A wireless sensor network is composed of a large number of sensor nodes and one or more sink nodes (base stations). The sensor nodes are deployed inside the area of interest to collect useful information from the surrounding environment and report it to a base station located generally at the extremity of the area of interest for example, the sensor nodes can monitor and report certain events like the movement of objects. The role of the base station is to gather the information sent by the sensor nodes and send it back to the user (control node), and eventually send queries to the sensor nodes.

Wireless sensor network may be static or dynamic. In static wireless sensor network senor nodes are fixed where as in dynamic wireless sensor network nodes are moveable. Nodes in the network may be deployed either randomly or manually, it depends on the application. Sometimes there are more than one sink nodes in the network especially when the coverage area is very large.
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 Characteristics of Wireless Sensor Network

Researchers Jun Zheng, Abbass Jalamari [1], Mohammad A.Matin[30] mentioned various 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.

 Issues of 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. Akkaya,M. Younis [2], Akyildiz [3] ,have mentioned about  various issues of wireless sensor 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. Some routing protocols have been developed to face this problem. Fault tolerance requirements depend on application.
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. To maintain the security of data different algorithms have been developed using encryption and decryption concept.

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.


[1] Jun Zheng ,Abbbas Jamalipour , Wireless sensor Network, A Networking  Perspective,  IEEE Book WILY publication ,2009.
[2] Akkaya, M. Younis, A survey on routing protocols for wireless sensor networks, Elsevier Journal of Ad Hoc Networks 3 (3) (2005) 325–349.
[3] Akyildiz, W. Su, Y. Sankarasubramaniam, E. Cayirci. A survey on sensor networks. IEEE Communications Magazine. 40 (8) (2002) 102–114