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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
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Outline Introduction Structure-Less Aggregation Experiments and Simulation Conclusion
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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
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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
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Dynamic Structure for Aggregation Create a structure dynamically Pros Optimization for source nodes Cons High maintenance cost Sink
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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
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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
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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
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DAA and RW Example Sink 1 2 3 4 Not guarantee aggregation of all packets from a single event !!
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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Dynamic Forwarding Example Example C3 C1C2 Sink Rule 0Rule 2Rule 3
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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
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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.
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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)
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Experiments (2) Event Size Better Performance: More chance of being aggregated Long Stretch Problem
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Experiments (3) Delay Stable: Random Delay Better Performance: Heavily depends on delay
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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
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Experiments (5) Event Size Best but not consider overhead
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Experiments (6) Scalability (Event with different distance to sink) Event Size: 400m Event Area: 400m x 800m Area Distance to Sink : 200m ~ 1400m
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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
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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.
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