ELEG 467/667

Sensor Networks

Spring 2006

Before anything happens

• Add / drop– Does anyone know of anyone who is dropping?– A few people can add! (Don’t tell anyone)

• Meeting time

Basics

• Instructor: Stephan Bohacek• TA: none, but Vinay Sridhara will help with

Lab stuff.• Web page: http://www.eecis.udel/~bohacek

– But nothing is there yet.

• Textbook: none. A reader will be available soon.

Course Focus

• Comprehensive introduction to sensor networks.• Network protocols at various layers.

– MAC, routing, transport. (there is cross-over here with wired and other wireless)

• Application issues.– Data fusion, localization

• Analytical methods in networks– Optimization (routing, and power control)– Modeling (analysis of MAC)

• Unique issues: energy efficiency, self managing, data driven, arbitrarily large scale, etc.

• And everything

Prerequisites

• Basic networking– For example, you know

• what the transport layer does• what exponential back-off is• at least one MAC protocol• what flooding is and how it works

• Programming– For example, you can implement flooding.

Format of class• Seminar / project oriented

– Lectures• The first couple of months and some others.

– Reading• Assigned reading with in class discussion

– Homework– Projects

• Many small/moderate projects and a large one

– Presentations • Grad students must give half-hour talks on a topic.

– The talk should be on a paper related to the lecture the week before. – The talk should be fairly short and should add to the prior week’s discussion.

• Project discussions– Group talk about project. The talk covers:

» The problem to be solved» The related papers (if the project is base on a paper, then the talk MUST include discussion from both the paper that

the project is based on, and some related papers)» Key challenges» Schedule

– Final project presentation

– Final project write-up

Projects

• Moderate and final projects will be group projects. Groups should have 2 grad and 2 under grads.

• Project selection and presentation (by 3/30)– Projects may be related to grad student lecture topic

• Project progress report (4/20)

• Project presentation (5/18 and 5/25)

Grading

• Class discussion/reading (10%)• Small/moderate projects (30%)• Homework (10%)• Presentations (20%)• Final project (30%)

Reading material• Reader available from the web site and papers listed on web site.• Also:

– Wireless sensor networks. Editors: Ragavendra, Sivalingam, and Znati

– Wireless sensor networks. Feng Zhao and Leonidas Guibas

– Protocols and Architecture for Wireless Sensor Networks. Karl and Willig\

– Wireless Sensor Networks. Bulusu and Jha

– Topology control in wireless ad hoc and sensor networks. Santi

– Handbook of sensor networks: algorithms and architectures. Ed. Stojmenovic

– Ad Hoc wireless networks. Murthy and Manoj.

– Wireless Sensor Networks. Edgar and Callaway

– Handbook of sensor networks. Editors: Ilyas and Mahgoub

Topics (approximate)

• Overview – today• Propagation of wireless

signals• Energy• MAC protocols• Routing• Transport

• Applications– Localization– Tracking– Time synchronization– Data

• Gathering

• Processing

• Compression

• Fusion

• Architecture

Today: Introduction

• Sensor networks.– Definition, motivation, examples

• Challenges.• Architecture and design Issues.

What are sensor networks?

• Networks of devices that are able to sense the environment, perform on-board computation, (and communicate)

• Why? Because we can: Technology– Circuit integration.

• Ability to integrate more functions into chip with lower energy

– Wireless communication.• Better communication theory• Better devices• Bit-rates are slowly increasing• Transmission power is decreasing

– Sensor technology

Result: sensing nodes

PC-104+

UCLA TAG

UCB Mote

Embedded Networked Sensing

• Micro-sensors,– on-board processing,– wireless interfaces – small scale and low cost => large numbers of sensor fields can be

deployed– monitor phenomena “up close”.

• Enables spatially and temporally dense monitoring.– Nyquist Sampling – you must sample often enough (in time or

space)– Inverse problems are very difficult, e.g., by sensing the temperature

at a few places, determine the temperature everywhere (numerically unstable, i.e., errors are amplified). Instead, directly sense the temperature everywhere.

– Wireless interface allow little infrastructure – easy deployment– Wireless interface allow cooperation and distributed computing

Instability of an inverse problem

for T=1:.25:10 G=zeros(21,21); for i=-10:10 G(i+11,:) = normpdf([-10:10]-i,0,T); end u0=sin([-10:10]/10*pi)'; %rand(21,1); u2=G*u0; u0hat = inv(G)*u2; u0NoiseEst = inv(G)*(u2+rand(size(u2))*.01); figure(1) clf plot(u0) hold on plot(u2,'g') plot(u0hat,'r') legend('original','observed','inverse') axis([0,21,-1,1]); title(sprintf('Inverse problem at %.2f ',T)) T pauseend

Contaminate sources Air sensors

Objective: use the sensors to determine the source of the contaminate.

Assume that the density at (x,y) is

x

yGx,y 1

2 y2exp 1

2xy

2U

Wind direction

where U is the flow into the environment at the source

Solution: put the sensor near to the source

Vision

Embed numeroussensing nodes tomonitor and interactwith physical world

Network these devicesso that they canexecute more complex tasks.

Note that these two problems are very different. Different scales and different requirements. It is difficult to lump them into a single “sensors” group.

Sensor networks applications…

Vision: “Embed the World”

• Buildings self-detect and self-correct from structural faults (e.g., weld cracks).

• Schools detect airborne toxins at low concentrations, trace contaminant transport to source.

• Buoys alert swimmers to dangerous bacterial levels (or an excellent set of waves coming, brah).

• Earthquake-rubbled building infiltrated with robots and sensors: locate survivors, evaluate structural damage.

• Ecosystems infused with chemical, physical, acoustic, image sensors to track global change parameters.

• More??

Buoys along the east coast

Sensor to assist surfers!

Sensors are not always small…

• For example, Umass’s CASA (Collaborative Adaptive Sensing of the Atmosphere). Network of meteorological radars to observe, detect, and predict atmospheric phenomena.

Deployments

• Ecological Habitat Monitoring– UCB/Intel Berkeley: Great Duck Island– UCLA-CENS: James Reserve– Princeton: ZebraNet in Kenya

• Structural Monitoring– UCLA-CENS: Factor Building– USC: Networked SHM – UCB/Intel Berkeley: SF Golden Gate Bridge– UD

• Biomedical Applications– Artificial retina– “Bio-monitors”

• Industrial and Commercial Apps– Ember Corp: Thermal Process Control, Shipment Tracking– CCM

Environmental monitoring

• Petrel habitat on Great Duck Island in Maine.• Questions to answer:

– Usage pattern of nesting burrows over the 24-72 hour cycle.– Changes in the burrow and surface environmental

parameters.– Differences in the micro-environments with and without large

numbers of nesting petrels.

Hierarchical deployment

Sensors

• Mica platform– Atmel AVR w/ 512kB Flash– 916MHz 40kbps RFM Radio

• Range: max 100 ft• Affected by obstacles, RF propogation

– 2 AA Batteries, boost converter• Mica weather board – “one size fits all”

– Digital Sensor Interface to Mica• Onboard ADC sampling analog photo, humidity and passive IR sensors• Digital temperature and pressure sensors

– Designed for Low Power Operation• Individual digital switch for each sensor

– Designed to Coexist with Other Sensor Boards• Hardware “enable” protocol to obtain exclusive access to connector

resources

• Packaging– Conformal sealant + acrylic tube

– Placement– Place above ground and in burrows (propagation?)

Gateway

• Communicate with sensor and base station.• Solar powered (sensors are just battery

powered).• Directional antenna pointed toward base

station.

Base station

• Laptops

• In lighthouse keepers house.

• Log all data and transmit via satellite to D.C. and then on to the Internet.

Smart Dust

• Design goals– Cubic millimeter.– Very low energy.

• Result: sensor package containing:– Sensors– Optical transmitter (passive and active) and receiver– Signal processing– Solar power source

Smart dust applications

• Environmental monitoring.– Insects.– Meteorological phenomena.

• Special operations.

Smart dust components

1-2 mm

Thick-Film Battery

Solar Cell

Power Capacitor

Analog I/O, DSP, Control

Active Transmitter with Laser

Diode and Beam SteeringPassive Transmitter with

Corner-Cube Retroreflector

Sensors

Receiver with Photodetector

Smart dust: passive transmission

High power laser emitted from BS for downlink and uplink communication.

DownlinkLaser

Uplink

CCD Corner-Cube

Uplink

DataIn

Data

ImageSensor

Retroreflector

DataIn

Photo-

DownlinkDataOut

detector

Base-StationTransceiver

DustMote

Signal Selectionand Processing

UplinkData ...

OutNOut1

Array

UnmodulatedInterrogation

ModulatedReflected

ModulatedDownlinkDataor

BeamforUplink

BeamforUplink

Lens

Lens

Passive transmissions

• Reflect illuminating beam (from BS) back encoding data.

• BS decodes data by reading the “on” and “off” reflections.

• Rates of up to 1 kbps over 150m.• Low power sensor power• But, uninterrupted LoS with BS.• A single CCD can decode multiple communications at

the same time. – Each CCD element can “see” a small region of space. Each

element can decode one communication.– Spatial multiplexing.

RFID

• RFID uses backscatter - Another passive transmission technique.• RADAR

– Send a beam and receive reflections.– Physical radar

• Put my hand out and hit what is there.• Instead of DC, I could use AC and move my hand back and forth. I could sense

things just the same. However, if what I am hitting resonates at the frequency my hand moves, then the thing I am hitting will start oscillating.

• A receiving antenna is not just a receiver. If current is moving along the antenna, then it is transmitting as well.

• If the circuit that the antenna is attached to has resonates at the carrier frequency, then this circuit will oscillate. These oscillation will cause RF transmissions.

• The RFID can switch the circuit to modulate the transmission.

Biomedical applications

• Health monitors.– Glucose level.– Digestive system.– Vascular system, etc.

• Artificial retina.

Sensors for vision

Today: Introduction

• Sensor networks.– Definition, motivation, examples

• Challenges.• Architecture and design Issues.

Challenges

• Energy• Self-configuring/adapting• Data processing• Scalability

Energy

• Sensors may require long life times– Great Duck Island required 9 months

– If embedded in highways, 10 years is required

– Pacemaker (not just a sensor!) last 5-10 years• (supervisory control and data acquisition (SCADA))

Energy

• Sensors may require long life times– Great Duck Island required 9 months– If embedded in highways, 10 years is required– Pacemaker (not just a sensor!) last 5-10 years

• (supervisory control and data acquisition (SCADA))

• Approaches– Low duty cycle systems.

• Sleep: deep sleep (everything off), listening but not transmitting, periodic listening

• Increases delay => impacts QoS

• Nodes must be synchronized– Requires good clocks (which require more power)– As battery power drops, clocks may experience severe drift, reducing effective lifetime

Energy

• Sensors may require long life times– Great Duck Island required 9 months

– If embedded in highways, 10 years is required

– Pacemaker (not just a sensor!) last 5-10 years• (supervisory control and data acquisition (SCADA))

• Approaches– Low duty cycle systems.

• Sleep: deep sleep (everything off), listening but not transmitting, periodic listening• Increases delay => impacts QoS• Nodes must by synchronized

– Requires good clocks (which require more power)– As battery power drops, clocks may experience severe drift, reducing effective lifetime

– Low bit-rate• Low power transmissions require low bit-rate

Energy

• Sensors may require long life times– Great Duck Island required 9 months

– If embedded in highways, 10 years is required

– Pacemaker (not just a sensor!) last 5-10 years• (supervisory control and data acquisition (SCADA))

• Approaches– Low duty cycle systems.

• Sleep: deep sleep (everything off), listening but not transmitting, periodic listening• Increases delay => impacts QoS• Nodes must by synchronized

– Requires good clocks (which require more power)– As battery power drops, clocks may experience severe drift, reducing effective lifetime

– Low bit-rate• Low power transmissions require low bit-rate

– Complicated communication schemes• Nodes can cooperate to transmit far with low power.• Advanced data compression• However, CPU uses power as well. But CPU power usage is decreasing as technology

advances.

Energy

• Sensors may require long life times– Great Duck Island required 9 months– If embedded in highways, 10 years is required– Pacemaker (not just a sensor!) last 5-10 years

• (supervisory control and data acquisition (SCADA))

• Approaches– Low duty cycle systems.

• Sleep: deep sleep (everything off), listening but not transmitting, periodic listening• Increases delay => impacts QoS• Nodes must by synchronized

– Requires good clocks (which require more power)– As battery power drops, clocks may experience severe drift, reducing effective lifetime

– Low bit-rate• Low power transmissions require low bit-rate

– Complicated communication schemes• Nodes can cooperate to transmit far with low power.• Advanced data compression• However, CPU uses power as well. But CPU power usage is decreasing as technology advances.

– Efficient protocols• Avoid retransmissions

– Avoid end-to-end retransmission with link layer retransmissions and data caching

• Avoid contention – FDM– TDM

Energy

• Approaches– Renewable power/scavenging.

– Solar energy.

– Mechanical vibrations (sneakers)

– Radio-Frequency inductance (RFID)

– Infrared inductance (passive optical)

Self-configuring/adapting

• Ad hoc deployment; inaccessible areas; – E.g, dropped from airplane

• Adapt to unpredictable environment.– E.g., nodes break, crash, run out of power (consider a deployment

where each node will last only a week, but nodes come out of hibernation at random times so the network has a lifetime of several months (see HW on next page))

• Unattended, untethered– There is no one to reboot …

• Fault tolerant and robust– approaches

• Each sensor operate autonomously from neighbors.• Overlapped services area.• No single point of failure.

Homework

• Suppose that there are N sensor.• Each sensor turns on at time t where t is a random

variable that is uniformly distributed between 0 and T.• Once on, the sensor will remain on for seconds.• Find N (as a function of T and ) such that at time

T/2, the average number of on sensors is M.• Hint:

– What is the probability that a particular sensor will be on at time T/2?

– Use binomial distribution

Challenges for embedded biomedical sensors:

• Limited capabilities: power, processing, storage, and communication.

• Continuous operation.• Robustness and fault tolerance.• Scalabilty.• Self-configuring, self-managing, self-healing.• Data-related issues.

Application-specific issues

• Material Constraints.– Bio-Compatibility.– Inconspicuous.

• Imitative of the environment.• Detect-proof: e.g. stealth flight.

• Security.– Privacy.– Interference.

• Regulatory issues– Such as FDA requirements.

Data processing

• Cooperation – Exploit computation near data to reduce communication.– Collaborative signal processing

• E.g., nearby images are combined to determine the exact location of an object. The position of the object is the data sent to the base station, not the images.

– Data aggregation/compression• If data is spatially correlated, then data can be aggregated and compressed,

taking advantage of correlation

Data processing

• Cooperation – Exploit computation near data to reduce communication.– Collaborative signal processing

• E.g., nearby images are combined to determine the exact location of an object. The position of the object is the data sent to the base station, not the images.

– Data aggregation/compression• If data is spatially correlated, then data can be aggregated and compressed,

taking advantage of correlation

• Limited computation and storage capabilities

• Error propagation – decrease fault tolerance– The more sensors a process uses, the lower the robustness but more

efficient.

• Complicated cooperation may require intensive communication.

• Heterogeneous power dissipation and lifetime– Nodes closer to base station must carry more data and are also in a better

position to aggregate data. But these will expend energy.

• Trade-off between latency and energy

Data processing• Distributed representation/storage• Data Centric Protocols, in-network processing:

– Interpretation of spatially distributed data (Per-node processing alone is not enough).

– Network performs in-network processing based on distribution of data.– Queries automatically directed towards nodes that maintain relevant/matching data.– Pattern-triggered data collection

• Multi-resolution data storage and retrieval.– E.g., many nodes hold general information, thus it is easy to get.– More detailed information requires contacting more nodes– Highly detailed information requires commutation with a large number of nodes.– If mostly low resolution data is required, then such a scheme is very useful

» low resolution = the enemy is crossing the border» Medium resolution = the enemy is in the north» High resolution = there is a enemy truck at the corner of Delaware and S. Chapel st.

• Distributed edge/feature detection.• Index data for easy temporal and spatial searching.• Finding global statistics (e.g., distribution).

Traditional approach: warehousing

• Data extracted from sensors, stored on server.

• Query processing takes place on server.

Sensor Database System

• Sensor Database System supports distributed query processing over sensor network

Sensor database system

• Characteristics of a sensor network:– Streams of data.– Large number of nodes– Multi-hop network.– No global knowledge

about the network.– Node failure and

interference is common.– Energy scarce.– Limited memory– No administration,

• Can existing database techniques be reused?

• What are the new problems and solutions?– Representing sensor data.– Representing sensor queries. – Processing query fragments

on sensor nodes.– Distributing query fragments.– Adapting to changing network

conditions. – Dealing with site and

communication failures.– Deploying and managing a

sensor database system.

Time and location

• Unlike the Internet, time and spatial location essential for some applications.– E.g., localization and time synchronization needed to detect

events, compare detections across nodes, perform collaborative processing, geo forwarding/routing, etc.

– GPS provides solution (with differential GPS providing finer granularity).

– GPS not always available.– Resolution is not very good (10’s of meters)– Other approaches?

• To correlate events, the time of the event must be known.• For coordinated sleeping, the sensors must be

synchronized.

Coverage

• Area coverage:fraction of area covered by sensors– Detectability: probability that

sensors detect moving objects

– Overlap: fraction of sensors covered by other sensors

– Control:• Where to add new nodes

for max coverage.• How to move existing

nodes for max coverage.

Why not Internet protocols?

• Traditional networks have hosts and routers.• Sensor nodes are both hosts and routers• Internet protocols were designed following an e2e approach.• For efficiency, need to use information from lower-layers.• Sensor networks are data-driven; end points don’t matter. • IP are bidirectional• Sensor networks are directional - Data flow and control flow• TCP/IP is wasteful, lazy convergence – it will work eventually• Sensor networks must be very thrifty - energy constraint issues • IP – the system admin is never very far away• Sensor network - Self-organization, self-management.• IP – local networks are fairly small• Sensor networks could contain of an arbitrarily large number of small

sensors generating data.

Today: Introduction

• Sensor networks.– Definition, motivation

• Challenges.• Architecture and Design Issues.

Sample layered architecture

Resource constraints call for more tightly integrated layers

Open Question:What are definingarchitecturalprinciples?