable for event-based network Sink

5. Dynamic Structure for Aggregation Create a structure dynamically Pros  Optimization for source nodes Cons  High maintenance cost Sink

6. Structure-Free Aggregation No structure  No structure maintenance cost Aggregation without structure  Where to transmit?  Wait for whom? Improve aggregating by transmitting packets to the same node at the same time  Spatial Convergence  Data Aware Anycast  Temporal Convergence  Random Waiting

7. Data Aware Anycast Anycast  One-to-any forwarding Anycast t o neighbor having packets for aggregating  Class A: Nodes closer to the sink with data for aggregation  Class B: Nodes with data for aggregation  Class C: Nodes closer to the sink Class B Canceled CTS RTS CTS Sender Class A Nbr Class B Nbr Class C Nbr Class A Nbr Class AClass C

8. Random Waiting Fixed Delay  Nodes close to sink pick high delay. Random Delay  Source nodes pick random delay between 0 and τ before transmission. … Sink τ =n τ =n-1 τ =n-2 τ =1 τ =0

9. DAA and RW Example Sink 1 2 3 4 Not guarantee aggregation of all packets from a single event !!

10. Structure-Less Aggregation Structure-free aggregation does not guarantee all packets are completely aggregated to one.  High cost for un-aggregated or partial-aggregated packets Structure-Less Aggregation (2 Phases)  1 st : Based on structure-free aggregation (DAA & RW) Aggregate packets form sources to aggregators locally  2 nd : Further aggregation on an implicitly constructed structure Aggregate packets from aggregators to sink Tree on Directed Acyclic Graphic (ToD)

11. Definition  Contiguous events  Cell: A square area with side length greater than the diameter which an event can span  F-cluster: First cluster, composed of multiple cells  S-cluster: Second cluster, composed of multiple cells (interleaved with F- cluster) 1D Construction of ToD F-clusterS-cluster

12. Tree on Directed Acyclic Graphic(ToD) sink F-clusters F-cluster-head Shortest Path a b c d F6 sink S-cluster S-cluster-head Shortest Path a bc d S5 S6 sink a b c d Shortest Path Tree F6 S6S5

13. Dynamic Forwarding for 1D (1) Forwarding Rules  Rule 0: Forward packets to F-aggregator by structure-free data aggregation protocol.  Rule 1: Event spans two cells in a F-cluster, forward to sink  Rule 2: Event spans one cells, forward to appropriate S-aggregator sink

14. Dynamic Forwarding for 1D (2) Property 1. Packets will be aggregated at a F-aggregator, or will be aggregated at a S-aggregator.  If only nodes in one cell are triggered and generate the packets  Aggregated at one F-aggregator (all nodes in a cell resides in the same F-cluster)  If nodes in two cells are triggered and generate the packets. Two cells are in the same F-cluster  aggregated at the F-aggregator Two cells are in different F-clusters  aggregated at the S-aggregator

15. Tree on Directed Acyclic Grahpic(ToD) 2D Construction A1A2B1B2C1C2 A3A4B3B4C3C4 D1D2E1E2F1F2 D3D4E3E4F3F4 G1G2H1H2I1I2 G3G4H3H4I3I4 A B D E GH F I C (a) F-clusters(b) Cells A1A2B1B2C1C2 A3A4B3B4C3C4 D1D2E1E2F1F2 D3D4E3E4F3F4 G1G2H1H2I1I2 G3G4H3H4I3I4 (c) S-clusters S1S2 S3S4 S3S4 S2S1

16. Dynamic Forwarding for 2D (1) Event may span multiple cells in a F-cluster  Assume the region spanned by an event is contiguous.  Maximum 4 cells (a) 1 Cell(a) 2 Cells(a) 3 Cells(a) 4 Cells No other F-cluster will have packets  Forward to sink Forward to other S-aggregators

17. Dynamic Forwarding for 2D (2) Forwarding Rules  Rule 0: Forward packets to F-aggregator by structure-free data aggregation protocol.  Rule 1: Event spans three or four cells in a F-cluster, forwards to sink.  Rule 2: Event spans a cell in a F-cluster, forward to a S-aggregator. F-cluster Corresponding S-cluster Cell generating packets

18. Dynamic Forwarding for 2D (2)  Rule 3: Event spans two cells, forward to two S-aggregators in order. C1C2 F-cluster X F-cluster Y S-cluster I S-cluster II C C  Forward to 1st S-aggregator (near sink), then forward to 2nd S-aggregator Sink F-aggregator S-aggregator

19. Dynamic Forwarding Example Example C3 C1C2 Sink Rule 0Rule 2Rule 3

20. Aggregator Selections Nodes play the role of F-aggregator in turn.  With probability based on residual energy  Hash current time to a node within that cluster Delegate the role of S-aggregator to F-aggregator  Select the F-aggregator in the F-cluster near sink as the S-aggregator Sink F-aggregator and S-aggregator (Right-top S-cluster) Sink

21. Dynamic Forwarding for 2D (3) Property 2. Packets will be aggregated at the F- aggregator, at the 1 st S-aggregator, or at the 2 nd S- aggregator.

22. Experiments (1) Experiments Environment  105 Mica2-based nodes  7 x 15 grid network  Node spacing: 3 feet  Transmission range: 2 grid-neighbor  2 F-clusters  Fixed event location Protocols  Dynamic Forwarding over ToD (ToD)  Data Aware Anycast (DAA)  Shortest Path Tree (SPT)  Shortest Path Tree with Fixed Delay (SPT-D)

23. Experiments (2) Event Size Better Performance: More chance of being aggregated Long Stretch Problem

24. Experiments (3) Delay Stable: Random Delay Better Performance: Heavily depends on delay

25. Experiments (4) Large Simulation Environment  2000m x 1200m area  1938 nodes (grid network)  Node spacing: 35m  Transmission range: 50m  Cell side length = Event diameter  Event with random way-point model at 10m/s for 400 seconds Protocols  ToD  DAA  SPT  OPT

26. Experiments (5) Event Size Best but not consider overhead

27. Experiments (6) Scalability (Event with different distance to sink)  Event Size: 400m  Event Area: 400m x 800m  Area Distance to Sink : 200m ~ 1400m

28. Experiments (7) Cell Size  Event Size: 200m, 400m, 600m  Best Cell Size: 200m Event  100m Cell 400m Event  200m Cell 600m Event  200m Cell  Future Work: Select appropriate cell size

29. Conclusion The paper proposes a semi-structured approach (ToD) that locally uses a structure-less technique followed by Dynamic Forwarding. ToD avoids the long stretch problem in fixed structured approach and eliminates the overhead of maintenance of dynamic structure.