Many industrial systems have specific requirements derived from the applications they execute. Specifically, the interaction of a distributed embedded control system (DECS) with the real-world imposes strict real-time (RT) and reliability requirements. For a system to be RT, it has to produce a proper result in a bounded time. On top of that, for a system to be reliable, it has to operate continuously during its mission time, and in cases in which very high reliability is needed, fault tolerance (FT) techniques are used. Moreover, these systems are often deployed in dynamic environments where the operational conditions may change in an unpredictable manner. Therefore, there is an increasing interest in creating DECSs that are capable of modifying their behavior autonomously and dynamically in response to unexpectedly changing requirements or conditions. In recent years, there is a growing trend toward using Ethernet as the network technology for DECSs. Unfortunately, the original specification of this technology lacks appropriate services to fulfill the most demanding requirements of industrial systems. In this regard, many Ethernet-based protocols and standards have been proposed along the past years to deal with these limitations. In this paper, we survey solutions that have been proposed to achieve FT in Ethernet-based DECSs, considering faults both in their nodes and communication subsystem. In addition, we discuss adaptive FT techniques that can be used to increase the flexibility of adaptive DECS. Finally, we identify future trends and open challenges to build highly reliable DECS in the future.

Time-Sensitive Networking (TSN) is a set of technical standards that is being developed to provide Ethernet with hard real-time, reliability and flexibility services. In the last years, there has been a growing interest in increasing the connectivity of all kind of devices. This trend has reached industrial environments, where the demanding timing and reliability constraints imposed the use of specialised networks with specific features to support these requirements. Moreover, the industry has shown interest in using Ethernet as the network technology in industrial environments, due to its low cost, high bandwidth and extensive use. The ability of TSN to support both, data-oriented and traditional control traffic over the same network makes it an appealing technology to implement the next generation of industrial networks with high connectivity. Nevertheless, TSN does not cover some reliability aspects important for its deployment in critical systems. In this work we propose the implementation of time redundancy of frames in order to tolerate temporary faults in the channel and, therefore, increase the reliability of the network.

Industry 4.0 and Autonomous Driving are emerging resource-intensive distributed application domains that deal with open and evolving environments. These systems are subject to stringent resource, timing, and other non-functional constraints, as well as frequent reconfiguration. Thus, real-time behavior must not preclude operational flexibility. This combination is motivating ongoing efforts within the Time Sensitive Networking (TSN) standardization committee to define admission control mechanisms for Ethernet. Existing mechanisms in TSN, like those of AVB, its predecessor, follow a distributed architecture that favors scalability. Conversely, the new mechanisms envisaged for TSN (IEEE 802.1Qcc) follow a (partially) centralized architecture, favoring short reconfiguration latency. This paper shows the first quantitative comparison between distributed and centralized admission control architectures concerning reconfiguration latency. Here, we compare AVB against a dynamic real-time reconfigurable Ethernet technology with centralized management, namely HaRTES. Our experiments show a significantly lower latency using the centralized architecture. We also observe the dependence of the distributed architecture in the end nodes’ performance and the benefit of having a protected channel for the admission control transactions.

In recent years the Time-Sensitive Networking (TSN) Task Group (TG) has been working on proposing a series of standards to provide Ethernet with hard real-time guarantees, online management of the traffic and fault tolerance mechanisms. In this way the TG expects to create the network technology of future novel applications with real-time and reliability requirements. TSN proposes using spatial redundancy to increase the reliability of Ethernet networks, but using spatial redundancy to tolerate temporary faults is not a cost-effective solution. For this reason, we propose to use time redundancy to tolerate temporary faults in the links of TSN-based networks. Specifically, we have proposed the Proactive Transmission of Replicated Frames (PTRF) mechanism, which consists in transmitting several copies of each frame in a preventive manner. In this article we present for the first time a detailed description of the mechanism, with the three different approaches we have designed. We also present the implementation of PTRF in a real TSN prototype. Furthermore, we carry out a qualitative comparison of the different approaches of the mechanism and we experimentally evaluate the approaches of the mechanism in a quantitative manner from three perspectives: the end-to-end delay, the jitter and the bandwidth consumption.

The Time-Sensitive Networking (TSN) Task Group (TG) is providing Ethernet with timing guarantees, reconfiguration services and fault tolerance mechanisms. Some of TSN’s targeted applications are real-time critical applications, which must provide a correct service continuously. To support these applications the TSN TG standardised a spatial redundancy mechanism. Even though spatial redundancy can tolerate permanent and temporary faults, it is not cost-effective. Instead, temporary faults can be tolerated using time redundancy. We proposed the Proactive Transmission of Replicated Frames (PTRF) mechanism to tolerate temporary faults in the links. In this work we present a new PTRF approach, a PTRF simulation model and a comparison of the approaches using exhaustive fault injection.

The IEEE Audio Video Bridging (AVB) Task Group (TG) was created to provide Ethernet with soft real-time guarantees. Later on, the TG was renamed to Time-Sensitive Networking (TSN) and its scope broadened to support hard real-time and critical applications. The Stream Reservation Protocol (SRP) is a key work of the TGs as it allows reserving resources in the network, guaranteeing the required quality of service (QoS). AVB's SRP is based on a distributed architecture, while TSN's is based on centralised ones. The distributed version of SRP is supported and used in TSN. Nevertheless, it was not designed to provide properties that are important for critical applications. In this work we model SRP using UPPAAL and we study the termination and consistency. We verify that SRP does not provide such properties. Furthermore, we propose an improved protocol called Consistent Stream Reservation Protocol (CSRP) and we formally verify its correctness using UPPAAL.