The art of fault-tolerant system reliability modeling

A step-by-step tutorial of the methods and tools used for the reliability analysis of fault-tolerant systems is presented. Emphasis is on the representation of architectural features in mathematical models. Details of the mathematical solution of complex reliability models are not presented. Instead the use of several recently developed computer programs--SURE, ASSIST, STEM, PAWS--which automate the generation and solution of these models is described

Addressing Complexity and Intelligence in Systems Dependability Evaluation

Engineering and computing systems are increasingly complex, intelligent, and open adaptive. When it comes to the dependability evaluation of such systems, there are certain challenges posed by the characteristics of “complexity” and “intelligence”. The first aspect of complexity is the dependability modelling of large systems with many interconnected components and dynamic behaviours such as Priority, Sequencing and Repairs. To address this, the thesis proposes a novel hierarchical solution to dynamic fault tree analysis using Semi-Markov Processes. A second aspect of complexity is the environmental conditions that may impact dependability and their modelling. For instance, weather and logistics can influence maintenance actions and hence dependability of an offshore wind farm. The thesis proposes a semi-Markov-based maintenance model called “Butterfly Maintenance Model (BMM)” to model this complexity and accommodate it in dependability evaluation. A third aspect of complexity is the open nature of system of systems like swarms of drones which makes complete design-time dependability analysis infeasible. To address this aspect, the thesis proposes a dynamic dependability evaluation method using Fault Trees and Markov-Models at runtime.The challenge of “intelligence” arises because Machine Learning (ML) components do not exhibit programmed behaviour; their behaviour is learned from data. However, in traditional dependability analysis, systems are assumed to be programmed or designed. When a system has learned from data, then a distributional shift of operational data from training data may cause ML to behave incorrectly, e.g., misclassify objects. To address this, a new approach called SafeML is developed that uses statistical distance measures for monitoring the performance of ML against such distributional shifts. The thesis develops the proposed models, and evaluates them on case studies, highlighting improvements to the state-of-the-art, limitations and future work

Experimental analysis of computer system dependability

This paper reviews an area which has evolved over the past 15 years: experimental analysis of computer system dependability. Methodologies and advances are discussed for three basic approaches used in the area: simulated fault injection, physical fault injection, and measurement-based analysis. The three approaches are suited, respectively, to dependability evaluation in the three phases of a system's life: design phase, prototype phase, and operational phase. Before the discussion of these phases, several statistical techniques used in the area are introduced. For each phase, a classification of research methods or study topics is outlined, followed by discussion of these methods or topics as well as representative studies. The statistical techniques introduced include the estimation of parameters and confidence intervals, probability distribution characterization, and several multivariate analysis methods. Importance sampling, a statistical technique used to accelerate Monte Carlo simulation, is also introduced. The discussion of simulated fault injection covers electrical-level, logic-level, and function-level fault injection methods as well as representative simulation environments such as FOCUS and DEPEND. The discussion of physical fault injection covers hardware, software, and radiation fault injection methods as well as several software and hybrid tools including FIAT, FERARI, HYBRID, and FINE. The discussion of measurement-based analysis covers measurement and data processing techniques, basic error characterization, dependency analysis, Markov reward modeling, software-dependability, and fault diagnosis. The discussion involves several important issues studies in the area, including fault models, fast simulation techniques, workload/failure dependency, correlated failures, and software fault tolerance

Computer aided reliability, availability, and safety modeling for fault-tolerant computer systems with commentary on the HARP program

Many of the most challenging reliability problems of our present decade involve complex distributed systems such as interconnected telephone switching computers, air traffic control centers, aircraft and space vehicles, and local area and wide area computer networks. In addition to the challenge of complexity, modern fault-tolerant computer systems require very high levels of reliability, e.g., avionic computers with MTTF goals of one billion hours. Most analysts find that it is too difficult to model such complex systems without computer aided design programs. In response to this need, NASA has developed a suite of computer aided reliability modeling programs beginning with CARE 3 and including a group of new programs such as: HARP, HARP-PC, Reliability Analysts Workbench (Combination of model solvers SURE, STEM, PAWS, and common front-end model ASSIST), and the Fault Tree Compiler. The HARP program is studied and how well the user can model systems using this program is investigated. One of the important objectives will be to study how user friendly this program is, e.g., how easy it is to model the system, provide the input information, and interpret the results. The experiences of the author and his graduate students who used HARP in two graduate courses are described. Some brief comparisons were made with the ARIES program which the students also used. Theoretical studies of the modeling techniques used in HARP are also included. Of course no answer can be any more accurate than the fidelity of the model, thus an Appendix is included which discusses modeling accuracy. A broad viewpoint is taken and all problems which occurred in the use of HARP are discussed. Such problems include: computer system problems, installation manual problems, user manual problems, program inconsistencies, program limitations, confusing notation, long run times, accuracy problems, etc

Resilience of an embedded architecture using hardware redundancy

In the last decade the dominance of the general computing systems market has being replaced by embedded systems with billions of units manufactured every year. Embedded systems appear in contexts where continuous operation is of utmost importance and failure can be profound. Nowadays, radiation poses a serious threat to the reliable operation of safety-critical systems. Fault avoidance techniques, such as radiation hardening, have been commonly used in space applications. However, these components are expensive, lag behind commercial components with regards to performance and do not provide 100% fault elimination. Without fault tolerant mechanisms, many of these faults can become errors at the application or system level, which in turn, can result in catastrophic failures. In this work we study the concepts of fault tolerance and dependability and extend these concepts providing our own definition of resilience. We analyse the physics of radiation-induced faults, the damage mechanisms of particles and the process that leads to computing failures. We provide extensive taxonomies of 1) existing fault tolerant techniques and of 2) the effects of radiation in state-of-the-art electronics, analysing and comparing their characteristics. We propose a detailed model of faults and provide a classification of the different types of faults at various levels. We introduce an algorithm of fault tolerance and define the system states and actions necessary to implement it. We introduce novel hardware and system software techniques that provide a more efficient combination of reliability, performance and power consumption than existing techniques. We propose a new element of the system called syndrome that is the core of a resilient architecture whose software and hardware can adapt to reliable and unreliable environments. We implement a software simulator and disassembler and introduce a testing framework in combination with ERA’s assembler and commercial hardware simulators

Development of a non-invasive method to detect pericellular spatial oxygen gradients using FLIM

PhDExtracellular oxygen concentrations affect cellular metabolism and influence tissue function. Detection methods for these extracellular oxygen concentrations currently have poor spatial resolution and are frequently invasive. Fluorescence Lifetime Imaging Microscopy (FLIM) offers a non-invasive method for quantifying local oxygen concentrations. However, existing FLIM methods also show limited spatial resolution >1 μm and low time-resolved accuracy and precision, due to widefield time-gate. This study describes a new optimised approach using FLIM to quantity extracellular oxygen concentration with high accuracy (±7 μmol/kg) and spatial resolution ( ≅ 0.3 μm). An oxygen sensitive fluorescent dye, tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate [Ru(bipy)3]+2, was excited with a multi-photon laser and fluorescence lifetime was measured using time-correlated single photon counting (TCSPC). The system was fully calibrated with optimised techniques developed for avoiding artefacts associated with photon pile-up and phototoxicity, whilst maximising spatial and temporal resolution. An extended imaging protocol (1800 sec) showed no phototoxic effects on cells at dye concentrations of <0.4 mM. Extracellular spatial oxygen gradients were identified around isolated chondrocytes, seeded in three-dimensional agarose gel. The technique was validated by regulating oxygen cellular consumption and thus confirming that the oxygen gradient was governed by cellular consumption. The technique identified a subpopulation of cells exhibiting statistically significant spatial oxygen gradients at the cell perihery. The subpopulation was shown to be significantly larger in cell diameter correlating with what that expected from chondrocytes in the deep zone. This technique provides an exciting opportunity to non-invasively quantify pericellular spatial oxygen gradients from within three-dimensional cellular constructs without prior manipulation of the cells. Thus by examining cellular metabolisms it will advance our understanding of the optimal cellular environment for tissue engineering and regenerative medicine

Design of a modular digital computer system, DRL 4

The design is reported of an advanced modular computer system designated the Automatically Reconfigurable Modular Multiprocessor System, which anticipates requirements for higher computing capacity and reliability for future spaceborne computers. Subjects discussed include: an overview of the architecture, mission analysis, synchronous and nonsynchronous scheduling control, reliability, and data transmission

Application of 1H HR-MAS-NMR spectroscopy in spatial tissue metabolic profiling

HR-MAS-NMR of intact tissue biopsies is a well-established method resulting in one NMR spectrum per whole biopsy showing all detectable metabolites at once. The aim of this project was to explore the possibility and usefulness of monitoring specific locations within the biopsy using HR-MAS-NMR. Firstly, the method was applied to a classic toxicology situation. Many drug development compounds fail because of preclinical animal liver toxicity conventionally detected using histology. Usually, only one of the murine liver lobes is used for this and is assumed to be representative of the whole organ. In this work, a metabolic variation across murine liver lobes has been investigated via a set of biopsies across all lobes. Using HR-MAS-NMR spectra analysed by various types of multivariate analysis, no lobe-specific metabolic variation could be found, confirming the general validity of the representative lobe approach. To increase location specificity, a spatially-resolved NMR pulse sequence (slice local- ized spectroscopy (SLS)) was modified and its respective effectiveness was explored. The pulse sequence was first validated using artificially created samples (phantoms), and practical examples were layered fruit separated by paraffin film and milled phantoms produced from materials which were magnetic-susceptibility-matched to the HR-MAS rotor. The HR-MAS SLS sequence was then applied to a mixed mouse renal tissue biopsy, and renal cortex and medulla successfully assigned to individual slices from spatially-resolved spectra using pure cortex and medulla reference HR-MAS-NMR spectra and orthogonal projection to latent structures discriminant analysis (OPLS-DA) to establish metabolic markers differentiating the two. Together, this work shows the potential of HR-MAS-NMR as applied to tissue biopsies. Particularly, spatially-resolved methods hold potential for improved biochemical and mechanistic understanding and the methodology could be expanded to applications in many areas of biomedical relevance.Open Acces

Complexity, Emergent Systems and Complex Biological Systems:\ud Complex Systems Theory and Biodynamics. [Edited book by I.C. Baianu, with listed contributors (2011)]

An overview is presented of System dynamics, the study of the behaviour of complex systems, Dynamical system in mathematics Dynamic programming in computer science and control theory, Complex systems biology, Neurodynamics and Psychodynamics.\u