Application of Parallel Redundancy in a Wi-Fi- based WNCS using OPNET Mostafa Hendawy, Mohamed ElMansoury, Karim N. Tawfik, Mohamed M. ElShenawy, Alia H. Nagui, Ahmed T. ElSayed Electronics Engineering Department American University in Cairo Cairo, Egypt {mhindawi; mansoury; karimnabil; shinnawy7; alianagui; a_t}@aucegypt.edu Hassan H. Halawa, Ramez M. Daoud, Hassanein H. Amer Electronics Engineering Department American University in Cairo Cairo, Egypt {hhhalawa; rdaoud}@ieee.org, hamer@aucegypt.edu Markus Rentschler Business Unit Networking Balluff GmbH Germany Markus.Rentschler@balluff.de Hany M. ElSayed Electronics and Communications Department Cairo University Giza, Egypt helsayed@ieee.org Abstract— A powerful approach to improve the performance of wireless communication is the parallel redundant transmission with dual-radio wireless devices. To further verify this approach in this work, an OPNET simulation is performed on a star- topology WNCS workcell with 30 sensor and actuator pairs that are equipped with dual-radios for parallel redundancy. The applied wireless simulation model is based on IEEE 802.11g (Wi- Fi) standard and a quantitative analysis of the effect of interference in an industrial environment, is presented. This study proved that parallel redundancy improves system performance under different interference environments. Keywords— diversity, parallel redundancy, wi-fi, 802.11, opnet I. INTRODUCTION Control networks are mainly intended for the communication of very small packets at a high transmission and reception rate [1, 2]. Since these networks are usually designed for a large number of nodes that involve real-time applications, timely packet reception is very important and hence, a high level of reliability and minimal losses must always be ensured [3]. Different network applications may use different protocols and depending on the application, it is determined whether retransmission is necessary [4]. In order to ensure high performance and reliability of control networks, deterministic network communication protocols like CAN and Profibus are utilized [1, 5]. IEEE 802.3 Ethernet was also introduced as a communication protocol for wired Networked Control Systems (NCSs) [3, 5, 6]. Wireless Networked Control Systems (WNCSs) became an important research topic for industrial applications due to several advantages that include less cabling and ease of installation and maintenance. One of several WNCS solutions available today is Wireless Interface for Sensors and Actuators (WISA) that used a modified version of IEEE 802.15.1 (Bluetooth) as a communication protocol between sensors, actuators, and the controller. WISA can also provide wireless power for sensors and actuators. Other alternative WNCS solutions were discussed. Reference [7] proposed a system that applied IEEE 802.11b Wi-Fi [8] and Ethernet protocols without modifications in order to model an industrial WNCS. The system in [7] used those presented in [9, 10] as its benchmark where Wi-Fi, in particular, was chosen over Bluetooth and ZigBee [11, 12] because of its superiority in terms of wider range and larger data rate. The proposed system produced an improvement in meeting timing requirements when compared to [9, 10], even with utilizing standard off-the-shelf equipment in the presence of external interference. The performance and tolerance to interference of the workcell model proposed in [7] was further enhanced in [13]. This was achieved via adjusting the delay constraint conditions while maintaining the overall end-to-end delay requirement of [9]. The performance of a wireless communication system can be significantly improved by applying diversity, which is basically the redundant transmission of information over stochastically uncorrelated channels [14]. A possible diversity scheme utilizes parallel redundancy in the space and frequency domains, which is able to yield specific gains especially in packet transmission schemes [15]. A recently presented example is Parallel Redundant WLAN (PRP- WLAN), which experimentally used the Parallel Redundancy Protocol (PRP) according to IEC 62439-3 [16] as splitter and combiner units on the Ethernet level and could yield significant improvements [17, 18]. This was further verified through the OPNET simulations studied in [19]. The mentioned PRP principle can also directly be applied on the wireless system’s MAC packet transmission layer, achieving simpler implementations in dual-radio devices, an approach that will be followed in this work. The assessment of interference on real-time wireless communication systems has received widespread research interest. Reference [20] studied the effect of interference on real-time communication in IEEE 802.11-based mesh networks.