A wireless sensor network (WSN) is a network made of many small sensor nodes (or motes) and one or more base stations (also called sinks), which centralises the data gathered by sensor nodes.

Sensor nodes are small computers, extremely basic in terms of their interfaces and their components. They usually only consist of a processing unit with limited computational power and limited memory, sensors (including specific conditioning circuitry), a communication device (usually radio transceivers or alternativelly optical), and a power supply. Other possible inclusions are energy harvesting modules, secondary ASICs, and possibly secondary communication devices (RS232, USB...).

The base stations are one or more distinguished components of the WSN with much more computational, energy and communication resources. They act as a gateway between sensor nodes and the end user.

Applications of WSNs

The Applications of WSNs are many and varied. They could be used in industry to monitor dangerous/hermetically-sealed environments. They could be deployed in wilderness areas, where they would remain for many years (monitoring some environmental variable) without the need to recharge/replace their power supplies. They could form a perimeter about a property and monitor the progression of intruders (using advanced application transfer techniques?). There are simply a near infinite amount of uses for WSNs!!

Typical applications of WSNs include monitoring, tracking, and controlling. Some of the specific applications are habitat monitoring, object tracking, nuclear reactor controlling, fire detection, traffic monitoring etc. In a typical application, a WSN is scattered in a region where it is meant to collect data through its sensor nodes.

Area Monitoring

Area monitoring is a typical application of WSN's. In area monitoring, the WSN is deployed over a region where some phenomenon is to be monitored. As an example, a large quantity of tiny sensor nodes could be deployed over a battlefield to detect enemy intrusion instead of using landmines. When the sensors detect the event being monitored (heat, pressure, sound, light, elctro-magnetic field, vibration, etc...), the event needs to be reported to one of the base stations, which can take appropriate action (e.g., send a message on the internet, to a satellite, etc...). Depending, on the exact application, different objective functions will require different data-propagation strategies, depending on things such as need for real-time response, redundancy of the data (which can be tackled via data aggregation techniques), need for security, etc...

Challenges

There are many challenges in implementing a WSN ranging from hardware, software, mechanical and even human-related. Keeping the power usage sufficiently low so that they operate for enough time involves careful power management and in some cases managing charging. Radio communication hardware has to be small enough while using a suitable network algorithm. A high bit-rate saves power by reducing communication time but in order to obtain a good range, especially in wet environments high-power hardware is often needed. These two tasks illustrate the careful balance and compromises that are needed in WSN designs.

Hardware

The main challenge is to produce low cost and tiny sensor nodes. With respect to these objectives, current sensor nodes are mainly prototypes. Miniaturization and low cost are understood to follow from recent and future progress in the fields of MEMS and NEMS.

Software

Energy is the scarcest resource of WSN nodes, and it determines the lifetime of WSN's. WSN are meant to be deployed in large numbers, in an ad-hoc way, in remote, hostile, etc... regions. For this reason, the algorithms and protocols need to address the following isues:

Lifetime maximization
Robustness and fault tolerancy
Self-configuration

Amongst the hot topics in WSN software, the following can also be pointed out:

Security
Mobility (when sensor nodes or base stations are moving)
Middleware: the design of middle-level primitives between the Software and the Hardware

Algorithms

WSN's are composed of a large number of sensor nodes, therefore, an algorithm for a WSN is implicitely a distributed algorithm. In WSN's the scarcest resource is energy, and one of the most energy-expensive operation is data transmission. For this reason, algorithmic research in WSN mostly focuses on the study and design of energy aware algorithms for data transmission from the sensor nodes to the bases stations. Data transmission is usually multi-hop (from node to node, towards the base stations), due to the polynomial growth in the energy-cost of radio tranmission with respect to the tranmission distance.

The algorithmic approach to WSN differentiates itself from the protocol approach by the fact that the mathematical models used are more abstract, more general, but sometimes less realistic than the models used for protocol design.

Protocols

Protocols for WSN need to address the specificities of the WSN nodes hardware. They are usually more "down to earth", compared to the algorithmic approach, and more directly implementable in real world WSN's.

Middleware

There is a need and considerable research efforts currently invested in the design of middleware for WSN's. There are various research efforts in developing middleware for wireless sensor networks. A nice survey of those efforts can be found here [1]. In general approaches can be classified into distributed database, mobile agents, and event-based [2].

Visualization of Wireless Sensor Networks Data

The data gathered from wireless sensor networks is usually saved in the form of numerical data in a central base station. There are many programs, like TosGUI and MonSense, that facilitate the viewing of these large amounts of data. Additionally, the Open Geospatial Consortium (OGC) is specifying standards for interoperability interfaces and metadata encodings that enable real time integration of heterogeneous sensor webs into the Internet, allowing any individual to monitor or control Wireless Sensor Networks through a Web Browser.

WSN Research Centers

Examples of major academic centers for research in wireless sensor networks are CITRIS at Berkeley and CENS at UCLA, in the USA and the NCCR MICS at EPFL, in Switzerland.

Center for Embedded Networked Sensing (CENS)

The Center for Embedded Networked Systems (CENS) at the University of California, Los Angeles, directed by Deborah Estrin, is also a leading research center with $40 million in core funding from the National Science Foundation [3].

Center for Information Technology Research in the Interest of Society (CITRIS)

The Center for Information Technology Research (CITRIS) in the Interest of Society at the University of California, Berkeley, currently directed by S. Shankar Sastry, is a $300 million multicampus research center that includes research and development of wireless sensor networks, and has used them to study microclimate variations in individual redwood trees [4].

National Center of Competence in Research on Mobile Information and Communication Systems (NCCR MICS)

The NCCR MICS was launched in 2001 at EPFL. It is performing research in mobile information and communication systems, with a strong emphasis on wireless sensor networks and novel self-organizing networks and information systems.

Tyndall National Institute (formerly NMRC)

The Microelectronic Applications Integration (MAI), sector of the Tyndall National Institute in Cork, Ireland, headed by Dr. Cian O'Mathuna, is currently involved in developing microsensing and microactuation devices for use in miniaturised wireless sensor networks. In particular the Ambient Technology Group is developing modular interchangeable hardware layers for use in many sensor network applications.

Sensor devices

The following lists some of the sensor nodes on the market.

Wireless Sensor Network Simulators

There exists platforms specifically designed to simulate Wireless Sensor Networks, like TOSSIM which is a part of TinyOS. Traditional network simulations like NS-2 have also been used. Apart from the above mentioned simulators, there are other simulators in the literature.

Emstar - An Environment for Developing Wireless Embedded Systems Software
GlmoSim - GLobal MObile Information systems SIMulator
1. a scalable simulation environment for wireless and wired network systems
2. Written both in C and Parsec
SENS - A Sensor, Environment and Network Simulator
J-Sim - (formerly known as JavaSim) is a component-based, compositional simulation environment
SWAN - Simulator for Wireless Ad-Hoc Networks
SensorSim - This is a patch to Ns-2 simulator
Tython - A DYNAMIC SIMULATION ENVIRONMENT FOR SENSOR NETWORKS
WiseNet
ATEMU
OpSeNet
OMNeT++
OPTNET
Sidh
Avrora,
Shawn - discrete event simulator designed with the simulation of large wireless sensor networks in mind

Reference Books

Wireless Sensor Networks, Cauligi S. Raghavendra (Editor), Krishna M. Sivalingam (Editor), Taieb Znati.
Wireless Sensor Networks: Architectures and Protocols, Edgar H. Callaway, Jr. and Edgar H. Callaway, CRC Press, August 2003, 352 pages.
Information Processing in Sensor Networks, Feng Zhao, and Leonidas J. Guibas (Eds).
Handbook of sensor networks; algorithms and architectures, Edited by Ivan Stojmenovic, Wiley-Interscience, 2005, 531 pages.
Handbook of Sensor Networks: Compact Wireless and Wired Sensing Systems, Mohammad Ilyas, Imad Mahgoub, CRC Press.
Wireless Sensor Network A Systems Perspective, Nirupama Bulusu, Sanjay Jha, Artech House, Published July 2005, ISBN 1580538673
Protocols and Architectures for Wireless Sensor Networks, Holger Karl, Andreas Willig, ISBN: 0-470-09511-3, 526 pages, January 2006
Adhoc and Sensor Networks Theory and Applications, Carlos de Morais Cordeiro (Philips Research North America, USA) & Dharma Prakash Agrawal (University of Cincinnati, USA), March 2006.
Networking Wireless Sensors, Bhaskar Krishnamachari (University of Southern California), (ISBN-13: 9780521838474 | ISBN-10: 0521838479)
Energy Scavenging for Wireless Sensor Networks: With Special Focus on Vibrations, Shad Roundy, Paul Kenneth Wright, Jan M. Rabaey, 232 pages, Kluwer Academic Publishers; (January 1, 2004), ISBN: 1402076630.
Distributed Sensor Networks", S. S. Iyengar, R. R. Brooks, Chapman & Hall/CRC; (October 22, 2004), ISBN: 1584883839 .
Handbook of Sensor Networks: Compact Wireless and Wired Sensing Systems, Mohammad Ilyas, Imad Mahgoub, 672 pages CRC Press; (July 16, 2004), ISBN: 0849319684 .
Algorithmic Aspects Of Wireless Sensor Networks (Lecture Notes in Computer Science)", Sotiris Nikoletseas, Jose Rolim, Springer-Verlag; (September 30, 2004), ISBN: 3540224769 .

External links

Overview of wireless sensor networks from the IEEE Computer Society.
EnOcean EnOcean enables a wireless community of batteryless sensors to connect to just a few transceivers that need to be powered for continuous operation.
Software Infrastructure for Wireless Sensor Networks.
TECO Particle Web Site.
BiSNET: Biologically-inspired architecture for Sensor NETworks Web Site.

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Wireless sensor network