Sensor Networks
LECTURE 1
Introduction to Sensor Networks
We call wireless sensor networks (WSN) a number of independent systems, having each
one or more sensing devices. These systems are able to communicate together through
the use of wireless links.
These networks must be easy to deploy and auto-configurable, and are usually batteryoperated. The energy consumption of these systems is thus a very important factor. There
has been a lot of interest during the last years about energy-aware medium access and
routing protocols. In particular, multi-hop wireless ad hoc routing is seen as the best way
to efficient solve the routing problem.
Types of Nodes
Sensor
– Low resources
– Inexpensive
– Energy constraints
• Main challenge!!
Sink
– High resources
– AC power supply
– Internet connection
(typically)
Node , sink :
Each of these scattered sensor nodes has the capabilities to collect data and route data
back to the sink
Task manager node:
The sink may communicate with the task manager node via Internet or Satellite
Unstructured vs. Structured Sensor Network
Unstructured
– Dense
– Ad hoc
Structured
– Fewer sensors
– Strategic positions
Sensing and Sensors
Sensing is a technique used to gather information about a physical object or process,
including the occurrence of events (i.e., changes in state such as a drop in temperature or
pressure). An object performing such a sensing task is called a sensor
What is a Sensor Network?
A sensor network is a collection of communicating sensing devices, or nodes. All of the nodes
are not necessarily communicating at any particular time, and nodes can only communicate
with a few nearby nodes
SENSOR NODE HARDWARE
Location Finding System
Mobilizer
Processor
Transceiver
Sensor ADC
Memory
Power Unit
•
Power Generator
Location finding system.
– Most of the sensor network routing techniques and sensing tasks require the knowledge of
location with high accuracy.
• Mobilizer
– May be needed to move sensor nodes when it is required to carry out the assigned tasks.
Applications
The applications for WSNs are many and varied. They are used in commercial and industrial
applications to monitor data that would be difficult or expensive to monitor using wired sensors.
They could be deployed in wilderness areas, where they would remain for many years
(monitoring some environmental variable) without the need to recharge/replace their power
supplies. They could form a perimeter about a property and monitor the progression of intruders
(passing information from one node to the next). There are a many uses for WSNs.
Typical applications of WSNs include monitoring, tracking, and controlling. Some of the
specific applications are habitat monitoring, object tracking, nuclear reactor controlling, fire
detection, traffic monitoring, etc. In a typical application, a WSN is scattered in a region where it
is mean to collect data through its sensor nodes.
Application Types
• Monitoring
– Environmental, industrial and health monitoring
– Factory and process automation
• Tracking
– Tracking objects, animals, people and vehicles
– Military, business, public transportation networks
Application Requirements
• End-to-end delay
– Tracking, alerting applications
• Reliability
– Long-term monitoring for off-line analysis
Another application
1. Border Monitoring
2. Battlefield Observation
3. Forest Fire Detection
4. Environment and Habitat Monitoring
5. Infrastructure security
6. Industrial sensing
7. Medical Applications
System Issues and Standards
Operating System and Standards
• Standards for Low-Rate Wireless Personal Networks (LR-WPN)
– IEEE 802.15.4
• TinyOS, Contiki
– Operating systems designed for sensor networks
Challenges: Networking
Networking
– Efficient routing (i.e. path selection) in mutli-hop networks
• In terms of energy consumption / performance
– Duty cycling
• Sleeping schedule to save energy
Challenges: Localization
Localization
– The problem of determining a node’s position
• Challenging in unstructured topologies
– Important for applications, routing protocols (e.g. geographic routing)
– Straight forward solution: GPS
• But, requires line of sight to satellites, consumes energy, increases cost
– Alternative estimation approaches
• E.g. Received Signal Strength Indicator (RSSI) methods
Challenges: Synchronization
Synchronization
– The problem of assuring that different nodes have a common notion of time
– Important for applications (correlating data) and networking protocols (time scheduling,
coordinated duty cycles)
– Known problem of distributed systems
• Typical solutions are unsuitable due to the limited resources
Energy
Battery-powered WSNs
– Eventually will die and need battery replacement
• Often not even possible (e.g. underground sensors)
– Extracting energy from the environment
– Infinite lifetime but energy not always available
Classification of Energy Availability
• Uncontrollable but predictable
– E.g. Solar energy
• Uncontrollable and unpredictable
– E.g. Vibrations in an indoor environment
• Fully controllable
– E.g. Flush-lights used to generate energy