Wireless Sensor Networks

1
Wireless Sensor Networks

• - overview -

2
Wireless Sensor Networks

• Introduction
• Terminology
• Applications
• Technical Challenges
• Examples
• Conclusion

3
Introduction

• A Wireless sensor network (WSN) is a network
  that is formed when a set of small sensor devices
  that are deployed in an ad hoc fashion cooperate
  for sensing a physical phenomenon.
• Wireless Sensor Network consists of base
  stations and a number of wireless sensors.

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Wireless Sensor Network
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Introduction-basic features-

• Self-organizing capabilities
• Short-range broadcast communication and multihop
  routing
• Dense deployment and cooperative effort of sensor
  nodes
• Frequently changing topology due to fading and
  node failure
• Limitation in energy, transmit power, memory, and
  computing power

6
Terminology

• Sensor The device
• Observer The end user/computer
• Phenomenon The entity of interest to the
  observer

7
Applications

• General engineering
• Agriculture and enivronmental monitoring
• Civil engineering
• Military applications
• Health monitoring and surgery

8
Applications -general engineering-

• Automotive telematics (cars networked)
• Fingertip accelerometer virtual keybords
• Sensing and maintenance in industrial plants
• Aircraft drag reduction
• Smart office spaces
• Tracking of goods in retail stores
• Tracking of containers and boxes
• Social studies (human interaction and social
  behavior)
• Commercial and residential security

9
Applications -agriculture and environmental
monitoring-

• Precision agriculture (crop and livestock
  management)
• Planetary exploration (inhospitable environments)
• Geophysical monitoring (seismic activity)
• Monitoring of freshwater quality
• Zebranet project
• Habitat monitoring
• Disaster detection (forest fires and floods)
• Contaminant transport

10
Applications -civil enginneering-

• Monitoring of structures
• Urban planing (groundwater paterns, percent of
  CO2 cities are expelling,...)
• Disaster recovery (locating signs of life after
  earthquake)

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Applications -military applications-

• Asset monitoring and management
• Surveillance and battle-space monitoring
• Urban warfare (sensors in buildings, movement of
  friend and foe, localizing snipers,...)
• Protection (for sensitive objects)
• Self-healing minefields

12
Applications -health monitoring and surgery-

• Medical sensing (physiological data transmitted
  to a computer or physician, wireless sensing
  bandages worn of infection, sensors in the blood
  stream which prevent coagulation and thrombosis)
• Micro-surgery (swarm of MEMS-based robots)

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Technical challenges-performance metrics-

• Energy efficiency/System Lifetime
• Latency
• Accuracy
• Fault tolerance
• Scalability
• Transport capacity/throughput
• Production costs
• Sensor network topology
• Transmission media
• Power supply
• Communication architecture
• Security

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Technical challenges-sensor network topology-

• Hundreds of nodes require careful handling of
  topology maintenance.
• Predeployment and deployment phase
• Numerous ways to deploy the sensors (mass,
  individual placement, dropping from plane..)
• Postdeployment phase
• Factors are sensor nodes position change,
  reachability due to jamming, noise, obstacles
  etc, available energy, malfunctioning
• Redeployment of additional nodes phase
• Redeployment because of malfunctioning of units

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Technical challenges- transmission media -

• In a Multihop sensor network nodes are linked by
  Wireless medium
• Radio Frequency (RF)
• Most of the current sensor node HW is based on it
• Do not need Line of Sight
• Can hide these sensors
• Infrared (IR)
• License free
• Robust to interference
• Cheaper and easier to build
• Require line of sight
• Short Range Solution
• Optical media
• Require Line of sight

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Technical challenges-power supply-

• Power supply usually the limiting factor in terms
  of size and cost and life time
• Power sources can be classified as
• Energy Reservoir (Energy storage in form of
  chemical energy batteries)
• Power Distribution methods
• Power Scavenging methods

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Technical challenges-power supply (contd.)-

• Power distribution
• Distribution of power to nodes from a nearby
  energy rich source
• Wires (defeats purpose of wireless communication)
• Acoustic waves (very low power level)
• Light or lasers (Directed laser beams to large
  number of nodes very complicated )
• Electromagnetic (RF) power
• distribution
• Example µ- chip developed
• by Hitachi for RFID devices

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Technical challenges-power supply (contd.)-

• Power Scavenging
• Energy provided depends on how long the source is
  in operation
• Used usually to charge secondary batteries
• Photovoltaic Cells
• Temperature gradient
• Human Power (average human body burns 10.5 MJ of
  energy per day)
• Wind / Air flow
• Vibrations

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Technical challenges- power consumption -
Components of a sensor node

• Sensing
• Communication
• Data processing

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Technical challenges- power consumption (contd.)-

• Key to Low Duty Cycle Operation
• Sleep majority of the time
• Wakeup quickly start processing
• Active minimize work return to sleep

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Technical challenges -Communication architecture-

• Combines power and routing awareness,
• Integrates data with networking protocols,
• Communicates power efficiently through the
  wireless medium
• promotes cooperative efforts of sensor nodes.

The sensor network protocol stack
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Technical challenges -communication architecture
(contd.)-

• Application layer
• An application layer management protocol makes
  the
• hardware and software of the lower layers
  transparent to
• the sensor network management applications.
• Sensor management protocol (SMP)
• Task assignment and data advertisement protocol
  (TADAP)
• Sensor query and data dissemination protocol
  (SQDDP)

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Technical challenges -communication architecture
(contd.)-

• Transport layer
• This layer is especially needed when the system
  is planned to be accessed through Internet or
  other external networks.
• No attempt thus far to propose a scheme or to
  discuss the issues related to the transport layer
  of a sensor network in literature.

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Technical challenges -communication architecture
(contd.)-

• Network layer
• Routing the data supplied by the transport layer.
  
• Power efficiency is always an important
  consideration.
• Sensor networks are mostly data centric.
• Data aggregation is useful only when it does not
  hinder the collaborative effort of the sensor
  nodes.
• An ideal sensor network has attribute-based
  addressing and location awareness.

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Technical challenges -communication architecture
(contd.)-

• Routing
• Flooding
• Broadcast based
• -High Overhead
• -Data aggregation to reduce the overhead
• -Less complex
• Unicast
• Sensors can communicate with the observer
  directly or with the cluster head using one to
  one unicast.
• MultiCast
• Sensors form application-directed groups and use
  multicast to communicate among group members.

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Technical challenges -communication architecture
(contd.)-
Selecting an energy efficient route

• Maximum available power (PA) route Route 2
• Minimum energy (ME) route Route 1
• Minimum hop (MH) route Route 3
• Maximum minimum PA node route Route 3

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Technical challenges -communication architecture
(contd.)-

• Data link layer
• The data link layer is responsible for the
  medium access and error control. It ensures
  reliable point-to-point and point-to-multipoint
  connections in a communication network.
• MAC (Medium Access Control)
• Creation of the network infrastructure
• Fairly and efficiently share communication
  resources between sensor nodes
• Error control
• Forward Error Correction (FEC)
• Automatic Repeat Request (ARQ).

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Technical challenges -communication architecture
(contd.)-

• Physical layer
• The physical layer is responsible for
  frequency selection, frequency generation, signal
  detection, modulation and data encryption.

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Technical challenges-security-
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Technical challenges-designed protocols-
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Examples

• MIT d'Arbeloff Lab The ring sensor
• Monitors the physiological status of the wearer
  and transmits the information to the medical
  professional over the Internet
• Oak Ridge National Laboratory
• Nose-on-a-chip is a MEMS-based sensor
• It can detect 400 species of gases and transmit a
  signal indicating the level to a central control
  station

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Examples- iButton -

• A 16mm computer chip armored in a stainless steel
  can
• Up-to-date information can travel with a person
  or object
• Types of i-Button
• Memory Button
• Java Powered Cryptographic iButton
• Thermochron iButton
• Applications
• Caregivers Assistance
• Do not need to keep a bunch of keys. Only one
  iButton will do the work
• Elder Assistance
• They do not need to enter all their personal
  information again and again. Only one touch of
  iButton is sufficient
• They can enter their ATM card information and PIN
  with iButton
• Vending Machine Operation Assistance

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Examples- Berkeley Motes-

• Small (under 1 square) microcontroller
• It consists of
• Microprocessor
• A set of sensors for temperature, light,
  acceleration and motion
• A low power radio for communicating with other
  motes
• C compiler Inclusion
• Development ongoing

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Examples-iBadge UCLA-

• Investigate behavior of children/patient
• Features
• Speech recording / replaying
• Position detection
• Direction detection /estimation (compass)
• Weather data Temperature, Humidity, Pressure,
  Light

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Conclusion

• Wireless Sensor Networks are ideal for remote
  sensing in various applications
• Due to the severe power constraints there is a
  need for a new set of protocols for WSN
• Power consumption in hardware and OS must be
  minimal
• Data redundancy can be exploited to reduce power
  consumption
• Technology of the future!!!!