Wireless Technology

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IEEE 802.15.4: a wireless communication technology for large-scale ubiquitous computing applications Anis Koubâa, Mario Alves, Eduardo Tovar
PP-HURRAY! Research Group, Polytechnic Institute of Porto Rua Dr. Antonio Bernardino de Almeida, 431, 4200-072 Porto, PORTUGAL {akoubaa, emt}@dei.isep.ipp.pt, mjf@isep.ipp.pt

Abstract. Wireless Sensor Networks (WSNs) have been attracting increasing interest for supporting a new generation of ubiquitous computing systems with great potential for many applications such as surveillance, environmental monitoring, health care monitoring or home automation. However, the communication paradigms in WSNs differ from the ones associated to traditional wireless networks, triggering the need for new communication protocols. In this context, the IEEE 802.15.4 protocol presents some potentially interesting features for supporting large-scale ubiquitous computing applications, namely power-efficiency, timeliness and scalability. Nevertheless, when addressing applications with (soft/hard) timing requirements some inherent paradoxes emerge, such as power-efficiency versus timeliness. Consequently, there is the need of engineering solutions for an efficient deployment of IEEE 802.15.4 in such scenarios. In this paper, we present some of the most important results on the IEEE 802.15.4 protocol that have been achieved within the context of wireless sensor networks. The paper outlines the most relevant characteristics of the IEEE 802.15.4 protocol and presents the most important research challenges regarding time-sensitive WSN-based applications. Then, it presents some timing performance analysis that unveils some directions for resolving the previously mentioned paradoxes.

1. Introduction
Wireless Sensor Networks (WSNs) have revolutionized the design of emerging embedded systems and triggered a new set of potential applications. This particular form of distributed and ubiquitous computing raises many challenges in terms of real-time communication and coordination due to the large number of constraints that must be simultaneously satisfied, including limited power, CPU speed, storage capacity and bandwidth. These constraints trigger the need for new paradigms in terms of node/sensor design and network communication/coordination mechanisms. The design of wireless sensor networks is mainly concerned with power-efficiency issues, due to the severe limitation in terms of energy consumption (Aykildiz et al. 2002; Stankovic et al. 2003). However, the design complexity is even more significant when applications have, in addition, real-time and/or scalability requirements (Stankovic et al. 2003). Several research initiatives, aiming at providing different design solutions for WSNs protocols, have recently emerged (Lu et al. 2002; Bandyopadhyay and Coyle 2003; He et

al. 2003; Ye et al. 2004; Bacco et al. 2004). However, we believe that the use of standard technologies pushed forward by commercial manufacturers can speed-up a wider utilization of WSNs. In this context, the IEEE 802.15.4 protocol (IEEE 802.15.4 Standard 2003), recently adopted as a communication standard for Low-Rate Wireless Local Area Networks (LR-WPANs), shows up itself as a potential candidate for such a deployment. This protocol provides enough flexibility for fitting different requirements of WSN applications by adequately tuning its parameters, even though it was not specifically designed for WSNs. In fact, low-rate, low-power consumption and low-cost wireless networking are the key features of the IEEE 802.15.4 protocol, which typically fit the requirements of WSNs. Moreover, the ZigBee specification (ZigBee Alliance 2005) relies on the IEEE 802.15.4 Physical and Data Link Layers, building up the Network and Application Layer, thus defining a full protocol stack for LR-WPANs (refer to Section 2.1). More specifically, the IEEE 802.15.4 Medium Access Control (MAC) protocol has the ability to provide very low duty cycles (from 100%...
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