Abstract
Wireless Sensor Networks (WSNs) have become a leading solution in many important applications such as intrusion detection, target tracking, industrial automation, smart building and so on. The sensor nodes are generally unattended after their deployment in hazardous, hostile or remote areas. These nodes have to work with their limited and non replenish able energy resources. Energy efficiency is one of the main design objectives for these sensor networks. Medium Access Control MAC sub-layer is part of Data Link layer in WSN's protocol stack. The energy consumption of sensor nodes is greatly affected by MAC protocol which controls the node radio functionalities. In this paper, the design requirements of energy efficient MAC protocols for WSNs are reviewed and classified. Several MAC protocols for the WSNs are described emphasizing their strength and weakness. Also, the paper introduces cross-layer protocols as a concept that leads to benefit from the network resources as well as prolonging network lifetime. The paper is appended by comparison between existing protocols regarding protocol's type, cross-layer support, and MAC scheduling. Finally, future research directions in the MAC protocol design are proposed. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).