ecent advances in electrical engineering and computer science, esp ecially in micro-

electro-mechanical systems (MEMS), very-large-scale integration (VLSI) and op erating

system theory, have made wireless sensor network (WSN) technologies an imp ortant

and promising research area. As the circuitry b ecame smaller and cheap er, the idea

of co op eratively sensing physical or environmental conditions, such as magnetic eld

intensity, temp erature, light intensity, sound, vibration, pressure, motion or p ollutants

with the help of a network of sensor no des has b ecome feasible.