A sensor network is a computer network of many small and
inexpensive devices using sensors to gather information from
different locations, such as temperature, pressure, motion, or
pollutants. Usually sensors exchange data via wireless
communication links, although in some special environments,
sensors use cables as their communication media. In this thesis
the term "emph{sensor network}" only refers to emph{wireless
sensor network}. Sensor nodes are often randomly distributed in
the target area, for example, from aircrafts in military missions,
therefore a sensor network is also a distributed computer network
within a certain area.