On Reliable and Deadline-Constrained Communication in Wireless Industrial Networks

Sammanfattning: Along with the quick development of wireless communication technologies, industrial automation networks are also in unceasing evolution. Industrial wireless sensor and actuator networks (IWSAN) have been increasingly adopted in industrial automation systems. Although there are a number of advantages of replacing cables with wireless links, such as cost reduction, enhanced scalability and flexibility, the stringent requirements on communication reliability and meeting firm deadlines from industrial mission-critical applications must still be fulfilled. Also, transmissions over wireless channels in industrial environments are prone to noise and interferences, resulting in frequent erroneous packet deliveries. Although industrial automation systems are usually designed to be tolerant of certain communication errors, successive transmission failures may still cause downtime of industrial applications, which might lead to significant economic losses or even serious accidents. This thesis addresses the problems mentioned above and aims to provide reliable and deadline-constrained communication via IWSANs for industrial automation systems. On the MAC layer, existing IWSAN standards utilize automatic repeat request (ARQ) to improve reliability at the cost of additional transmission latency. An alternative method is to use Forward Error Correction (FEC) schemes to provide reliable communication by recovering erroneous data and avoiding unnecessary retransmissions. On the MAC layer, Time Division Multiple Access (TDMA) is usually applied in current IWSAN standards for collision-free and deterministic communication. An inappropriate scheduling scheme may introduce high transmission jitter and degrade the quality of control. On the network layer, routing protocols play an important role in both communication reliability and latency. Existing solutions are either hardly able to fulfill all stringent requirements from the mission-critical industrial applications, too complicated to be realized, or lack verification in reality. The contributions of this thesis consist of (i) the possibilities of using FEC schemes in IWSANs is explored under the requirements of existing standards on the MAC layer. A compatible and flexible FEC scheme on the MAC layer for IWSANs that does not violate the standard is proposed and evaluated by simulations. (ii) To guarantee an acceptable control quality, a TDMA scheduling scheme is presented aiming for low communication jitter, and it is compared to classic scheduling schemes. (iii) The advantages of exploiting flooding in IWSANs is discussed; a reliable controlled flooding-based routing protocol is proposed and compared to both traditional routing protocols and other flooding-based protocols. (iv) A complete IWSAN platform is built and the whole protocol stack is implemented. Measurements were conducted in a real industrial environment to verify the correctness of the proposed solution.