WIRELESS SENSOR NETWORKS

A wireless sensor network is a collection of nodes organized into a corporate

network. Each node consists of processing capability (one or more

microcontrollers, CPUs or DSP chips), may contain multiple types of memory

(program, data and flash memories), have a RF transreceiver (usually with a

single Omni directional antenna), have a power source (e.g., batteries and solar

cells), and accommodate various sensors and actuators. The nodes

communicate wirelessly and even self organize after being deployed in an ad

hoc fashion. Systems of 1000s or even 10000s nodes are anticipated. Such

systems can revolutionize our live and work.

Currently wireless networks are beginning to be deployed at an accelerated

pace. It is not un reasonable to expect that in 10-15 years that the world will be

covered with wireless sensor networks with access to them via the Internet.

This can be considered as the Internet becoming physical network. This new

 technology is exciting with unlimited potential for numerous application areas

including environmental, medical, military, transportation, entertainment, crisis

management  homeland defense, and smart spaces.

In computer science and telecommunications, wireless sensor networks are an active

research area with numerous workshops and conferences arranged each year. The

topology of the wireless sensor networks can vary from a simple star network to an

advanced multi-hop wireless mesh network.

CHARACTERISTICS:

The main characteristics of a WSN include:

·        Power consumption constrains for nodes using batteries or energy harvesting

·        Ability to cope with node failures

·        Mobility of nodes

·        Communication failures

·        Heterogeneity of nodes

·        Scalability to large scale of deployment

·        Ability to withstand harsh environmental conditions

·        Ease of use

HARDWARE AND SOFTWARE:

 One of the major challenge in a Wireless Sensor Network is to produce low cost and

tiny sensor nodes. Now a days, many companies are producing WSN and the

commercial situation can be compared to home computing in 1970s. Most of the

nodes are still in the research and development stage particularly their software.

 

 To determine the lifetime of Wireless Sensor Networks, energy is the scarcest

resource for the Wireless Sensor nodes. The key component for the software of a

Wireless Sensor Network is ad-hoc communications.

The algorithms and protocols need to address the following issues:

Ø Life time Maximization

Ø Robustness and fault tolerance

Ø Self-configuration

 The lifetime of a Wireless Sensor Network is based upon the limited energy resource.

In order to conserve power the node should shut off the radio power supply when

not in use.

APPLICATIONS:

Ø Area monitoring

Ø Environmental/Earth monitoring

Ø Air Quality monitoring includes Interior monitoring and Exterior monitoring

Ø Air pollution monitoring

Ø Forest fire detection

Ø Landslide detection

Ø Water quality monitoring

Ø Agriculture

Ø Industrial monitoring which includes machine health monitoring

 Simulation of Wireless Sensor Networks:

  At present, agent-based modeling and simulation is the only paradigm which allows

  the simulation of complex behavior in the environments of wireless sensors

 (such as flocking). Agent-based simulation of wireless sensor and ad-hoc networks

is a relatively new paradigm. Agent-based modeling was originally based on social

simulation.

Network simulators like OPNET, NetSim and NS2 can be used to simulate a wireless

sensor network.

Standards:

The prominent standards of Wireless Sensor networks include:

Ø Wireless HART

Ø IEEE 1451

Ø Zigbee/802.15.4

Ø Zigbee IP

Ø 6LowPAN

 The IEEE focuses on the physical and MAC layers and the Internet Engineering task

force works on the layer 3. Standards are not used much i.e. used very less in the case

of Wireless Sensor Networks.

MAC:

A medium access control (MAC) protocol coordinates actions over a shared channel.

The most commonly used solutions are contention-based. One general contention-

based strategy is for a node which has a message to transmit to test the channel to

see if it is busy, if not busy then it transmits, else if busy it waits and tries again later.

After colliding, nodes wait random amounts of time trying to avoid re-colliding. If two

or more nodes transmit at the same time there is a collision and all the nodes

colliding try again later. Many wireless MAC protocols also have a doze mode where

nodes not involved with sending or receiving a packet in a given timeframe go into

sleep mode to save energy. Many variations exist on this basic scheme.

 MAC protocol for wireless sensor networks must consume little power, avoid

collisions, be implemented with a small code size and memory requirements, be

efficient for a single application, and be tolerant to changing radio frequency and

networking conditions.

             One example of a good MAC protocol for wireless sensor networks is B-MAC.

  B-MAC is highly configurable and can be implemented with a small code and

 memory size. It has an interface that allows choosing various functionality and only

 that functionality as needed by a particular application.

 B-MAC contains four main parts :

Ø Clear Channel Assessment (CCA)

Ø Packet backoff

Ø Link layer acks

Ø Low power listening.

 For CCA, B-MAC uses a weighted moving average of samples when the channel is idle

in order to assess the background noise and better be able to detect valid packets

and collisions.

The packet backoff time is configurable and is chosen from a linear range as opposed

to an exponential backoff scheme typically used in other distributed systems. This

reduces delay and works because of the typical communication patterns found in a

wireless sensor network. B-MAC also supports a packet by packet link layer

acknowledgement. In this way only important packets need pay the extra cost.

A low power listening scheme is employed where a node cycles between awake and

sleep cycles. While awake it listens for a long enough preamble to assess if it needs to

stay awake or can return to sleep mode. This scheme saves significant amounts of

energy.

TYPES OF WIRELESS SENSOR NETWORKS:

1.Unstructured Wireless Sensor Network	2.Structured Wireless Network
·      Dense collection of nodes	·      Few and scarcely distributed nodes
·      Ad-hoc deployment	·      Pre planned deployment
·      Difficulty in network maintenance	·      Lower network maintenance

Diagramatically a Wireless sensor Network can be represented as

                   

FUTURE SCOPE:

The Wireless sensor networks helps the present technology to Blend and

interconnect individual smart devices, Enable access to information about the real

world and also interact and influence the real world. Also this Wireless Sensor

Network receives live information from the road authority about the state of the

Roads including traffic jams, road accident and weather. The capability and

 accessibility of the networks can also be increased to a large extent with the help of

this Wireless Sensor Networks.

REFERENCES:

Jennifer Yick, Biswanath Mukherjee, Dipak Ghosal, Wireless Sensor networks Survey,

Elsevier, 2008.