Supporting real-time distributed computer-controlled systems with multi-hop P-NET networks

Fieldbus communication networks aim to interconnect sensors, actuators and controllers within process control applications. Therefore, they constitute the foundation upon which real-time distributed computer-controlled systems can be implemented. P-NET is a fieldbus communication standard, which uses a virtual token-passing medium-access-control mechanism. In this paper pre-run-time schedulability conditions for supporting real-time traffic with P-NET networks are established. Essentially, formulae to evaluate the upper bound of the end-to-end communication delay in P-NET messages are provided. Using this upper bound, a feasibility test is then provided to check the timing requirements for accessing remote process variables. This paper also shows how P-NET network segmentation can significantly reduce the end-to-end communication delays for messages with stringent timing requirements.     

Hard Real-Time Communication with the Timed Token Protocol: Current State and Challenging Problems

With the increasing use of distributed real-time systems, the ability of communication networks to handle real-time traffic is becoming more and more important. The timed token medium access control protocol, which has been now incorporated into several network standards such as FDDI and SAFENET due to its special timing property of bounded medium access time, is one of the most suitable and attractive candidate communication protocols for supporting distributed hard real-time applications. Extensive research has been conducted on using the timed token protocol to guarantee timely transmission of messages in a communication environment with hard real-time requirements. This paper intends to present a comprehensive review on recent advances in hard real-time communication with the timed token protocol. In addition, several challenging problems are identified.     

工业实时通讯网络(现场总线)的基础理论研究与现状(上)

分布式计算机系统的实时能力是由工业实时通讯网络 (现场总线) 保证的．随着前 者在工业应用中的逐渐普及，现场总线的通讯模型和实时性能的研究也开始日益重要．按照 对实时的支持来划分，现场总线主要分为事件触发方式和时间触发方式两大类．其中，时间 触发方式通常采用令牌来直接支持实时．时间触发方式按照令牌的管理方式，进一步可以分 为集中式令牌、分布式令牌和虚拟令牌三类．在传统的通信网络的性能评价中，系统通过率 和平均通信延迟等是最重要的性能评价指标，而且研究方法主要集中在排队论和随机过程等 随机分析方法上．然而，实时应用则强调满足每一个信息的时间要求，通常借鉴实时调度理 论中的基本理论和分析方法．在本篇，作者从分布式计算机系统中任务的通信特征、现场总 线的实时要求和通讯模型，到相关的实时调度理论，给予了详细的阐述．在下一篇，作者重 点阐述如何利用实时调度理论来分析和改进FF和WorldFIP，两个最受欢迎的现场总线协议， 并且提出了相关的有待于解决的问题, 例如，统一的性能评价体系等．     

过程TPN模拟方法用于两种现场总线网络的性能比较研究

以具有过程说明的TPN为工具,对两种不同类型的Fieldbus网络性能进行了研究、分析 和比较.在轮询协议中,采用了P/C通信模型,由主节点管理轮询队列;在令牌协议中,分别 采用循环令牌和授权令牌来满足周期性和突发性通信需要,其中对网络响应时间、吞吐量和振 颤等性能进行了重点讨论,给出了它们的性能差异和响应界限.     

噪音环境下的FF Fieldbus通讯性能研究

通过噪音对令牌运行过程的作用,本文分析了在噪音环境下FF Fieldbus的性能变化 ．本文首先建立了令牌运行过程的自动机模型,然后利用此模型分析了噪音对令牌运行方式 的影响．根据噪音的影响,本文推导出令牌运行时间和令牌平均运行时间．最后利用解析表 达式分析出噪音分布对FF Fieldbus性能的影响．     

Response modeling runtime schedulers for timing analysis of self-timed dataflow graphs

Constituent tasks of modern day Embedded Streaming Applications (ESAs), such as engine control systems, multimedia and software defined radios often exhibit execution behaviors that do not conform to conventional task models. ESAs consist of iterative, pipelined sequences of tasks that are conditioned by intra- and inter-iteration dependencies, and often have strict throughput and latency requirements. We model ESAs as dataflow graphs, where actors represent computational units, and directed edges represent communication channels between actors. Due to practical constraints like cost-effectiveness, power consumption and chip-area, multiple ESAs are run on a shared (multi-processor) platform. Thus rigorous timing analysis is required to verify whether individual ESAs meet their respective timing requirements.We look at response modeling, a compositional timing analysis approach wherein the local worst-case influence of runtime scheduling is represented within the constructs provided in dataflow. These local representations (called response models) can be composed together to construct a global understanding of an ESA’s worst-case execution which is then used to verify whether its real-time requirements are met. This paper proposes a generic response modeling technique for runtime scheduling of ESAs. We focus on preemptive Fixed Priority Scheduling (FPS) but also demonstrate that we can apply our technique to a wide range of runtime schedulers. In our experiments, we present academic and industrial case-studies that highlight the effectiveness of our approach in the timing analysis of ESAs with unconventional execution behavior.     

Worst-case response time analysis of real-time tasks under fixed-priority scheduling with deferred preemption

Fixed-priority scheduling with deferred preemption (FPDS) has been proposed in the literature as a viable alternative to fixed-priority pre-emptive scheduling (FPPS), that obviates the need for non-trivial resource access protocols and reduces the cost of arbitrary preemptions. This paper shows that existing worst-case response time analysis of hard real-time tasks under FPDS, arbitrary phasing and relative deadlines at most equal to periods is pessimistic and/or optimistic. The same problem also arises for fixed-priority non-pre-emptive scheduling (FPNS), being a special case of FPDS. This paper provides a revised analysis, resolving the problems with the existing approaches. The analysis is based on known concepts of critical instant and busy period for FPPS. To accommodate for our scheduling model for FPDS, we need to slightly modify existing definitions of these concepts. The analysis assumes a continuous scheduling model, which is based on a partitioning of the timeline in a set of non-empty, right semi-open intervals. It is shown that the critical instant, longest busy period, and worst-case response time for a task are suprema rather than maxima for all tasks, except for the lowest priority task. Hence, that instant, period, and response time cannot be assumed for any task, except for the lowest priority task. Moreover, it is shown that the analysis is not uniform for all tasks, i.e. the analysis for the lowest priority task differs from the analysis of the other tasks. These anomalies for the lowest priority task are an immediate consequence of the fact that only the lowest priority task cannot be blocked. To build on earlier work, the worst-case response time analysis for FPDS is expressed in terms of known worst-case analysis results for FPPS. The paper includes pessimistic variants of the analysis, which are uniform for all tasks, illustrates the revised analysis for an advanced model for FPDS, where tasks are structured as flow graphs of subjobs rather than sequences, and shows that our analysis is sustainable.     

Period And Priority Assignment Method For Dcs Design

Distributed control systems (DCS) used in industry environments consist of sensors, actuators, and controllers that are connected with a fieldbus. It is difficult to design the DCS while guaranteeing the time‐critical requirements because of both communication delays due to the fieldbus and computation delays due to many tasks executed in one node. This paper proposes a priority assignment method and a period assignment method that find the shortest periods of control loops and guarantee the end‐to‐end constraints such as precedence constraints and timing constraints. Also, a DCS design method is presented by using the proposed two assignment methods. The presented design method considers the worst‐case response times of tasks and messages simultaneously and is applicable to a practical DCS, which consists of several constraints. The design method is validated by examples.     

基于自动机理论的分布式实时调度分析工具

分布式实时系统是广泛应用在众多关键领域的一类复杂实时系统.为保证其上运行任务的实时性,传统基于最坏响应时间的调度分析方法往往包含了实际系统运行过程中无法达到的最坏情况,因此在这些情况下的分析结果过于悲观.基于自动机理论的模型检测方法的好处在于能够穷尽地搜索整个系统状态空间,得到精确的分析结果.为了利用形式化方法的优势来...     

Engineering PROFIBUS networks with heterogeneous transmission media

A significant number of process control and factory automation systems use PROFIBUS as the underlying fieldbus communication network. The process of properly setting up a PROFIBUS network is not a straightforward task. In fact, a number of network parameters must be set for guaranteeing the required levels of timeliness and dependability. Engineering PROFIBUS networks is even more subtle when the network includes various physical segments exhibiting heterogeneous specifications, such as bus speed or frame formats, just to mention a few. In this paper we provide underlying theory and a methodology to guarantee the proper operation of such type of heterogeneous PROFIBUS networks. We additionally show how the methodology can be applied to the practical case of PROFIBUS networks containing simultaneously DP (Decentralised Periphery) and PA (Process Automation) segments, two of the most used commercial-off-the-shelf (COTS) PROFIBUS solutions. The importance of the findings is however not limited to this case. The proposed methodology can be generalised to cover other heterogeneous infrastructures. Hybrid wired/wireless solutions are just an example for which an enormous eagerness exists.     

分布式控制系统的模型化可靠性分析方法

分布式控制系统（ＤＣＣＳ）是一种以通信网络为基本连接的多处理机控制系统。高可靠性和高可用性是ＤＣＣＳ系统的主要特点之一。在一些重要的过程控制应用中,为保证系统的高可靠性指标需要对分布式控制系统的可靠性分析与设计。本语在提出ＤＣＣＳ系统中的两个主要环节通信网络和智能化模块的可靠性数学模型基础上,讨论了ＤＣＣＳ系统的可靠性分析方法与一般设计步骤。     

A new algorithm to prove the schedulability of real-time systems

In hard real-time multiprocessor systems, tasks not only have timing constraints but also often have precedence constraints caused by communication among themselves. In this paper a new algorithm to prove the schedulability of real-time systems is proposed, in which both precedence constraints and communication costs are considered and represented by offsets and modified deadlines. To obtain a tight upper bound for the worst-case response time, the concepts of local critical instant and local worst-case response time are introduced. The proposed algorithm is compared with other algorithms using test cases. The comparison shows that the proposed algorithm has improved the performances to these compared algorithms.     

State-based real-time analysis of SDF applications on MPSoCs with shared communication resources

The timing predictability of Multi-Processor System on Chip (MPSoC) platforms with hard real-time applications is much more challenging than that of traditional platforms due to their large number of shared processing, communication and memory resources. Yet, this is an indispensable challenge for guaranteeing their safe usage in safety critical domains (avionics, automotive).In this article, a real-time analysis based on model-checking is proposed. The model-checking based method allows guaranteeing timing bounds of multiple Synchronous Data Flow Application (SDFA) implementations. This approach utilizes timed automata (TA) as a common semantic model to represent WCET of software components (SDF actors) and shared communication resource access protocols for buses, DMA, private local and shared memories of the MPSoC. The resulting network of TA is analyzed using the UPPAAL model-checker for providing safe timing bounds of the implementation. Furthermore, we will show the extension of our previous system model enabling single-beat inter-processor communication style beside the burst-transfer style and provide the implementation of the complete set of TA templates capturing the considered system model.We demonstrate our approach using a multi-phase electric motor control algorithm (modeled as SDFA) mapped to Infineon’s TriCore-based Aurix multicore hardware platform with both the burst and single-beat inter-processor communication styles. Our approach shows a significant precision improvement (up to a percentage improvement of 300%) compared with the worst-case bound calculation based on a pessimistic analytical upper-bound delays for every shared resource access. In addition, scalability is examined to demonstrate analysis feasibility for small parallel systems, up to 40 actors mapped to 4-tiles and up to 96 actors on a 2-tiles platforms.     

令牌多环网性能—可靠性综合评价

网络的性能与可靠性已分别得到广泛的研究,然而,怎样分析具有容错机制的环网的性能仍属有待开拓的研究领域,本文提出了一种分析具有自修复机制的多环网络系统的方法,并给出了报文传输时间分布的拉氏变换式,平均等待时间及平均队列长度公式,分析结果可用于评估故障与修复过程对实时令牌环网的实时响应特性的影响,对基于LAN的分布式实时系统设计具有指导意义。     

一种精确程序最坏执行时间分析方法

Java语言的动态特性使程序的最坏执行时间分析较悲观和难以预测,提出一种精确最坏执行时间分析方法,在高层分析中,引入一种标记方法,对带有标记的Java类文件进行反编译提取控制流程,得到每一个基本块中的Java 字节码指令的最坏情况下的执行次数,在底层分析中,建立结合流水线和高级缓存影响的时间模型,得到每条指令所对应的执行时间,最后结合高层分析和底层分析的结果得到程序的最坏情况下的执行时间。实验表明,该方法可以使对实时Java 程序的最坏情况执行时间预测更加安全和精确。     

Some features of two fieldbuses of the IEC 61158 standard

The IEC 61158 is on the way to become an international fieldbus standard after a long period of contrasts and misunderstandings. The final standard document results from the joining of eight among the most diffused fieldbus protocols.This paper takes into consideration one of the most popular fieldbus of the final standard document, Profibus, and examines the differences with the original IEC 61158 Technical Specification (which is also included in the standard). The analysis concerns the data link layer of the two fieldbuses: Profibus is based on a token passing technique, while the IEC fieldbus realises a mixed access combining token passing with a centralised scheduling procedure managed by a special node named Link Active Scheduler (LAS). Two parameters of the data link layers have been in particular considered: the cycle time and the medium access efficiency. In particular, the evaluation of the cycle times has been carried out using analytical models which are valid under the hypothesis that the token holding times of the stations are sufficiently long to satisfy all the service requests coming from the users, that is a typical situation of a network not overloaded. Considerations have also been made to investigate the behaviours of the cycle times when the network traffic increases.     

Causality,influence, and computation in possibly disconnected synchronous dynamic networks

In this work, we study the propagation of influence and computation in dynamic distributed computing systems that are possibly disconnected at every instant. We focus on a synchronous message-passing communication model with broadcast and bidirectional links. Our network dynamicity assumption is a worst-case dynamicity controlled by an adversary scheduler, which has received much attention recently. We replace the usual (in worst-case dynamic networks) assumption that the network is connected at every instant by minimal temporal connectivity conditions. Our conditions only require that another causal influence occurs within every time window of some given length. Based on this basic idea, we define several novel metrics for capturing the speed of information spreading in a dynamic network. We present several results that correlate these metrics. Moreover, we investigate termination criteria in networks in which an upper bound on any of these metrics is known. We exploit our termination criteria to provide efficient (and optimal in some cases) protocols that solve the fundamental counting and all-to-all token dissemination (or gossip) problems.     

Enforcing the timing behavior of real-time stations in legacy bus-based industrial Ethernet networks

This paper proposes a virtual token-passing mechanism, which allows the coexistence of legacy industrial Ethernet devices with enhanced (real-time) devices in the same bus network. The VTPE mechanism allows the implementation of real-time (RT) applications upon legacy bus-based industrial networks, enforcing the adequate timing behavior of the supported applications. It enables the traffic separation between legacy Ethernet stations and enhanced (RT) stations. A set of experiments has been performed, which allowed to measure the message access delay and the token rotation time. The achieved results clearly demonstrate that the VTPE architecture enables the support of RT communication upon legacy bus-based industrial Ethernet networks, even when the communication medium is shared with uncontrolled traffic sources.     

The design of distributed control computer systems

The top-down approach in the design of Distributed Control Computer Systems, DCCS, is presented. The starting point of the study is three statements: (i) the design approach should be production process control oriented; (ii) for real time process control applications, computer networks require an efficient communication subsystem, the most important features of which are high reliability and short message transmission times and (iii) recent research has shown that the tools and methodologies used in both computing and control should be complementary to each other in order to solve some of the distributed control computer system/distributed computer control system design problems. The paper contains the design problems of distributed control computer systems, the presentation of the methods and tools for the study and design of the DCCS, the top-down approach in the design of the DCCS, and examples of two design stages.     

片上网络二维和三维结构的通信性能分析

芯片集成技术的迅猛发展,使得片上网络从二维向三维扩展成为可能.研究表明三维片上网络因拓扑维度的增加而缩短了通信距离,极大地提升了网络的平均通信性能.本文对比分析了k-ary-2-mesh网络及其对应的三维网络在最差情形下的通信性能,得出了以下结论:三维网络的平均通信性能虽然更优,但受垂直信道影响其最差情形下的通信性能可...