Wireless sensor networks play an important rule in human lives, they have wide range of application such as environmental monitoring, industrial process monitoring, target detection and tracking and tactical systems. Wireless sensor networks are different with other wireless networks because they are constrained by their energy. Compared to other mobile devices, sensor nodes almost have batteries to provide energy. Hence, the limitation of batteries brings the necessity to achieve an efficient medium access protocol subject to power constraints. MAC protocols in wireless sensor networks play an important rule in energy consumption. In this paper, we first show that the properties of sensor network and its different with other wireless networks such as ad-hoc. Then, we analyze several previously proposed MAC protocols, pointing out their strengths and weaknesses. Finally, we discuss the exist problems of MAC protocols and the future research on MAC layer design.
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