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Wireless Sensor Network


November 1, 2016
Published By : Pratik Kataria
Categorised in:


“A wireless sensor network (WSN) is a wireless network consisting of spatially distributed autonomous devices using sensors to cooperatively monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants, at different locations.” – Wikipedia


A very low cost low power computer- on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals.
It performs tasks, processes data and controls the functionality of other components in the sensor node.
Monitors one or more sensors.
A Radio Link to the outside world.
Are the building blocks of Wireless Sensor Networks (WSN).


The Microcontroller Unit (MCU) is the primary choice for in-
node processing.
Power consumption is the key metric in MCU selection.
The MCU should be able to sleep whenever possible, like the radio.
Memory requirements depend on the application
ATmega128L and MSP430 are popular choices

Communication Device

Medium options

  • Electromagnetic, RF
  • Electromagnetic, optical
  • Ultrasound


Transceiver Characteristics

Service to upper layer: packet, byte, bit
Power consumption
Supported frequency, multiple channels
Data rate
Power control
Communication range

Transceiver States

Transceivers can be put into different operational states, typically:

  • Transmit
  • Receive
  • Idle – ready to receive, but not doing so
  • Sleep – significant parts of the transceiver are switched off

Wakeup Receivers

When to switch on a receiver is not clear

  • Contention-based MAC protocols: Receiver is always on
  • TDMA-based MAC protocols: Synchronization overhead, inflexible

Desirable: Receiver that can (only) check for incoming messages

  • When signal detected, wake up main receiver for actual reception
  • Ideally: Wakeup receiver can already process simple addresses
  • Not clear whether they can be actually built

Main categories:

  • Passive, omnidirectional
    • Examples: light, thermometer, microphones, hygrometer, …
  • Passive, narrow-beam
    • Example: Camera
  • Active sensors
    • Example: Radar

Characteristics of WSN

Requirements: small size, large number, and low cost.
Constrained by
– Energy, computation, and communication
Small size implies small battery
Low cost & energy implies low power CPU, radio with minimum bandwidth and range
Ad-hoc deployment implies no maintenance or battery replacement
To increase network lifetime, no raw data is transmitted

Distinguishing Features

WSNs are ad hoc networks (wireless nodes that self-organize into an infrastructure less network).
Sensing and data processing are essential
WSNs have many more nodes and are more thickly deployed
Hardware must be cheap; nodes are more prone to failures
WSNs operate under very strict energy constraints
WSN nodes are typically static.
The communication scheme is many-to-one (data collected at a base station) rather than peer-to-peer

Nodes are battery-powered.
Nobody is going to change the batteries. So, each operation brings the node closer to death.

“Lifetime is crucial!”
To save energy:

  • Sleep as much as possible.
  • Acquire data only if crucial.
  • Use data synthesis and compression.
  • Transmit and receive only if necessary. Receiving is just as costly as sending.

Scalability and Reliability

WSNs should

  • self-configure and be robust to topology changes (e.g., death of a node)
  • maintain connectivity: can the Base Station reach all nodes?
  • ensure coverage: are we able to observe all phenomena of interest?


  • Reprogramming is the only practical kind of maintenance.
  • It is highly desirable to reprogram wirelessly.

Data Collection

  • Centralized data collection puts extra burden on nodes close to the base station. Clever routing can ease that problem
  • Clustering: data from groups of nodes are compound before being transmitted, so that fewer transmissions are needed.
  • Often getting measurements from a particular area is more important than getting data from each node.
  • Security and authenticity should be guaranteed. However, the CPUs on the sensing nodes cannot handle fancy encryption schemes.

Power Supply

  • AA batteries power  the  vast  majority  of  existing  platforms. They dominate the node size.
  • Alkaline batteries offer a high energy density at a cheap price. The discharge curve is far from flat, although.
  • Lithium coin cells are more compact and boast a flat discharge curve.
  • Rechargeable batteries: Who does the recharging?
  • Solar cells are an option for some applications.
  • Fuel cells may be an alternative in the future.
  • Energy scavenging techniques  are  a  hot  research  topic (mechanical, thermodynamical, electromagnetic).


  • Commercially-available chips
  • Available bands: 433 and 916MHz, 2.4GHz ISM bands
  • Typical transmit power: 1 milliwatt =0 decibal milliwatt(dBm).

Power control

  • Sensitivity: as low as -110dBm
  • Narrowband (FSK) or Spread Spectrum communication. DS-SS (e.g., ZigBee) or FH-SS (e.g., Bluetooth)
  • Relatively low rates (<100 kbps) save power.

Ad Hoc Wireless Networks

It is decentralized type of wireless network.
Each node participates in routing by forwarding data for other nodes, so the determination of which nodes forward data is made dynamically on the basis of network connectivity.
Large number of self-organizing static or mobile
nodes that are possibly randomly deployed.
Near(est)-neighbor communication.
Sensor Networks and Sensor-Actuator Networks are a prominent example.

Wireless Sensor Networks

Formed by hundreds or thousands of motes that communicate with each other and pass data along from one to another
Research done in this area focus mostly on energy aware computing and distributed computing


Types of Sensors

1.Acoustic, sound, vibration

a.Geophone (converts ground movement (displacement)

into voltage)

b.Hydrophone(listening to underwater sound)

c.Microphone (converts sound in air into an electrical


2.Automotive, transportation

a.Radar gun (to detect the speed of other objects)

b.Parking sensors (to alert the driver of unseen

obstacles during parking military exercises)


4.Electric current, electric potential, magnetic, radio

5.Environment, weather, moisture, humidity

6.Flow, fluid velocity

7.Ionizing radiation, subatomic particles

8.Navigation instruments

9.Position, angle, displacement, distance, speed, acceleration

10.Optical, light, imaging, photon

11.Pressure, Force, density, level

Sensor Network Architecture

The two basic kinds of sensor network architecture

  • Layered Architecture
  • Clustered Architecture

1. Layered Architecture

A layered architecture has a single powerful base station, and the layers of sensor nodes around it correspond to the nodes that have the same hop-count to the BS.

In the in-building scenario, the BS acts an access point to a wired network, and small nodes form a wireless backbone to provide wireless connectivity.

The advantage of a layered architecture is that each node is involved only in short-distance, low-power transmissions to nodes of the neighboring layers.

2. Clustered Architecture

A clustered architecture organizes the sensor nodes into clusters, each governed by a cluster-head. The nodes in each cluster are involved in message exchanges with their cluster- heads, and these heads send message to a BS.
Clustered architecture is useful for sensor networks because of its inherent suitability for data fusion. The data gathered by all member of the cluster can be fused at the cluster-head, and only the resulting information needs to be communicated to the BS.
The cluster formation and election of cluster-heads must be an autonomous, distributed process.

WSN Applications

  • Building Automation
  • Sensors and Robots
  • Healthcare
  • Military surveillance
  • Environmental/Habitat monitoring
  • Inventory tracking

Building Automation Application

Measuring temperature and humidity
Controlling heating, ventilation, air-conditioning unit, shades blinds and lighting
Controlling access and providing security etc.

Robot Application

A ring of robots to fight fires.
Robots that are connected to communicate with each other by Wireless Sensor Network to fight the fire by sensing the fire and locate it to make a ring shape around it and each Fire Fighter Robot will fight fire from one direction so that the fire will be easily stopped.

Healthcare Applications

In Medical health care field, WSN are used with embedded systems to monitor the health of the patients in the hospital

Or outside the hospital through the internet.

  • Wireless pulse Oximeter sensor
  • Wireless muscle movements monitor

Military Applications

Shooter Localization
Perimeter Defense (Oil pipeline protection)
Insurgent Activity Monitoring (MicroRadar)
Sensors measuring: electromagnetic energy / signals, light, pressure, sound – explosions
Also: chemical, biological and explosive vapor; presence of people or objects
Use of WSNs can reduce uncertainty: where enemy forces will be deployed; their role
OTW (Other than War): Areas at risk of natural disaster; location of population to be protected

What is RFID?

RFID = Radio Frequency IDentification.
An ADC (Automated Data Collection) technology that:

  • uses radio-frequency waves to transfer data between a reader and a movable item to identify, categorize, track..
  • Is fast and does not require physical sight or contact between reader/scanner and the tagged item.
  • Performs the operation using low cost components.
  • Attempts to provide unique identification and backend integration that allows for wide range of applications.

Other ADC technologies: Bar codes, OCR(optical character Reorganization).

RFID system components


RFID tags

Tags can be attached to almost anything:

  • Items, cases or pallets of products, high value goods
  • vehicles, assets, livestock or personnel

Passive Tags

  • Do not require power – Draws from Interrogator Field
  • Lower storage capacities (few bits to 1 KB)
  • Shorter read ranges (4 inches to 15 feet)
  • Usually Write-Once-Read-Many/Read-Only tags
  • Cost around 25 cents to few dollars

Active Tags

  • Battery powered
  • Higher storage capacities (512 KB)
  • Longer read range (300 feet)
  • Typically can be re-written by RF Interrogators
  • Cost around 50 to 250 dollars

Tag block diagram


RFID tag memory

Read-only tags

  • Tag ID is assigned at the factory during manufacturing
    • Can never be changed
    • No additional data can be assigned to the tag

Write once, read many (WORM) tags

  • Data written once, e.g., during packing or manufacturing
    • Tag is locked once data is written
    • Similar to a compact disc or DVD


  • Tag data can be changed over time
    • Part or all of the data section can be locked

RFID readers

Reader functions:

  • Remotely power tags
  • Establish a bidirectional data link
  • Inventory tags, filter results
  • Communicate with networked server(s)
  • Can read 100-300 tags per second

Readers (interrogators) can be at a fixed point such as

  • Entrance/exit
  • Point of sale

Readers can also be mobile/hand-held

RFID applications

  • Manufacturing and Processing
    • Inventory and production process monitoring
    • Warehouse order fulfillment
  • Supply Chain Management
    • Inventory tracking systems
    • Logistics management
  • Retail
    • Inventory control and customer insight
    • Auto checkout with reverse logistics
  • Security
    • Access control
    • Counterfeiting and Theft control/prevention
  • Location Tracking
    • Traffic movement control and parking management
    • Wildlife/Livestock monitoring and tracking

RFID advantages over bar-codes

No line of sight required for reading
Multiple items can be read with a single scan

Each tag can carry a lot of data (read/write)
Individual items identified and not just the category

Passive tags have a virtually unlimited lifetime
Active tags can be read from great distances

Can be combined with barcode technology