Hybrid MAC Protocols Characteristics in Multi-hops Wireless Sensor Networks *M. Miladi, T. Ezzedine and R. Bouallegue Abstract—In the current decade, wireless sensor networks are emerging as a peculiar multi-disciplinary research area. By this way, energy efficiency is one of the fundamental research themes in the design of Medium Access Control (MAC) protocols for wireless sensor networks. Thus, in order to optimize the energy consumption in these networks, a variety of MAC protocols are available in the literature. These schemes were commonly evaluated under simple network density and a few results are published on their robustness in realistic network’s size. We, in this paper, provide an analytical study aiming to highlight the energy waste sources in wireless sensor networks. Then, we experiment three energy efficient hybrid CSMA/CA based MAC protocols optimized for wireless sensor networks: Sensor-MAC (SMAC), Time-out MAC (TMAC) and Traffic aware Energy Efficient MAC (TEEM). We investigate these protocols with different network densities in order to discuss the end-to-end performances of these schemes (i.e. in terms of energy efficiency, delay and throughput). Through Network Simulator (NS- 2) implementations, we explore the behaviors of these protocols with respect to the network density. In fact, this study may help the multi- hops sensor networks designers to design or select the MAC layer which matches better their applications aims. Keywords—Energy efficiency, medium access control, network density, wireless sensor networks. I. I NTRODUCTION N OWADAYS, with the enormous technological innova- tions, the low-cost sensor devices development has be- come possible. Moreover, it is feasible to deploy wireless sensor networks that are able to sense and report several physical phenomena in a real time manner. A wireless sensor network consists of a sensor nodes group that are miniaturized computers’ systems, interconnected by a wireless technology in an ad hoc fashion. In fact, theses networks are practical in various applications such as environmental supervision, medical care and military domain ... Practically speaking, the recent wireless sensor networks pose many challenges as for as communication protocols are con- cerned. The communication character in a wireless sensor network has a huge impact in use. However, the proposed protocols for wireless networks aren’t suitable for the sensor networks and don’t meet their needs. Since wireless sensor nodes have limited battery supply, it becomes clear that, renewing or recharging the battery is not practical taking into consideration the wireless sensor network applications’ nature. Mohamed Miladi (*Corresponding author) and Tahar Ezzedine are with the Syscom laboratory, at the National Engineering School of Tunis, University Tunis el Manar, Tunisia. E-mails: mohamed.miladi@enit.rnu.tn, tahar.ezzedine@enit.rnu.tn. Ridha Bouallegue is with the Systel Laboratory, University of 7th November at Carthage, Higher School of Communication of Tunis, Tunisia. E-mail: ridha.bouallegue@supcom.rnu.tn. Manuscript received November, 2008; revised ..., 2008. As a matter of fact, the sensor node shall conserve energy as much as possible to extend its lifetime and so the whole net- work lifetime. For this reason, it is crucial to design techniques that are able to reduce the node’s energy consumption. In fact, many researches have been directed to focus on developing communication protocols taking into account the stated energy constraints. Among others, the MAC layer which specifies how nodes access to the shared channel has a great influence on the energy consumption. This is because it emerges as a major responsible for managing the radio system which presents the most energy consumer component in the sensor node. In this area of research, the wireless sensor network community has proposed, implemented and evaluated several MAC protocols as being energy efficient techniques. Further, these schemes can be subdivided into three classes which are reservation-based, contention-based and hybrid approaches. According to the literature, the latest category is the most promised way to fulfill the wireless sensor networks require- ments. However, these approaches are commonly evaluated under simple network density which doesn’t present the real sensor application deployment that shall contain various nodes, typically dispersed at high densities. Accordingly, the node’s density impacts the network characteristics and more precisely affects many end-to-end network properties such as energy, latency and throughput [1]. Many recent publications on the performance evaluation have pointed out miscellaneous MAC protocols characteristics in context of the wireless sensor networks [2], [3]. To the best of our knowledge, few has addressed the real network deployment impacts. Unlike these studies, we, in this paper, examine the network’s density effect on the MAC layer characteristics in the wireless sensor networks. To do this, we adopt the square grid-based pattern which presents an efficient way to guarantee sensing coverage in wireless sensor networks [4], [5]. In addition, this regular deployment allows us to concentrate on the inherent properties of the investigated MAC protocols. Accordingly, through NS-2 implementations, we review three prominent hybrid CSMA/CA based MAC protocols proposed specifically for wireless sensor networks : SMAC [6], TMAC [7] and TEEM [8]. The rest of this paper is structured as follows. The major energy waste sources in wireless sensor networks are analytically investigated in sec- tion II. Section III will cover the studied MAC protocols’ basic mechanisms. Section IV will present the simulations’ results and their interpretations followed by concluding findings in section V which also will explore our future works. International Journal of Electrical and Computer Engineering 3:3 2008 207