Wireless sensor networks are composed of nodes that communicate wirelessly and self-organize after deployment. Each node contains processing capability, memory, an RF transceiver and antenna, a power source, and sensors. Systems can include thousands or tens of thousands of nodes communicating to monitor environments. It is expected that within 10-15 years, wireless sensor networks will cover the world and connect to the Internet, making the Internet a physical network. Research in this area includes workshops and conferences each year focused on algorithms and protocols to maximize network lifetime while ensuring robustness, fault tolerance, and self-configuration.

1. 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.

2. 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.

3. The algorithms and protocols need to address the following issues:
Life time Maximization
Robustness and fault tolerance
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:
Area monitoring
Environmental/Earth monitoring
Air Quality monitoring includes Interior monitoring and Exterior monitoring
Air pollution monitoring
Forest fire detection
Landslide detection
Water quality monitoring
Agriculture
Industrial monitoring which includes machine health monitoring

4. 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:
Wireless HART
IEEE 1451
Zigbee/802.15.4
Zigbee IP
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.

5. 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.

6. B-MAC contains four main parts :
Clear Channel Assessment (CCA)
Packet backoff
Link layer acks
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.

7. 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

8. this Wireless Sensor Networks.
REFERENCES:
Jennifer Yick, Biswanath Mukherjee, Dipak Ghosal, Wireless Sensor networks Survey,
Elsevier, 2008.