Query Driven Data Collection and Data Forwarding in Intermittently

Connected Mobile Sensor Networks

Wei WU1, Hock Beng LIM2, Kian-Lee TAN1

1National University of Singapore2Nanyang Technological University

DMSN 2010, 2010-09-13

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Outline

Sparse mobile sensor networkProblem: long query answering timeProposal: query-driven data collection and

location-based data forwarding System model and assumptions Query-driven data collection Location-based data forwarding Simulation results

Conclusion

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Mobile Sensor Networks (MSNs)

Dynamic networks formed by mobile sensors.

Mobile sensors On ground, aerial, wearable

NASA UAV

NASA

Nokia

UTK

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Applications of MSNs

Reconnaissance Disaster rescueEnvironment monitoring

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Sparse MSNs

A MSN is sparse when The number of sensors is small,

Mobile sensors are more expensive than stationary sensors

Task field is big, Wireless communication range is

limited.

Characteristics The topology is dynamic. The connection between nodes is

intermittent.

BS

21 4

3

5 67

8

910

11

12

Query

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Problem

The query response time at the base station can be long. Data forwarding to BS is done in a carry-and-forward manner.

Only a small portion of data objects are forwarded to the base station.

Limited data availability at the base station. The base station cannot disseminate queries to

the sensor nodes.

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Mobile Data Collector

Mobile data collector A mobile node that moves to the sensors to

collect data objects and returns to the base station.

Can be a normal mobile sensor or a special mobile unit.

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Our Study

Purpose: reduce the query response time at the base station.

Basic idea: use a mobile data collector (MDC) to do query-driven data collection. The base station sends a pending query to a

mobile data collector (MDC). The MDC moves to the sensors to find query

answer and returns to the base station.

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Challenge

Challenge The sensor net is disconnected. How can the MDC find the sensor that has query result?

Solution Use spatial predicate in the query to direct the MDC’s m

ovement. The mobile sensors collaborate with the MDC, forward d

ata objects to neighbors along data objects’ collection path.

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Sparse MSNs System Model

One base station Receives queries from users; Receives data objects from mobile nodes.

A number of mobile sensors Move in a task field; acquire data objects periodically; Forward data towards BS when connected.

One mobile data collector (MDC) Gets query from BS; Moves to collect data objects from mobile

sensors; Returns to BS.

BS

C

21 4

3

5 67

8

910

11

12

Query

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Assumptions

Mobile sensors have GPS units. Data objects have location metadata. Queries have spatial predicates.

A query requests for a data object acquired at a specific location.

A data object can be used to answer the query if the data object was acquired at (or very close to) query location.

Sensors spend most energy on movement. Sensors have enough storage.

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Collection Path and Forwarding Region

Qp: a query requesting for a data object acquired at (near) p.

Dp: a data object acquired at location p.

Collection Path for Dp. Path(Dp) The shortest physical path in the field fro

m the base station to p.

Forwarding Region for Dp. Region(Dp) The union of the points in the field whose

distances to Path(Dp) are shorter than sensors’ communication range r.

The area covered by the MDC’s wireless signal when it moves on Path(Dp).

BS

C r

Collection Path of Dp

Forwarding Region of Dpp

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Collection Path and Forwarding Region (Cont’d) Each data object has a collection

path and a forwarding region.

The MDC moves along Path(Dp) to collect data for Qp.

Mobile sensors facilitate the MDC’s data collection. Mobile sensors (best effort) forwar

d data objects to neighbors in data objects’ forwarding regions.

Increase the chance that MDC will meet a sensor that carries Dp.

Forward data objects towards the base station.

Reduce the MDC’s move distance.

BS

C r

Collection Path of Dp

Forwarding Region of Dpp

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Simple Examples

BS

p21

3

BS

p

21

3

BS

p1

2

3

p2

Example 1 Example 2 Example 3

S2 forwards Dp to S3. S2 forwards Dp to S1. S2 forwards Dp1 to S3.

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Location-based Data Forwarding

Sensors forward data objects to sensors in data objects’ forwarding region.

Also forward data objects towards the base station. Decisions are made based on sensors’ locations and data objects’ l

ocation metadata.

Sensor Si carrying data object Dp encounters a sensor Sj that does not have Dp.

If Si is in Region(Dp) If Sj is also in Region(Dp) and Sj is closer to the BS

Si forwards Dp to Sj Else

If Sj is in Region(Dp) or Sj is closer to Region(Dp) Si forwards Dp to Sj

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Query-driven Data Collection

The BS has a pending query Qp that requests for a data object acquired at p in the field.

The BS sends Qp to the MDC. The MDC moves along Path(Dp) to collect Dp.

When connected to a sensor, it checks whether the sensor has a data object that can answer Qp.

If so, the MDC gets the data object and moves back to BS. The MDC also collects other data objects during the course.

Mobile sensors forward data objects to the MDC when connected.

MDC’s route is determined by query location.

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Performance Study

Simulation setup Follows the system model. 600m* 600m task field. Mobility model: random waypoint. Bandwidth: 2Mbps.

Parameter Unit Default Range

Number of sensors n 30 20-50

Move speed v Meters/s 2 1-8

Data object size D KB 500 100-1000

Sense interval Ts Second 20 10-60

Query interval Tq Second 20 10-60

Communication range r Meter 100 50-150

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Experiment Result

Effect of the number of sensors

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Experiment Result (2)

Effect of move speed

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Experiment Result (3)

Effect of data object size

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Summary

In sparse mobile sensor networks, query response time at base station may be long.

We propose Query-driven data collection.

Make use query’s location predicate. Location-based data forwarding to facilitate query-driven

data collection. Make use data objects’ location metadata. Improve data availability along data collection path.

Simulation results show that our solution can help reduce the average query response time at the base station.

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Future Work

Take query and data objects’ temporal information also into account.

Take nodes’ trajectory into account.Multiple path forwarding.Data collection for multiple queries.

Thank you!

Questions?

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Query-driven Data Collection and Location-based Data Forwarding Query-driven data collection.

The MDC collects data objects for queries that it gets from the base station.

The collection path is determined by the query.

Location-based Data forwarding for data collection data forwarding decision based on data objects’ location meta

data and sensors’ location. Prioritize data objects for forwarding, to reduce MDC’s collecti

on distance.

Objective: reduce the query response time at the base station by reducing the distance that the MDC needs to move before finding query answer.

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Prioritize Data Objects in Data Forwarding A sensor needs to decide what data objects to

forward. A sensor may (have acquired and) carry many data

objects. It can only forward a small number of data objects to a

neighbor (due to limited connection time).

Two situations: Forward to a neighboring sensor.

Prioritize based on quantitative measure of collection distance reduction.

Forward to the MDC. Prioritize based on collection path distance.

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Data caching

A sensor caches a data object locally after forwarding it to a neighboring sensor. It does not forward it any more.

Objective: improve data availability among mobile sensors. When the MDC encounters a sensor, it is more likely to get query answer from the sensor.

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Forwarding to a Neighboring Sensor

A sensor’s collection distance w.r.t a data object Dp. cd(si,dp) The distance that the MDC ne

eds to move to get Dp.

If Circle(si, r) intersects Path(Dp), cd(si,dp) is the distance from BS to the intersection that is nearer to BS.

If Circle(si,r) does not intersect Path(Dp), cd(si,dp) is the length of Dp.

BS

Path(Dp)

p

1 r

2 r

I2

I1

i Sensor i

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Forwarding to a Neighboring Sensor

A sensor’s collection distance w.r.t a data object Dp. cd(si,dp) The distance that the MDC ne

eds to move to get Dp.

If Circle(si, r) intersects Path(Dp), cd(si,dp) is the distance from BS to the intersection that is nearer to BS.

If Circle(si,r) does not intersect Path(Dp), cd(si,dp) is the length of Dp.

BS

Path(Dp)

p

1 r

2 r

I2

I1

i Sensor i

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Forwarding to a Neighboring Sensor (Cont’d) Si, Sj, Dp

Delta-collection-distance:cd(Si,Dp) – cd(Sj,Dp)

It is the moving distance of MDC that is saved by the data forwarding if the MDC goes to collect Dp.

Si prioritizes the data objects for forwarding to Sj based on their delta-collection-distances. The data object with the largest delta-collection-distance is forwarded first.

For the data objects whose delta-collection-distances are zero. Si considers the data objects that are outside their forwarding regions. Si forwards Dp to Sj if Sj is closer to Dp’ forwarding region.

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Forwarding to the MDC

When a sensor is connected to the MDC, the sensors forwards data objects to the MDC. Objects are prioritized based on the lengths of

their collection paths. Objects whose locations are the furthest from

the BS are forwarded to the MDC first.

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Effect of Sense Interval

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Effect of Query Interval

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Effect of communication range