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

Outline

Introduction Structure-Less Aggregation Experiments and Simulation Conclusion

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

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

Dynamic Structure for Aggregation Create a structure dynamically

Pros Optimization for source nodes

Cons High maintenance cost

Sink

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

Data Aware Anycast

Anycast One-to-any forwarding

Anycast to 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

Canceled CTS

RTS

CTS

Sender

Class A Nbr

Class B Nbr

Class C Nbr

Class A Nbr

Class A Class C

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

DAA and RW Example

Sink

1

2

3

4

Not guarantee aggregation of all packets from a single event !!

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) 1st : Based on structure-free aggregation (DAA & RW)

Aggregate packets form sources to aggregators locally

2nd : Further aggregation on an implicitly constructed structure Aggregate packets from aggregators to sink Tree on Directed Acyclic Graphic (ToD)

Tree on Directed Acyclic Graphic(ToD) 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-cluster S-cluster

Tree on Directed Acyclic Graphic(ToD)

sink

F-clusters

F-cluster-head

Shortest Path

a b c d

F6

sink

S-cluster

S-cluster-headShortest Path

a b c d

S5 S6

sink

a b c d

Shortest Path Tree

F6

S6S5

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

sink

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

Tree on Directed Acyclic Grahpic(ToD) 2D Construction

A1 A2 B1 B2 C1 C2

A3 A4 B3 B4 C3 C4

D1 D2 E1 E2 F1 F2

D3 D4 E3 E4 F3 F4

G1 G2 H1 H2 I1 I2

G3 G4 H3 H4 I3 I4

A B

D E

G H

F

I

C

(a) F-clusters (b) Cells

A1 A2 B1 B2 C1 C2

A3 A4 B3 B4 C3 C4

D1 D2 E1 E2 F1 F2

D3 D4 E3 E4 F3 F4

G1 G2 H1 H2 I1 I2

G3 G4 H3 H4 I3 I4

(c) S-clusters

S1 S2

S3 S4S3 S4

S2S1

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

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

Dynamic Forwarding for 2D (2)

Rule 3: Event spans two cells, forward to two S-aggregators in order.

C1 C2

F-cluster X

F-cluster Y

S-cluster I S-cluster IIC C

Forward to 1st S-aggregator (near sink), then forward to 2nd S-aggregator

Sink

F-aggregator

S-aggregator

Dynamic Forwarding Example Example

C3

C1 C2

Sink

Rule 0 Rule 2 Rule 3

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

Dynamic Forwarding for 2D (3) Property 2. Packets will be aggregated at the F-

aggregator, at the 1st S-aggregator, or at the 2nd S-aggregator.

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)

Experiments (2)

Event Size

SourcesngContributiofNumber

onsTransmissiTotalofNumber

onTransmissi ofNumber Normalized

Better Performance:More chance of being aggregated

Long Stretch Problem

Experiments (3)

Delay

Stable: Random Delay

Better Performance: Heavily depends on delay

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

Experiments (5)

Event Size

Best but not consider overhead

Experiments (6)

Scalability (Event with different distance to sink)

Event Size: 400m Event Area: 400m x 800m Area Distance to Sink

: 200m ~ 1400m

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

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.