Dual Path Communications over Multiple Spanning Trees for Networked Control Systems Sylvain Kubler a,∗ , J´ er´ emy Robert a , Jean-Philippe Georges a , ´ Eric Rondeau a a Research Centre for Automatic Control of Nancy, Campus sciences, BP-70239, F-54506 Vandœuvre-l` es-Nancy Cedex, France Abstract The switched Ethernet networks are more and more deployed in industry. The Spanning Tree Protocol implemented in the switches enables management of the link connectivity. But the reconfiguration time of the Spanning Tree Protocol (STP) when link failure occurs is not adapted to satisfy industrial constraints. The objective of this paper is to propose a method based only on standard, mitigating the probability of disconnection between nodes having hard real-time properties. The approach developed in this paper consists of duplicating frames and of forwarding them on different paths. These paths are optimized and specified by using genetic algorithms. OPNET simulations show the interest of this proposal on a particular Networked Control System. Keywords: Real-time systems, Fault tolerance, Spanning-tree, Genetic Algorithm, Switched Networks 1. Introduction The modern plant architectures implement communi- cation networks to control and to monitor their remote and distributed applications. The major interest facing the point to point architectures is to mitigate the wire costs and to enable easier information sharing. In the framework of embedded systems and industrial systems, the commu- nication requirements are to guarantee bounded end-to- end delays and to maintain the connectivity between all network nodes. In the 1980’s, many networks named field- buses were developed to respect these strong constraints. More recently, the trend in fieldbuses was to use Ethernet protocol (IEC 61784 gathers the different fieldbuses stan- dards). The advantages are that Ethernet is a well known protocol, widely implemented (ensuring its timelessness), and its performance is continually increasing with technol- ogy evolution (especially its bandwidth). Consequently, Ethernet is not a determinist network since it implements the carrier-sense multiple access with collision detection (CSMA/CD) mechanism to resolve the problem of con- tention in case of simultaneous data transmission [1]. How- ever, the fully switched architectures allow inhibiting the collisions and making possible delay estimations in the worst cases [2]. Moreover, the IEEE 802.1d introduces algorithms to face loops’ issues when redundant links are used. In such cases, the Spanning Tree Protocol (STP) de- fines a hierarchical logical tree topology by inhibiting some * Corresponding author Email addresses: Sylvain.Kubler@cran.uhp-nancy.fr (Sylvain Kubler), Jeremy.Robert@cran.uhp-nancy.fr (J´ er´ emy Robert), jean-philippe.georges@cran.uhp-nancy.fr (Jean-Philippe Georges), Eric.Rondeau@cran.uhp-nancy.fr ( ´ Eric Rondeau) switch ports. STP is then able to react to a link failure by activating inhibited ports. Nevertheless, the standard STP reconfiguration time period (in seconds) is too long and is not compatible with the industry requirements (of- ten less than 1s). This is especially the case for Networked Control System where a network is used in feedback con- trol loops. In this field, several research works [3] focus on the way to reconfigure the network when failures occur, by proposing either new protocols than STP (such that native devices are not anymore supported) or specific/dedicated physical network organizations. The goal of this paper is both to retain only native protocols (like STP) and to sig- nificantly increase the network dependability by reducing the reconfiguration time, even to zero in particular cases. This dependability is evaluated in terms of probability of disconnection between nodes. The practical approach is to duplicate the messages with real-time properties and to send them to the remote nodes by several paths. If one path fails, messages will still arrive at the destination by another path in hiding the STP reconfiguration time period on the trouble path. This article is organized in the following way: Section 3 describes the Spanning Tree Protocol standards and the related works. Section 4 formalizes the fitness value to be optimized. Section 5 presents the approach based on ge- netic algorithms to increase the dependability of switched network architectures. Finally, Section 6 illustrates the interest of this study in the Networked Control Systems framework. 2. Networked Control Systems Feedback control systems in which the control loops are closed through a real-time network are called Networked Preprint submitted to Elsevier April 22, 2013