A Study on MAC Protocols for Wireless Sensor Networks Md. Kamrul Hasan and Muhammad Sajjadur Rahim Department of Information and Communication Engineering University of Rajshahi, Rajshahi-6205, Bangladesh email: (hasan_ict_4506, sajid_83)@yahoo.com Abstract – Wireless Sensor Networks (WSNs) naturally apply to a broad range of applications that involve system monitoring and information tracking. However, the applications for wireless sensor networks have notably different characteristics and requirements from standard wireless local area network (WLAN) applications. Low energy consumption is the most important consideration for WSN. The low message rate that typical for sensor network applications and the relaxed latency requirements allow for significant reduction in energy consumption of the radio used in the sensor nodes. In this paper, we carried out a study of several energy-efficient Medium Access Control (MAC) protocols designed for wireless sensor networks such as Sensor-MAC (S-MAC) and Time- out MAC (T-MAC). Keywords – Energy-efficient, MAC protocols, WSN 1. INTRODUCTION Recent advances in wireless communications and electronics, specially in MEMS (Micro Electro- Mechanical Systems), have enabled the development of low-cost, low-power, multi-functional sensor nodes that are small in size and can communicate untethered in short distances. These tiny sensor nodes, which consist of sensing, data processing and communicating components, leverage the idea of sensor networks. A typical sensor network generally consists of a large number of sensor nodes deployed in the environment being sensed and controlled, as shown in Fig. 1. Fig. 1 A typical sensor network Energy management is a challenging problem in designing communication protocols for a wireless sensor network. Studies reveal that energy wastage occurs mainly from collision (two interfering nodes transmit at the same time), overhearing (a node receives a packet not destined for it), control packet overhead and idle listening (the radio of a node is active even when there is nothing to transmit or receive). Hence, MAC protocols designed for WSN must address these major energy consumers. 2. MAC PROTOCOLS FOR WSN Recently, a number of MAC protocols have been developed explicitly targeted at multi-hop sensor networks. 2.1 S-MAC One of the most important MAC is the S-MAC protocol which addresses the overheads caused by idle listening, collisions, overhearing and control packet overhead [1]. The basic idea of this contention-based protocol is that time is divided into relatively large frames. Every frame has two parts: an active part and a sleeping part. During the sleeping part, a node turns off its radio to preserve energy. During the active part, it can communicate with its neighbors and send any messages queued during the sleeping part, as shown in Fig. 2. Fig. 2 The S-MAC and T-MAC duty cycles; the arrows indicate transmitted and received messages; TA denotes the activity time-out period Sink Task manager User A B C D E Internet and Satellite Sensor nodes Sensor field