Wireless Sensor Networks
The title contains three words. All of them have a meaning individually and the combined term has come to be known quite well in the wireless community. Let us look at the three words separately before discussing about its combined meaning.
Wireless - Whatever we are going to build is wireless. Typically, it shall have batteries to power it up and therefore will not be connected to anything using wires. Battery powered devices have their own world; their own challenges. Power saving is one of them. But that is not the only one. Weight, minimum number of moving parts, graceful decay of the working as the battery power decreases etc are few other concerns.
Sensor - We wish to sense a real world quantity. Light, temperature, humidity, acceleration, pressure, speed are all real world quantities and the devices we shall build will have some sensing component. How often should these readings be taken? How accurately? The answers largely depend on the application.
Networks - There will be multiple such wireless sensors connected together in a network. This is to be a wireless network. We may have one centralized location to which all these devices are connected. But more often, we will have an ad-hoc network consisting only of these devices and an ability to connect to some centralized node when required.
Wireless Sensor Networks are thus envisioned to be a bunch of tiny nodes connected to each other using radio, powered by light weight batteries and which are continuously monitoring some part of the environment around them and sending important data to one another. We have not spoken about compute power yet; it is so because these devices are not meant to compute too much. But this does not mean that they cannot, or that people do not make them compute. Much of the times, the sensed data needs to be sent to other nodes only when such need arises. Some intelligence on the part of the nodes goes a long way in reducing the total communication overhead incurred by the nodes. Therefore, many techniques have been developed to make use of whatever compute power the node has and reduce the load on the communication network.
Areas of Research
Broadly speaking, the areas of research in wireless sensor networks can be divided into the following catagories:
Routing algorithms
MAC level communication
Time Synchronization
Data storage
Power optimization
Localization
Each of this now contains many paper publications and some of them are real world implementations and not just simulations. Purushottam Kulkarni's site for the CS647 course provides valuable reading material for reading into each of these sections.