A Uniform Treatment of Architectures in Decentralized Discrete-Event System

Solutions to decentralized discrete-event systems problems are characterized by the way local decisions are fused to yield a global decision. A fusion rule is colloquially called an architecture. This paper provides a uniform treatment of architectures in decentralized discrete-event systems. Current approaches neither provide a direct way to determine problem solvability conditions under one architecture, nor a way to compare existing architectures. Determining whether a new architecture is more general than an existing known architecture relies on producing examples ad hoc and on individual inspiration that puts the conditions for solvability in each architecture into some form that admits comparison. From these research efforts, a method based on morphisms between graphs has been extracted to yield a uniform approach to decentralized discrete-event system architectures and their attendant fusion rules. This treatment provides an easy and direct way to compare the fusion rules -- and hence to compare the strength or generality of the corresponding architectures

05241 Abstracts Collection -- Synthesis and Planning

From 12.06.05 to 17.06.2005 the Dagstuhl Seminar 05241 ``Synthesis and Planning\u27\u27 was held in the International Conference and Research Center (IBFI), Schloss Dagstuhl. During the seminar, several participants presented their current research, and ongoing work and open problems were discussed. Abstracts of the presentations given during the seminar as well as abstracts of seminar results and ideas are put together in this paper. The first section describes the seminar topics and goals in general. Links to extended abstracts or full papers are provided, if available

Fundamentals of Large Sensor Networks: Connectivity, Capacity, Clocks and Computation

Sensor networks potentially feature large numbers of nodes that can sense their environment over time, communicate with each other over a wireless network, and process information. They differ from data networks in that the network as a whole may be designed for a specific application. We study the theoretical foundations of such large scale sensor networks, addressing four fundamental issues- connectivity, capacity, clocks and function computation. To begin with, a sensor network must be connected so that information can indeed be exchanged between nodes. The connectivity graph of an ad-hoc network is modeled as a random graph and the critical range for asymptotic connectivity is determined, as well as the critical number of neighbors that a node needs to connect to. Next, given connectivity, we address the issue of how much data can be transported over the sensor network. We present fundamental bounds on capacity under several models, as well as architectural implications for how wireless communication should be organized. Temporal information is important both for the applications of sensor networks as well as their operation.We present fundamental bounds on the synchronizability of clocks in networks, and also present and analyze algorithms for clock synchronization. Finally we turn to the issue of gathering relevant information, that sensor networks are designed to do. One needs to study optimal strategies for in-network aggregation of data, in order to reliably compute a composite function of sensor measurements, as well as the complexity of doing so. We address the issue of how such computation can be performed efficiently in a sensor network and the algorithms for doing so, for some classes of functions.Comment: 10 pages, 3 figures, Submitted to the Proceedings of the IEE

A Hybrid Approach to Fault Diagnosis in Teams of Autonomous Systems

Discrete event systems (DES) are dynamical systems equipped with a discrete state set and an event driven state transition structure. An event in a DES occurs instantaneously causing transition from one state to another. DES models have emerged to provide a formal treatment of many man-made systems such as automated manufacturing systems, computer systems, communication networks and air traffic control systems. In this thesis, we study fault diagnosis in teams of autonomous systems. In particular, one consider a team of two spacecraft in deep space. The spacecraft cooperate with each other in leader-follower formation flying. Formation flying demonstrates the capability of spacecraft to react to each other in order to maintain a desired relative distance autonomously without human intervention. In the system considered here, instruments (actuators and sensors) may fail and cause error. Because of the communication delays in deep space, each entity should be able to diagnose the failure and decide how to reconfigure itself. Basically, fault diagnosis in such systems requires information exchange between the autonomous elements of the team. The exchanged information for example may include position and velocity data. Our goal in the thesis is to propose a method for fault diagnosis with reduced information exchange. One solution is to transmit only discrete event information between autonomous systems. Transmission of discrete event data occurs less frequently than the transmission of continuous streams of data. The discrete event data may include high level supervisory commands issued every now and then and discretized values of continuous data that are transmitted only when a continuous-variable data (such as angle or acceleration) crosses the threshold. The fault diagnosis scheme proposed in this thesis is an adaptation of hybrid fault diagnosis for distributed autonomous systems. This system is simulated using MATLAB/SIMULINK Software and DECK Toolbox. We examined different maneuvers for spacecraft and investigated the effect of faults on the overall system and the performance of our designed fault diagnoser

FAST : a fault detection and identification software tool

The aim of this work is to improve the reliability and safety of complex critical control systems by contributing to the systematic application of fault diagnosis. In order to ease the utilization of fault detection and isolation (FDI) tools in the industry, a systematic approach is required to allow the process engineers to analyze a system from this perspective. In this way, it should be possible to analyze this system to find if it provides the required fault diagnosis and redundancy according to the process criticality. In addition, it should be possible to evaluate what-if scenarios by slightly modifying the process (f.i. adding sensors or changing their placement) and evaluating the impact in terms of the fault diagnosis and redundancy possibilities. Hence, this work proposes an approach to analyze a process from the FDI perspective and for this purpose provides the tool FAST which covers from the analysis and design phase until the final FDI supervisor implementation in a real process. To synthesize the process information, a very simple format has been defined based on XML. This format provides the needed information to systematically perform the Structural Analysis of that process. Any process can be analyzed, the only restriction is that the models of the process components need to be available in the FAST tool. The processes are described in FAST in terms of process variables, components and relations and the tool performs the structural analysis of the process obtaining: (i) the structural matrix, (ii) the perfect matching, (iii) the analytical redundancy relations (if any) and (iv) the fault signature matrix. To aid in the analysis process, FAST can operate stand alone in simulation mode allowing the process engineer to evaluate the faults, its detectability and implement changes in the process components and topology to improve the diagnosis and redundancy capabilities. On the other hand, FAST can operate on-line connected to the process plant through an OPC interface. The OPC interface enables the possibility to connect to almost any process which features a SCADA system for supervisory control. When running in on-line mode, the process is monitored by a software agent known as the Supervisor Agent. FAST has also the capability of implementing distributed FDI using its multi-agent architecture. The tool is able to partition complex industrial processes into subsystems, identify which process variables need to be shared by each subsystem and instantiate a Supervision Agent for each of the partitioned subsystems. The Supervision Agents once instantiated will start diagnosing their local components and handle the requests to provide the variable values which FAST has identified as shared with other agents to support the distributed FDI process.Per tal de facilitar la utilització d'eines per la detecció i identificació de fallades (FDI) en la indústria, es requereix un enfocament sistemàtic per permetre als enginyers de processos analitzar un sistema des d'aquesta perspectiva. D'aquesta forma, hauria de ser possible analitzar aquest sistema per determinar si proporciona el diagnosi de fallades i la redundància d'acord amb la seva criticitat. A més, hauria de ser possible avaluar escenaris de casos modificant lleugerament el procés (per exemple afegint sensors o canviant la seva localització) i avaluant l'impacte en quant a les possibilitats de diagnosi de fallades i redundància. Per tant, aquest projecte proposa un enfocament per analitzar un procés des de la perspectiva FDI i per tal d'implementar-ho proporciona l'eina FAST la qual cobreix des de la fase d'anàlisi i disseny fins a la implementació final d'un supervisor FDI en un procés real. Per sintetitzar la informació del procés s'ha definit un format simple basat en XML. Aquest format proporciona la informació necessària per realitzar de forma sistemàtica l'Anàlisi Estructural del procés. Qualsevol procés pot ser analitzat, només hi ha la restricció de que els models dels components han d'estar disponibles en l'eina FAST. Els processos es descriuen en termes de variables de procés, components i relacions i l'eina realitza l'anàlisi estructural obtenint: (i) la matriu estructural, (ii) el Perfect Matching, (iii) les relacions de redundància analítica, si n'hi ha, i (iv) la matriu signatura de fallades. Per ajudar durant el procés d'anàlisi, FAST pot operar aïlladament en mode de simulació permetent a l'enginyer de procés avaluar fallades, la seva detectabilitat i implementar canvis en els components del procés i la topologia per tal de millorar les capacitats de diagnosi i redundància. Per altra banda, FAST pot operar en línia connectat al procés de la planta per mitjà d'una interfície OPC. La interfície OPC permet la possibilitat de connectar gairebé a qualsevol procés que inclogui un sistema SCADA per la seva supervisió. Quan funciona en mode en línia, el procés està monitoritzat per un agent software anomenat l'Agent Supervisor. Addicionalment, FAST té la capacitat d'implementar FDI de forma distribuïda utilitzant la seva arquitectura multi-agent. L'eina permet dividir sistemes industrials complexes en subsistemes, identificar quines variables de procés han de ser compartides per cada subsistema i generar una instància d'Agent Supervisor per cadascun dels subsistemes identificats. Els Agents Supervisor un cop activats, començaran diagnosticant els components locals i despatxant les peticions de valors per les variables que FAST ha identificat com compartides amb altres agents, per tal d'implementar el procés FDI de forma distribuïda.Postprint (published version

Blockchain-based end-to-end encryption for Matrix instant messaging

Privacy and security in online communication is an important topic today, especially in the context of instant messaging. A lot of progress has been made in recent years to ensure that conversations are secure against attacks by third parties, but privacy from the service provider itself remains difficult. There are a number of solutions offering end-to-end encryption, but most of them rely on a centralized server, proprietary clients, or both. In order to have fully secure instant messaging conversations, a decentralized and end-to-end encrypted communication protocol is needed. This means there is no single point of control, and each message is encryped directly on the user's device such that only the recipient can decrypt it. This work proposes an end-to-end encryption system for the Matrix protocol based on blockchain technology. Matrix is a decentralized protocol and network for real-time communication that is currently mostly used for instant messaging. This protocol was selected because of its versatility and extensibility. Using the Secret Store feature in OpenEthereum, the proposed system encrypts data using keys stored on the Ethereum blockchain. Access control to the keys is also handled by the Secret Store via a smart contract. The proposed encryption system has multiple advantages over alternative schemes: The underlying blockchain technology reduces the risk of data loss because of its decentralized and distributed nature. Thanks to the use of smart contracts this system also allows for the creation of an advanced access control system to decryption keys. In order to test and analyze the proposed design, a reference implementation was created in the form of a library. This library can be used for future research, but also as a building block for different applications to easily implement end-to-end encryption based on blockchain technology