Pods:Pods:Issues in the Issues in the

Design of Practical Design of Practical Ad-HocAd-Hoc

Sensor NetworksSensor NetworksEdoardo BiagioniEdoardo Biagioni

Information and Computer Information and Computer SciencesSciences

University of Hawaii at ManoaUniversity of Hawaii at Manoaesb@hawaii.eduesb@hawaii.edu

OutlineOutline

The PODS wireless ad-hoc sensor The PODS wireless ad-hoc sensor networknetwork

Issues for sensor networksIssues for sensor networks ScalabilityScalability EnergyEnergy Other issues, including collaborative Other issues, including collaborative

event detection, position detection, event detection, position detection, mobilitymobility

Closing commentsClosing comments

Practical Sensor Practical Sensor NetworksNetworks

Deployed in the real worldDeployed in the real world Often in hostile environmentsOften in hostile environments Must function unattended for a long timeMust function unattended for a long time Reliability: network must survive node Reliability: network must survive node

failure(s)failure(s) Should be invisible or at least unobtrusiveShould be invisible or at least unobtrusive Variety of tasks, including video, images, Variety of tasks, including video, images,

sound, IR, and event detectionsound, IR, and event detection

The PODS Wireless Sensor The PODS Wireless Sensor NetworkNetwork

University of HawaiiUniversity of Hawaii Information and Information and

Computer Sciences, Computer Sciences, BotanyBotany

Study endangered Study endangered plants, of which plants, of which little is knownlittle is known

Example: Example: Silene Silene Hawaiiensis Hawaiiensis (at (at right)right)

Wireless Ad-Hoc Sensor Wireless Ad-Hoc Sensor NetworksNetworks

Use wireless data transmission Use wireless data transmission (802.11/Wi-Fi, Bluetooth, other radio (802.11/Wi-Fi, Bluetooth, other radio or infrared) to communicate dataor infrared) to communicate data

Each node generates data and also Each node generates data and also forwards dataforwards data

Networks self-configure to deliver Networks self-configure to deliver data where needed, typically to one data where needed, typically to one or more base stationsor more base stations

PODS goalsPODS goals

Study endangered plant species Study endangered plant species in situin situ Do not disturb environment: camouflage Do not disturb environment: camouflage

equipment, limit human visits, use equipment, limit human visits, use wireless ad-hoc networkswireless ad-hoc networks

Collect high resolution images: Collect high resolution images: phenologyphenology Weather sensing: low resolution, many Weather sensing: low resolution, many

sensorssensors Near real-time data available on the web: Near real-time data available on the web:

http://www.pods.hawaii.edu/http://www.pods.hawaii.edu/

High Resolution Images:High Resolution Images:Hawaiian RainforestHawaiian Rainforest

PODS strategiesPODS strategies

Collect data at regular intervals: once/hour Collect data at regular intervals: once/hour for images (daytime), every 10 minutes for for images (daytime), every 10 minutes for weatherweather

Camouflage everythingCamouflage everything Pods may “sleep” to conserve energyPods may “sleep” to conserve energy Use all possible routes (i.e. multipath Use all possible routes (i.e. multipath

routing) to send data to the base stationrouting) to send data to the base station Use a two-level hierarchy to decrease Use a two-level hierarchy to decrease

traffictraffic Visualization turns Visualization turns datadata into into informationinformation

Wireless Sensor Network Wireless Sensor Network IssuesIssues

ScalingScaling Energy Consumption and EfficiencyEnergy Consumption and Efficiency

Event Detection and Inter-Node Event Detection and Inter-Node cooperationcooperation

Node Position Detection and ProcessingNode Position Detection and Processing Mobility: Fixed-Mobile, Self-PositioningMobility: Fixed-Mobile, Self-Positioning

ScalabilityScalability Today’s wireless ad-hoc sensor networks: at Today’s wireless ad-hoc sensor networks: at

most a few hundred nodesmost a few hundred nodes Tomorrow’s wireless ad-hoc sensor networks: Tomorrow’s wireless ad-hoc sensor networks:

thousands to millions of nodesthousands to millions of nodes Current MANET (Mobile Ad-hoc NETwork) Current MANET (Mobile Ad-hoc NETwork)

protocols broadcast requests to find routes, protocols broadcast requests to find routes, which is not scalablewhich is not scalable

Many protocols only work with limited Many protocols only work with limited network diameternetwork diameter (maximum number of hops) (maximum number of hops)

Data transfer does not scale [Gupta and Data transfer does not scale [Gupta and Kumar]Kumar]

Scalable TechniquesScalable Techniques Only broadcast where absolutely necessary, e.g. Only broadcast where absolutely necessary, e.g.

to establish routes to the base station(s)to establish routes to the base station(s) Combine, compress, and reduce the data to be Combine, compress, and reduce the data to be

sent, e.g. by only sending when an event is sent, e.g. by only sending when an event is detecteddetected

Almost free: localized distributed computationAlmost free: localized distributed computation Almost free (but not always possible): Almost free (but not always possible):

geographic routinggeographic routing Reduce congestion by running different Reduce congestion by running different

networks in the same area, using multiple paths networks in the same area, using multiple paths when possible [Chen]when possible [Chen]

Scalable Hierarchical Scalable Hierarchical NetworkNetwork

If two wireless ad-hoc networks use If two wireless ad-hoc networks use different frequenciesdifferent frequencies

They can run in the same area without They can run in the same area without conflictconflict

A A backbonebackbone network of more powerful network of more powerful processors can take pictures, provide processors can take pictures, provide redundant communicationredundant communication

A network of low-power, simple A network of low-power, simple processors can carry sensors, do simple processors can carry sensors, do simple in-network processingin-network processing

Hierarchical NetworkHierarchical Network

Energy EfficiencyEnergy Efficiency A low-power x86 system PC fed by a A low-power x86 system PC fed by a

large lead-acid battery might run for two large lead-acid battery might run for two weeksweeks

Can use Solar Panels, but expensive and Can use Solar Panels, but expensive and hard to camouflagehard to camouflage

Energy might be the dominant cost and Energy might be the dominant cost and weight in a large sensor network: weight in a large sensor network: processor $10-$100, 10-100g, radio processor $10-$100, 10-100g, radio similar, batteries $100 (and processors similar, batteries $100 (and processors are getting smaller and cheaper faster are getting smaller and cheaper faster than batteries)than batteries)

Addressing Energy Addressing Energy EfficiencyEfficiency

Don’t use x86:Don’t use x86:

ARM can be twice as efficient, PIC ARM can be twice as efficient, PIC can be 1000 times more efficientcan be 1000 times more efficient

Use sleep mode whenever possible – Use sleep mode whenever possible – protocols must accomodate thisprotocols must accomodate this

Use directional antennas to gain Use directional antennas to gain greater range without additional greater range without additional power (power to go a distance power (power to go a distance dd can can increase as fast as increase as fast as dd44))

Event DetectionEvent Detection

Detection done by a single-node, e.g. a change Detection done by a single-node, e.g. a change in weather value or a bird song: relatively in weather value or a bird song: relatively simplesimple

Distributed event detection requires knowing Distributed event detection requires knowing your neighbors and relative positionsyour neighbors and relative positions

Event detection is a way of compressing dataEvent detection is a way of compressing data A posterioriA posteriori event detection: reprogramming event detection: reprogramming

the network (after deployment) to try and the network (after deployment) to try and detect events not known before deploymentdetect events not known before deployment

Position DetectionPosition Detection

GPS is still expensive ($) and energy GPS is still expensive ($) and energy intensive (minutes to a first fix, with intensive (minutes to a first fix, with relatively large energy draw)relatively large energy draw)

Equip a few nodes with GPS, let the Equip a few nodes with GPS, let the others detect their position from radio others detect their position from radio range or signal strength: [Savarese, range or signal strength: [Savarese, Robinson, Bulusu]Robinson, Bulusu]

For manual deployments: record position For manual deployments: record position when deploying, configure node to when deploying, configure node to remember positionremember position

Mobility: Fixed-Mobile Mobility: Fixed-Mobile NetworkNetwork

Most of the network is fixed but a few Most of the network is fixed but a few units may move: e.g. researcher in the units may move: e.g. researcher in the field, Berkeley UAV (unmanned aerial field, Berkeley UAV (unmanned aerial vehicle) data collectionvehicle) data collection

MANET protocol would be wasteful: MANET protocol would be wasteful: too much routing overheadtoo much routing overhead

On-demand routing is usefulOn-demand routing is useful Optimize for the many fixed nodes, not Optimize for the many fixed nodes, not

the few mobile nodesthe few mobile nodes

Mobile Sensor NodesMobile Sensor Nodes

Air-dropped sensor networks: Air-dropped sensor networks: aircraft can return for multiple aircraft can return for multiple passes until the desired area is passes until the desired area is coveredcovered

Mobile sensor nodes: how should Mobile sensor nodes: how should they place themselves to both cover they place themselves to both cover the area and communicatethe area and communicate

ReferencesReferences [Chen]: Multipath On-Demand Routing in [Chen]: Multipath On-Demand Routing in

Sensor Network Topologies Sensor Network Topologies [Gupta and Kumar]: The Capacity of [Gupta and Kumar]: The Capacity of

Wireless NetworksWireless Networks [Savarese]: Savarese, Rabaey, Beutel, [Savarese]: Savarese, Rabaey, Beutel,

Locationing in distributed ad-hoc wireless Locationing in distributed ad-hoc wireless sensor networkssensor networks

[Robinson]: Robinson and Marshall, An [Robinson]: Robinson and Marshall, An iterative approach to locating simple iterative approach to locating simple devices in an ad-hoc networkdevices in an ad-hoc network

[Bulusu]: Bulusu, Heidemann, Estrin, [Bulusu]: Bulusu, Heidemann, Estrin, Adaptive Beacon PlacementAdaptive Beacon Placement

SummarySummary

Main issues: scalability, energy efficiencyMain issues: scalability, energy efficiency These issues affect the entire network These issues affect the entire network

design, including hardware design, design, including hardware design, protocol design, and what data that can protocol design, and what data that can be collectedbe collected

Other issues: event detection and node Other issues: event detection and node collaboration, determination of position, collaboration, determination of position, and mobilityand mobility

Pressing real-world problems really help!Pressing real-world problems really help!

Acknowledgements and Acknowledgements and URLsURLs

DARPADARPA Kim Bridges and Brian Chee, Kim Bridges and Brian Chee,

University of HawaiiUniversity of Hawaii Students at the University of Hawaii: Students at the University of Hawaii:

Shu Chen, Fengxian Fan, Michael Shu Chen, Fengxian Fan, Michael Lurvey, Dan Morton, and many moreLurvey, Dan Morton, and many more

http://www.pods.hawaii.eduhttp://www.pods.hawaii.edu http://www2.ics.hawaii.edu/~esbhttp://www2.ics.hawaii.edu/~esb