Latency Analysis of Multiple Classes of AVB Traffic in TSN with Standard Credit Behavior using Network Calculus

Time-Sensitive Networking (TSN) is a set of amendments that extend Ethernet to support distributed safety-critical and real-time applications in the industrial automation, aerospace and automotive areas. TSN integrates multiple traffic types and supports interactions in several combinations. In this paper we consider the configuration supporting Scheduled Traffic (ST) traffic scheduled based on Gate-Control-Lists (GCLs), Audio-Video-Bridging (AVB) traffic according to IEEE 802.1BA that has bounded latencies, and Best-Effort (BE) traffic, for which no guarantees are provided. The paper extends the timing analysis method to multiple AVB classes and proofs the credit bounds for multiple classes of AVB traffic, respectively under frozen and non-frozen behaviors of credit during guard band (GB). They are prerequisites for non-overflow credits of Credit-Based Shaper (CBS) and preventing starvation of AVB traffic. Moreover, this paper proposes an improved timing analysis method reducing the pessimism for the worst-case end-to-end delays of AVB traffic by considering the limitations from the physical link rate and the output of CBS. Finally, we evaluate the improved analysis method on both synthetic and real-world test cases, showing the significant reduction of pessimism on latency bounds compared to related work, and presenting the correctness validation compared with simulation results. We also compare the AVB latency bounds in the case of frozen and non-frozen credit during GB. Additionally, we evaluate the scalability of our method with variation of the load of ST flows and of the bandwidth reservation for AVB traffic

Impact of AS6802 Synchronization Protocol on Time-Triggered and Rate-Constrained Traffic

TTEthernet is an Ethernet-based synchronized network technology compliant with the AFDX standard. It supports safety-critical applications by defining different traffic classes: Time-Triggered (TT), Rate-Constrained (RC), and Best-Effort traffic. The synchronization is managed through the AS6802 protocol, which defines so-called Protocol Control Frames (PCFs) to synchronize the local clock of each device. In this paper, we analyze the synchronization protocol to assess the impact of the PCFs on TT and RC traffic. We propose a method to decrease the impact of PCFs on TT and a new Network Calculus model to compute RC delay bounds with the influence of both PCF and TT traffic. We finish with a performance evaluation to i) assess the impact of PCFs, ii) show the benefits of our method in terms of reducing the impact of PCFs on TT traffic and iii) prove the necessity of taking the PCF traffic into account to compute correct RC worst-case delays and provide a safe system

Design Optimization of Cyber-Physical Distributed Systems using IEEE Time-sensitive Networks (TSN)

In this paper we are interested in safety-critical real-time applications implemented on distributed architectures supporting the Time-SensitiveNetworking (TSN) standard. The ongoing standardization of TSN is an IEEE effort to bring deterministic real-time capabilities into the IEEE 802.1 Ethernet standard supporting safety-critical systems and guaranteed Quality-of-Service. TSN will support Time-Triggered (TT) communication based on schedule tables, Audio-Video-Bridging (AVB) flows with bounded end-to-end latency as well as Best-Effort messages. We first present a survey of research related to the optimization of distributed cyber-physical systems using real-time Ethernet for communication. Then, we formulate two novel optimization problems related to the scheduling and routing of TT and AVB traffic in TSN. Thus, we consider that we know the topology of the network as well as the set of TT and AVB flows. We are interested to determine the routing of both TT and AVB flows as well as the scheduling of the TT flows such that all frames are schedulable and the AVB worst-case end-to-end delay is minimized. We have proposed an Integer Linear Programming (ILP) formulation for the scheduling problem and a Greedy Randomized Adaptive Search Procedure (GRASP)-based heuristic for the routing problem. The proposed approaches have been evaluated using several test cases

Scheduling Rate Constrained traffic in End Systems of Time-Aware Networks

Nowadays, most of cyber-physical systems in avionics, automotive or recent Industry 4.0 domains require networked communication for mixed-critical applications. Ethernet-based networks such as AFDX, TTEthernet or TSN are capable to support transmission of both safety-critical and non-critical flows. This paper focuses on the TTEthernet network compliant with the avionics ARINC 664-P7 standard supporting time-triggered communication (TT) together with rate-constrained (RC) and best-effort (BE) traffic. Due to a global synchronization, TTcommunication with low latency and minimal jitter is ensured with static schedules computed offline. For event-triggered RC flows, bounded jitter at the source and end-to-end latency are guaranteed with worst-case analysis methods. With the increasing demands of applications, flows with Quality of Service (QoS) requirements such as video or audio may be transmitted as BE flows. However, on current configurations, no guarantees are offered to BE flows. In this paper, we aim at increasing the maximum RC utilization and improving the QoS of BE flows to allow the transmission of video or audio traffic with low jitter and end-to-end delay requirements. For this, we focus on the scheduling mechanisms and propose a scheduling approach based on a static slotted table that is applied at end systems. This table integrates the TT schedules usually obtained with Satisfiability Modulo Theories (SMT) approaches and establishes offsets of RC flows that reduce the end-to-end delay of BE flows. Several strategies for offset computations are proposed based on the distribution of flows locally at end system or globally at switch. We show that local strategies perform better than the global ones to reduce end-to-end delay of BE flows

Performance impact of the interactions between time-triggered and rate-constrained transmissions in TTEthernet

Switched Ethernet is becoming a de-facto standard in industrial and embedded networks. Many of today's applications benefit from Ethernet's high bandwidth, large frame size, multicast and routing capabilities through IP, and the availability of the standard TCP/IP protocols. There are however many variants of Switched Ethernet networks, just considering the MAC level mechanisms on the stations and communication switches. An important technology in that landscape is TTEthernet, standardized as SAE6802, which allows the transmission of both purely time-triggered (TT) traffic and sporadic (or rate-constrained-RC) traffic. To the best of our knowledge, the interactions between both classes of traffic have not been studied so far in realistic configurations. This work aims to shed some light on the kind of performances, in terms of latencies, jitters and useful bandwidth that can be expected from a mixed TT and RC configuration. The following issues will be answered in a quantified manner by sensitivity analysis: How do both classes of traffic interfere with each other? What are the typical worst-case latencies and useful bandwidth that can be expected for a RC stream for various TT traffic loads? What is the overall impact of TTEthernet integration policy for the RC traffic? This study builds on a worst-case traversal time analysis developed by the authors for SAE6802, and explores these questions by experiments performed configurations of various sizes

Modeling and Analysis of Mixed Synchronous/Asynchronous Systems

Practical safety-critical distributed systems must integrate safety critical and non-critical data in a common platform. Safety critical systems almost always consist of isochronous components that have synchronous or asynchronous interface with other components. Many of these systems also support a mix of synchronous and asynchronous interfaces. This report presents a study on the modeling and analysis of asynchronous, synchronous, and mixed synchronous/asynchronous systems. We build on the SAE Architecture Analysis and Design Language (AADL) to capture architectures for analysis. We present preliminary work targeted to capture mixed low- and high-criticality data, as well as real-time properties in a common Model of Computation (MoC). An abstract, but representative, test specimen system was created as the system to be modeled

Determinism Enhancement and Reliability Assessment in Safety Critical AFDX Networks

RÉSUMÉ AFDX est une technologie basée sur Ethernet, qui a été développée pour répondre aux défis qui découlent du nombre croissant d’applications qui transmettent des données de criticité variable dans les systèmes modernes d’avionique modulaire intégrée (Integrated Modular Avionics). Cette technologie de sécurité critique a été notamment normalisée dans la partie 7 de la norme ARINC 664, dont le but est de définir un réseau déterministe fournissant des garanties de performance prévisibles. En particulier, AFDX est composé de deux réseaux redondants, qui fournissent la haute fiabilité requise pour assurer son déterminisme. Le déterminisme de AFDX est principalement réalisé par le concept de liens virtuels (Virtual Links), qui définit une connexion unidirectionnelle logique entre les points terminaux (End Systems). Pour les liens virtuels, les limites supérieures des délais de bout en bout peuvent être obtenues en utilisant des approches comme calcul réseau, mieux connu sous l’appellation Network Calculus. Cependant, il a été prouvé que ces limites supérieures sont pessimistes dans de nombreux cas, ce qui peut conduire à une utilisation inefficace des ressources et augmenter la complexité de la conception du réseau. En outre, en raison de l’asynchronisme de leur fonctionnement, il existe plusieurs sources de non-déterminisme dans les réseaux AFDX. Ceci introduit un problème en lien avec la détection des défauts en temps réel. En outre, même si un mécanisme de gestion de la redondance est utilisé pour améliorer la fiabilité des réseaux AFDX, il y a un risque potentiel souligné dans la partie 7 de la norme ARINC 664. La situation citée peut causer une panne en dépit des transmissions redondantes dans certains cas particuliers. Par conséquent, l’objectif de cette thèse est d’améliorer la performance et la fiabilité des réseaux AFDX. Tout d’abord, un mécanisme fondé sur l’insertion de trames est proposé pour renforcer le déterminisme de l’arrivée des trames au sein des réseaux AFDX. Parce que la charge du réseau et la bande passante moyenne utilisée augmente due à l’insertion de trames, une stratégie d’agrégation des Sub-Virtual Links est introduite et formulée comme un problème d’optimisation multi-objectif. En outre, trois algorithmes ont été développés pour résoudre le problème d’optimisation multi-objectif correspondant. Ensuite, une approche est introduite pour incorporer l’analyse de la performance dans l’évaluation de la fiabilité en considérant les violations des délais comme des pannes.----------ABSTRACT AFDX is an Ethernet-based technology that has been developed to meet the challenges due to the growing number of data-intensive applications in modern Integrated Modular Avionics systems. This safety critical technology has been standardized in ARINC 664 Part 7, whose purpose is to define a deterministic network by providing predictable performance guarantees. In particular, AFDX is composed of two redundant networks, which provide the determinism required to obtain the desired high reliability. The determinism of AFDX is mainly achieved by the concept of Virtual Link, which defines a logical unidirectional connection from one source End System to one or more destination End Systems. For Virtual Links, the end-to-end delay upper bounds can be obtained by using the Network Calculus. However, it has been proved that such upper bounds are pessimistic in many cases, which may lead to an inefficient use of resources and aggravate network design complexity. Besides, due to asynchronism, there exists a source of non-determinism in AFDX networks, namely frame arrival uncertainty in a destination End System. This issue introduces a problem in terms of real-time fault detection. Furthermore, although a redundancy management mechanism is employed to enhance the reliability of AFDX networks, there still exist potential risks as pointed out in ARINC 664 Part 7, which may fail redundant transmissions in some special cases. Therefore, the purpose of this thesis is to improve the performance and the reliability of AFDX networks. First, a mechanism based on frame insertion is proposed to enhance the determinism of frame arrival within AFDX networks. As the network load and the average bandwidth used by a Virtual Link increase due to frame insertion, a Sub-Virtual Link aggregation strategy, formulated as a multi-objective optimization problem, is introduced. In addition, three algorithms have been developed to solve the corresponding multi-objective optimization problem. Next, an approach is introduced to incorporate performance analysis into reliability assessment by considering delay violations as failures. This allowed deriving tighter probabilistic upper bounds for Virtual Links that could be applied in AFDX network certification. In order to conduct the necessary reliability analysis, the well-known Fault-Tree Analysis technique is employed and Stochastic Network Calculus is applied to compute the upper bounds with various probability limits

Quantitative Performance Comparison of Various Traffic Shapers in Time-Sensitive Networking

Owning to the sub-standards being developed by IEEE Time-Sensitive Networking (TSN) Task Group, the traditional IEEE 802.1 Ethernet is enhanced to support real-time dependable communications for future time- and safety-critical applications. Several sub-standards have been recently proposed that introduce various traffic shapers (e.g., Time-Aware Shaper (TAS), Asynchronous Traffic Shaper (ATS), Credit-Based Shaper (CBS), Strict Priority (SP)) for flow control mechanisms of queuing and scheduling, targeting different application requirements. These shapers can be used in isolation or in combination and there is limited work that analyzes, evaluates and compares their performance, which makes it challenging for end-users to choose the right combination for their applications. This paper aims at (i) quantitatively comparing various traffic shapers and their combinations, (ii) summarizing, classifying and extending the architectures of individual and combined traffic shapers and their Network calculus (NC)-based performance analysis methods and (iii) filling the gap in the timing analysis research on handling two novel hybrid architectures of combined traffic shapers, i.e., TAS+ATS+SP and TAS+ATS+CBS. A large number of experiments, using both synthetic and realistic test cases, are carried out for quantitative performance comparisons of various individual and combined traffic shapers, from the perspective of upper bounds of delay, backlog and jitter. To the best of our knowledge, we are the first to quantitatively compare the performance of the main traffic shapers in TSN. The paper aims at supporting the researchers and practitioners in the selection of suitable TSN sub-protocols for their use cases