# An Adaptive Back-Off Mechanism for Wireless Sensor Networks
## 1 Introduction  
Wireless sensor networks (WSNs) are widely applied in many domains ranging from military applications to civilian applications and gradually play more important roles in disaster relief, urgent search, health care, environmental monitoring, and so on. Generally, WSNs consist of large numbers of low-cost, densely deployed sensor nodes that organize themselves into a multi-hop wireless network. In WSNs, data collection and transmission can be automatically carried out so as to gain effective environmental information. Meanwhile, WSNs also face many challenges such as insufficient coverage, scalability, a lack of robustness, congestion, and mobility support [1]. Furthermore, as sensor nodes are often deployed in hostile or inaccessible environments, battery charging and replacement are inconvenient and even impractical. As a result, WSNs are always faced with a seriously limited lifetime. Medium access control (MAC) protocols have the function of deploying wireless channel resources efficiently for network nodes in WSNs in order to improve channel utilization and avoid data collision. There are different categories of MAC protocols, such as schedule-based, contention-based, hybrid, cross-layer, and real-time. Meanwhile, different MAC protocols are designed for different types of applications [2]. Contention-based MAC protocols adopt specific back-off mechanisms to adjust the size of the contention window of each node before transmitting a message. The node with a smaller contention window size has a higher priority to access the wireless channel. Due to the back-off mechanism, the node preemptively occupying the channel sends data, when other nodes wait for another contention period until the channel is idle again. Thus, data collision is avoided to some extent. It is worth noting that the wireless network performance, i.e., throughput, delay, and energy efficiency, among other parameters, may vary with different back-off mechanisms. In general, the major cause of energy waste in conventional MAC protocols is idle listening, which is a requisite period of some MAC protocols such as IEEE802.11 or code division multiple access (CDMA) protocol. During this period, the nodes have to keep listening to the channel for the potential message directed to them. Relevant studies have shown that the energy consumed in idle listening is comparable to that consumed in receiving or transmitting [3]. As mentioned above, the WSNs are faced with a serious energy limitation, and these MAC protocols with long idle listening are not suitable for WSNs because of the high-energy consumption in idle listening. As a result, specially designed MAC protocols for WSNs are needed. Sensor-MAC (S-MAC) [4], which is initiated based upon the 802.11 protocol, is a typical kind of contention-based MAC protocol designed for WSNs. In March 2010, the newest version of S-MAC was proposed. In order to reduce energy consumption, S-MAC protocol adopts a periodic listening/sleep working mechanism. This working mode has a low duty cycle. Sensor nodes periodically alternate between listening and sleep states. When listening, nodes are able to transmit or receive data; when sleeping, nodes turn off their radio to save energy. The core of the S-MAC protocol is to shorten the idle listening time and further reduce power consumption by controlling non-working nodes to go into a sleep state [5]. T-MAC [6] was proposed to improve the poor performance of S-MAC when traffic load is variable. In T-MAC, the idle listen period ends when no activation event has occurred in the channel for a fixed time threshold. Nodes will keep on updating their timeout values whenever an activation event occurs. When no events occur in a timeout period, the nodes will go to sleep. To eliminate energy waste caused by a continuous renewal of node timeout values in T-MAC, the concept of advertising for data contention is introduced in advertisement-based MAC (ADV-MAC) [7], advertisement-based time-division multiple access (ATMA) [8], and adaptive MAC (AdAMAC) [9].