1.1 Introduction 

Keywords: Characteristics of wireless Sensor network, issues of wireless Sensor network,Issues in Wireless Sensor Network, Characteristics of Wireless Sensor Network,Components of a Sensor Node
Wireless Sensor Network (WSN) is collection of different sensor nodes deployed in an area to monitor the environmental and physical conditions parameters like temperature, pressure, movement of objects, light etc. In a wireless sensor network sink node is also there which also known as base station. The development of wireless sensor networks is motivated by military applications.
The sensor nodes in Wireless Sensor Network gather information from atmosphere and transmit it to a base station. Information sent by all the senor nodes in the network gathers at base station and from there this collected information is sent to the user through internet. Generally, the base station is more powerful in terms of resources than the sensor nodes. The sensor node is equipped with low-power batteries suitable for its small size, which limits the ability of the sensor node in terms of processing, storage and transmission.



                                                  

1.2 Applications of Wireless Sensor Networks
Currently wireless sensor networks are successfully used in different application domains. Some of major applications of wireless sensor networks are as follows:
·         Wireless Sensor network are used to monitor the environmental activities such as air pollution in atmosphere and water quantity under water.
·         Wireless sensor network are also used in military applications and in remote sensing applications such as sensing of nuclear, biological and chemical weapon and for detection of terrorist attacks.
·         Patient monitoring,
·         Real time traffic monitoring.
·         Industrial monitoring and agriculture.
1.3 Components of a Sensor Node
Devices in wireless sensor network can also be equipped with actuators to act upon certain conditions. Such types of networks are also referred as Wireless Sensor and Actuator Networks. Sensor node is the main part of wireless sensor network, a sensor node consist of different components. Each component is designed to perform a specific task. 

Processing Unit
In a sensor node processing unit controls the other components of sensor node and performs the required computations. Processing unit is attached with storage unit. Processor in a sensor node can operates in different modes like idle, active or sleep. These different modes are required for power saving purpose. Storage unit with processor is a memory chip attached with microcontroller. In a sensor node mostly flash memory is used because of its low cost and storage capacity. Memory requirements are application dependent.
Sensing Unit
Sensing unit is the main component of sensor node. Sensing unit measures the environmental conditions and physical parameters. Sensing unit produces the analog signal which contains the sensed data. ADC is also attached with sensing unit which converts the analog signal sensed by sensing unit to digital signals and the digitized data is transmitted to controller for further processing.
Transmission Unit
Transmission unit in sensor node consist a transceiver which is also known as radio. Transceiver can transmit and receive the data. A Sensor node uses transceiver to interact with base station and other sensor nodes in the network. Transceiver can be in different states during its operation such as transmit, receive, idle and sleep.
Power Unit
In wireless sensor networks power is supplied to sensor node by a battery attached with it. Sensor nodes have limited power therefore every operation perform by a sensor node should be energy efficient. Apart from the above components some extra components can be used with sensor nodes such as position finding system and power generator, it depends on application requirements


1.4 Characteristics of Wireless Sensor Network
Some important characteristics of wireless sensor network which affects the performance of wireless sensor network are as follows:
· Scalability and Dense Node Deployment: In a wireless sensor network sensor nodes are densely deployed in the area of interest for application. Nodes can be deployed either randomly or manually. Number of sensor nodes can be grow according to need of application and can be in the order of hundreds or thousands.
· Application Specific: Wireless sensor network is application dependent. A wireless sensor network is designed and deployed according to need of application.
· Battery Powered Nodes: In a wireless sensor network nodes are powered with battery. It is difficult to replace the battery or recharge the battery. In some applications external power generator can be used.
· Self Configurable Nature of Nodes: In a wireless sensor network sensor nodes can autonomously configure themselves during the communication in the network.
Node Failure: Once the nodes are deployed in the network then after some time a node may be damage or fail due to some reason for e.g. energy depletion.
1.5 Issues in Wireless Sensor Network
Wireless sensor network is really useful in different areas to achieve the desire objectives, but still there are some issues concerned with wireless sensor network that need to be solve for the better performance of the network. Some of these challenging issues in wireless sensor network are as follows:
·         Energy Consumption: Sensor nodes in wireless sensor network have limited energy capacity. Energy consumption in wireless sensor network depends on different operations performed by sensor nodes such as receiving and transmitting the data.  Energy constraint in sensor network is also affected by the development and design of hardware and software, routing protocol and architecture used in sensor networks. Several algorithms and protocols have been developed to minimize the energy consumption and make the network energy efficient.
·         Nodes Deployment:  Wireless sensor network consist of several nodes deployed either randomly or manually inside an area. Deployment of sensor nodes requires a handling of topology very carefully.  This deployment of nodes is either deterministic or self organizing. Sensor network topology is prone to frequent changes after deployment.  In dynamic wireless sensor network some additional sensor nodes can be added to the network at any time to replace the faulted node or due to any change in the task dynamic. Addition of new nodes needs reorganization of the network very carefully.
·         Production Cost: Since the sensor networks consist of large number of nodes and the cost of a single node is very important to justify the cost of overall network. Sensor nodes with advanced features are very costly. As a result cost of each sensor node has kept being low. Wireless sensor network also consist of some additional components so keeping the cost low is a challenging task.
·         Failure Tolerance: Sensor nodes fail due to some hardware problems or physical damage and energy depletion. Nodes failure in wireless sensor network is higher as compare to wired network. Protocol used in wireless sensor network should be able to detect the node failure as soon as possible and should be robust to handle the failure with maintaining the working of network.
·         Hardware Constraints: Generally a sensor node has sensing unit, power unit, processing unit and transmission unit. In some application for extra functionality there is requirement of some extra hardware components which may increase the cost of deployment, energy consumption and size of sensor node.
·         Security: Maintaining the security of data sensed by sensor nodes is another challenge in wireless sensor network. Data may be captured during the transmission from sensor node to base station.


·         Compression and Aggregation: Data compression and aggregation are the techniques to increase the quality of data and reduce the data size. Developing energy efficient compression and aggregation techniques in wireless sensor networks is a challenging task.