Wireless Sensor Network introduction
Efficient design and implementation of wireless sensor networks has become a hot area of research in recent years, due to the vast potential of sensor networks to enable applications that connect the physical world to the virtual world. By networking large numbers of tiny sensor nodes, it is possible to obtain data about physical phenomena that was difficult or impossible to obtain in more conventional ways. In the coming years, as advances in micro-fabrication technology allow the cost of manufacturing sensor nodes to continue to drop, increasing deployments of wireless sensor networks are expected, with the networks eventually growing to large numbers of nodes.
Wireless Sensor Networks (WSNs) have been widely considered as one of the most important technologies for the twenty – first century. Enabled by recent advances in microelectronic mechanical systems (MEMS) and wireless communication technologies, tiny, cheap, and smart sensors deployed in a physical area and networked through wireless links and the Internet provide unprecedented opportunities for a variety of civilian and military applications, for example, environmental monitoring, battle field surveillance, and industry process control.
Wireless Sensor Networks (WSNs) can be defined as a self-configured and infrastructureless wireless networks to monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants and to cooperatively pass their data through the network to a main location or sink where the data can be observed and analyzed. A sink or base station acts like an interface between users and the network. One can retrieve required information from the network by injecting queries and gathering results from the sink. Typically a wireless sensor network contains hundreds of thousands of sensor nodes.
The sensor nodes can communicate among themselves using radio signals. A wireless sensor node is equipped with sensing and computing devices, radio transceivers and power components. The individual nodes in a wireless sensor network (WSN) are inherently resource constrained: they have limited processing speed, storage capacity, and communication bandwidth. After the sensor nodes are deployed, they are responsible for self-organizing an appropriate network infrastructure often with multi-hop communication with them. Then the onboard sensors start collecting information of interest. Wireless sensor devices also respond to queries sent from a “control site” to perform specific instructions or provide sensing samples. The working mode of the sensor nodes may be either continuous or event driven. Global Positioning System (GPS) and local positioning algorithms can be used to obtain location and positioning information. Wireless sensor devices can be equipped with actuators to “act” upon certain condition.
Wireless Sensor Network Characteristics
A WSN typically consists of a large number of low – cost, low – power, and multifunctional sensor nodes that are deployed in a region of interest. These sensor nodes are small in size, but are equipped with sensors, embedded microprocessors, and radio transceivers, and therefore have not only sensing capability, but also data processing and communicating capabilities. They communicate over a short distance via a wireless medium and collaborate to accomplish a common task, for example, environment monitoring, battlefield surveillance, and industrial process control. Compared with traditional wireless communication networks, for example, cellular systems and MANET, sensor networks have the following unique characteristics and constraints:
- Dense Node Deployment: Sensor nodes are usually densely deployed in a field of interest. The number of sensor nodes in a sensor network can be several orders of magnitude higher than that in a MANET.
- Battery: Powered Sensor Nodes. Sensor nodes are usually powered by battery. In most situations, they are deployed in a harsh or hostile environment, where it is very difficult or even impossible to change or recharge the batteries.
- Severe Energy: Computation, and Storage Constraints. Sensor nodes are highly limited in energy, computation, and storage capacities.
- Self Configurable: Sensor nodes are usually randomly deployed without careful planning and engineering. Once deployed, sensor nodes have to autonomously configure themselves into a communication network.
- Application Specific: Sensor networks are application specific. A network is usually designed and deployed for a specific application. The design requirements of a network change with its application.
- Unreliable Sensor Nodes: Sensor nodes are usually deployed in harsh or hostile environments and operate without attendance. They are prone to physical damages or failures.
- Frequent Topology Change: Network topology changes frequently due to node failure, damage, addition, energy depletion, or channel fading.
- Many-to-One Traffic Pattern: In most sensor network applications, the data sensed by sensor nodes flow from multiple sources sensor nodes to a particular sink, exhibiting a many – to – one traffic pattern.
- Data Redundancy: In most sensor network applications, sensor nodes are densely deployed in a region of interest and collaborate to accomplish a common sensing task. Thus, the data sensed by multiple sensor nodes typically have a certain level of correlation or redundancy.
Wireless Sensor Network Architecture
The number of devices connected to the Internet has been steadily increasing. It is estimated that several hundreds of millions of devices are connected today. Under the on-going proliferation of the Internet of Things (IoT), the next-generation Internet where a wide variety of things can be connected, and big data that allows everything around us to be digitized for visualization, some people are even anticipating the explosive growth of the connected devices, with as many as several tens of billions of devices being connected in near future. Wireless sensor networks have become important elements for making the most of IoT and big data.
Figure 1: WSN Model
Devices with a built-in sensor (sensor nodes) are scattered to any locations where sensing is required, from your vicinity to houses and buildings, and even remote mountains and forests. Those scattered sensor nodes also have a small built-in radio communication unit, which can transmit data obtained from the sensor over wireless communications. Such communications are carried out not only between a gateway and a sensor node but also between a sensor node and another sensor node. The feature of wireless sensor networks is that they can realize the same function as the Internet over wireless communications without using cables. Repeating data relays, those sensor nodes securely send data to the gateway. The gateway in turn sends the data to a server or the cloud. The server or cloud analyzes the data for convenient use.
 Buratti, Chiara, et al. “An overview on wireless sensor networks technology and evolution”, Sensors 9.9 (2009): pp. 6869-6896.
 Zheng, Jun, and Abbas Jamalipour, “Introduction to wireless sensor networks”, Wireless Sensor Networks: A Networking Perspective 1 (2009): pp. 1-18.
 Matin, M. A., and M. M. Islam, “Overview of wireless sensor network”, Wireless Sensor Networks-Technology and Protocols, InTech, 2012.
 Shio Kumar Singh, M P Singh, and D K Singh, “Routing Protocols in Wireless Sensor Networks – A Survey”, International Journal of Computer Science & Engineering Survey (IJCSES) Vol.1, No.2, November 2010
 Taiyo Yuden, “wireless sensor network”, available online at: http://www.yuden.co.jp/ut/solutions/wsn/