A wireless sensor network (WSN) is a network consisting of spatially distributed autonomous devices, and use them to cooperatively monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants at different locations. The applications for WSNs are many and varied, typically involved some kind of monitoring, tracking and controlling. Specific applications for WSNs include habitat monitoring, object tracking, nuclear reactor control, fore detection, and traffic monitoring. Mobile phones could also be used as kind of wireless sensors. They have the advantage of numerous users around the world. People could collect data when they travel around by mobile phones, which is a rising and promising field called ”participatory sensing”.
In a typical application, a WSN is scattered in a region to collect data through its sensor nodes. Although individual sensor node is not very expensive, large deployment of sensor nodes in the network makes the total cost considerable expensive. In the meantime, there are many mobile phone users around the world, who could act as data collector sometimes. As a result, quite a lot of money could be saved by decreasing the number of of sensor nodes in locations where mobile phone users exist. However, the quality of sensing results by human differ much from each other, which may cannot satisfy the requirement of applications. So wireless sensors still need to be deployed as a complementary approach to participatory sensing. And the locations of sensors deployed should be calculated carefully. The aim is to minimize the number of sensors, while providing enough satisfactory quality of data.