International Journal of Computer Networks & Communications (IJCNC) Vol.3, No.5, Sep 2011 DOI : 10.5121/ijcnc.2011.3517 235           Lamia CHAARI and Lotfi KAMOUN SFAX University, National Engineering School (ENIS), TUNISIA lamia.chaari@tunet.tn ABSTRACT In recent years, very interesting researches are concentrated to develop several protocols for Wireless sensor networks (WSN) and to explore WSN in various scenarios. The standardization of the IEEE 802.15.4 protocol as a flexible communication solution for low-rate wireless sensor networks leads to the widespread deployment of this technology in many potential applications with different requirements especially in industrial automation. The IEEE 802.15.4 specifies physical and media access control layers that could be optimized to ensure low-power consumption and to guarantee real-time constraints by using its guaranteed time slot (GTS) mechanism and enabling Beacon mode. Our focus in this paper is on real-time capabilities and reliability. We analyzed and compared the performance of the IEEE 802.15.4 standard using OMNET++ Simulator. Among the objectives of our study is to outline which degree the standard accomplish real time requirements. Different application scenarios were been evaluated. The analysis gives a full understanding of the behavior of the IEEE 802.15.4 access mechanism concerning data delivery delay, goodput, throughput and packet errors rate metrics. We have focused on single sink scenarios and we have analyzed some network performance according to nodes number. Our simulation results pave the way for an efficient dimensioning and deploying of an IEEE 802.15.4 cluster. KEYWORDS IEEE 802.15.4; Zigbee; GTS, Medium Access control, OMNET++; Real time, Simulation. 1. INTRODUCTION Recent development of communication technology has facilitated the expansion of the wireless sensors networks (WSN) [1][2][3]. The applicable area of WSN includes military sensing, data broadcasting [4], environmental monitoring [6], Intelligent Vehicular Systems [7], multimedia [8], patient monitoring [9], agriculture [10] [11], industrial automation [12] [13] [14] and audio [15] etc. WSN networks have not yet achieved widespread deployments, although they have been proven capable to meet the requirements of many applications categories. WSN has some limitations as lower computing power, smaller storage devices, narrower network bandwidth and very lower battery power. Real-time applications of a WSN [5] require the sensor network paradigm to provide mechanisms that guarantee a certain level of quality of service (QoS). Whereas the main objective in sensor network research is minimizing energy consumption, mechanisms that deliver application-level QoS efficiently and map these requirements to network-layer metrics, such as jitter and latency, have not get major concerns. Data delivery delay in WSN presents specific system design challenges, which is the object of this article. On the hand, the joint efforts of the Zigbee Alliance [16] and the IEEE 802.15.4 [17] task group lead to the specification of a standard protocol stack for low rate wireless sensor networks , which becoming a universal solution for low cost, low power monitoring and control devices in industrial automation. Furthermore, the IEEE 802.15.4 is quite flexible for industrial