Selection of a checkpoint interval in a critical-task environment

The selection of an optimal checkpointing strategy has most often been considered in the transaction processing environment where systems are allowed unlimited repairs. In this environment an optimal strategy maximizes the time spent in the normal operating state and consequently the rate of transaction processing. This paper seeks a checkpoint strategy which maximizes the probability of critical-task completion on a system with limited repairs. These systems can undergo failure and repair only until a repair time exceeds a specified threshold, at which time the system is deemed to have failed completely. For such systems, a model is derived which yields the probability of completing the critical task when each checkpoint operation has fixed cost. The optimal number of checkpoints can increase as system reliability improves. The model is extended to include a constraint which enforces timely completion of the critical task     

A redundant system with non-instantaneous switchover and “preparation time” for the repair facility

This paper discusses two models of two-unit standby redundant systems in which the switchover time is a random variable and the repair facility is not available for a random time immediately after each repair completion. In model I the probability distributions of the life time of the online unit and switchover time are general while all the other distributions are exponential. Model II is a cold standby system in which the probability distributions of the “preparation time” of the repair facility is exponential and all the other distributions are general. Using the regeneration point technique the availability functions of the two systems are determined. Several special cases are also discussed.     

Modeling and analyzing the effects of periodic inspection on theperformance of safety-critical systems

This paper presents a method for incorporating into Markov models of safety-critical systems, periodic inspections and repairs which occur deterministically in time. Both perfect and imperfect inspections and repairs can be modeled. Based on this new modeling technique, closed-form solutions are derived for a variety of important performance indexes including MTTF, MTTF_D, MTTF_S, average availability, and average probability of failing dangerously. The solutions are applied to an example system to illustrate how the method can be used to study the effects on performance of: (a) choices of the time-length between periodic inspections and repairs, and (b) improvements in inspection and repair techniques     

ESCAF for MTBF, Time Evolution, Sensitivity Coefficients, Cut-Set Importance, and Non-Coherence of Large Systems

ESCAF - an "Electronic Simulator to Compute and Analyze Failures" ? is a commercially available desktop apparatus, now in wide use, which employs special electronic circuit boards to simulate and analyze a system by methods and procedures that are easy to learn and apply by engineers with only very limited knowledge of electronics or computing. The apparatus has been used mainly to determine the cut sets and failure probabilities of very large systems, with up to 416 components, offering possibilities well beyond those of conventional methods of analysis. This article defines new parameters that can be calculated by ESCAF, including novel parameters (degree of non-coherence). It also explains how ESCAF determines, for systems of any complexity, measures that were previously inaccessible for other than very simple systems: ? Mean time between failures, mean time to failure, mean time to repair, failure and repair intensities, and equivalent failure or repair rates as a function of time, ? Time-derivative of the unavailability and/or failure probability, ? Sensitivity coefficients indicating the importance of each component in probability calculation, ? Importance of each minimal cut set (for coherent systems), ? Whether a system is coherent or not. The first three results can be obtained regardless of system coherency. All results are obtained very simply and very rapidly.     

Stochastic behaviour of a three-state complex repairable system with three types of failures

This paper deals with the operational availability of a three-state complex repairable system consisting of n distinct units arranged in series. Single service facility is available for three types of failures of the complex system. A mathematical model for the complex system under severe restrictions has been developed for exponential failures and general repairs. Laplace transforms of various state probabilities have been evaluated and availability is then obtained by the inversion process. Few graphical illustrations are also given in the end so as to explain the practical utility of the model.     

Evaluation of Generating System Reliability Indices Under Limited Repair Facility

A generating system having n units, where each unit is represented by a 2-state device, will have more than 2n possible states when repair facility of the system is limited, i.e. number of repair crews is less than the number of units. When the number of units failed is more than the number of repair crews available, the most recent failures queue for repairs. This paper gives a general equation to determine the number of system states under limited repair facility and an approximate technique to determine the probabilities of system states and reliability indices of the system.     

Failure Time for a Redundant Repairable System of Two Dissimilar Elements

The distribution of time to failure for a system consisting of two dissimilar elements or subsystems operating redundantly and susceptible to repair is discussed. It is assumed that the times to failure for the two system elements are independent random variables from possibly different exponential distributions, and that the repair times peculiar to each element are independently distributed in an arbitrary fashion. For this basic model a derivation is given of the Laplace-Stieltjes transform of the distribution function of time to system failure, i.e, the time until both elements are simultaneously down for repair, measured from an instant at which both are operating. An explicit formula is given for the mean or expected time to system failure, a natural approximation to the latter is exhibited, and numerical comparisons indicate the quality of this approximation for various repair time distributions. In a second model the possibility of system failures due to overloading the remaining element after a single element failure is explicitly recognized. The assumptions made for the basic model are augmented by a stochastic process describing the random occurrence of overloads. Numerical examples are given. Finally, it is shown how the above models may be easily modified to account for delays in initiating repairs resulting from only occasional system surveillance, and to account for random catastrophic failures.     

Analysis of Maintenance Man Loading Via Simulation

This paper provides the results of a computer simulation performed to test compliance with a system maintenance requirement. Special attention is given to the selection of the simulation run time (sample size) and fitting a curve to the output data. The actual simulation program was written in the RCA Flow Simulation language which is similar to the IBM GPSS (General-Purpose Simualtion System) language. More specifically, the system requirement was as follows. The probability that the system maintenance-man loading exceeds 4-h repair time in a day (24 h) shall be less than 0.0025. Equipment failures and repairs were simulated via a computer. Equipment failure and repair probability distributions were used as input data. A year's operation was simulated in less than 2-min computer time. A simulation sample size of 360 days was selected on the basis of a statistical analysis using a nonparametric 0.60 confidence level. The results showed a maximum repair time in a day of 130 min, which indicates the system repair time distribution is considerably better than required. The output data were analyzed and the observed repair times were fit with an empirical distribution function.     

Reliability Modeling and Analysis of Computer Networks

The reliability analysis of computer communication networks is generally based on Boolean algebra and probability theory. This paper discusses various reliability problems of computer networks including terminal-pair connectivity, tree connectivity, and multi-terminal connectivity. This paper also studies the dynamic computer network reliability by deriving time-dependent expressions for reliability measures assuming Markov behavior for failures and repairs. This allows computation of task and mission related measures such as mean time to first failure and mean time between failures. A detailed analysis of the bridge network is presented.     

A bulk queueing system with random failures and two phase repairs

The paper studies a queueing system with bulk arrivals and bulk departures wherein the service channel is subject to random failures and the repairs are performed in two phases. Units wait until both phases of repairs of the service channel are complete. The probability generating functions of the queue length have been obtained for the various states of the system and the corresponding results for the steady state have been derived. Two particular cases have been discussed and the mean queue length has been obtained explicitly. Finally depenedence of the mean queue length on failure and repair parameters have been shown graphically.     

Stochastic analysis of systems with primary and secondary failures

This paper presents the stochastic analysis of repairable systems involving primary as well as secondary failures. To this end, two models are considered. The first model represents a system with two identical warm standbys. The failure rates of units and the system are constant and independent while the repair times are arbitrarily distributed. The second system modeled consists of three repairable regions. The system operates normally if all three regions are operating, otherwise it operates at a derated level unless all three regions fail. The failure rates and repair times of the regions are constant and independent. The first model is analyzed using the supplemental variable technique while the second model is analyzed using the regenerative point technique in the Markov renewal process. Various expressions including system availability, system reliability and mean time to system failure are obtained.     

A note on minimal repair

The cumulative function for the number of failures for a unit which is subjected to minimal repair after each failure is a nonstationary Poisson process. In this note, a simple method for obtaining this result is presented. A conditional probability approach is used for the derivation     

Availability and reliability of system with dependent componentsand time-varying failure and repair rates

System reliability depends not only on the reliabilities of components in the system but also on their interactions. Generally, in a system, not only s-independent failures but also s-dependent failures among components can occur; thus there are many studies where the s-dependencies among components are taken into account in system reliability and availability analysis, but in which the failure and repair rates were assumed constant. Whereas, from a practical viewpoint, the constant failure rate assumption for components has been, and is repeatedly challenged by knowledgeable reliability practitioners. Therefore, there are other studies which handled the problem of time-varying failure rates, among which all concerned repairable systems did not involve s-dependent failures. In most cases, however, to combine s-dependent failures and time-varying failure and repair rates in system reliability and availability analysis is the most appropriate for real systems. But it is very difficult to obtain the analytic solution and, in most cases, the closed-form solution for system reliability and availability does not exist, so that numerical or simulation methods must be used. This paper studies one kind of system that endures environmental shocks, and where one or more components can fail simultaneously due to a cumulative shock-damage process. An approach for reliability and availability analysis of such kinds of repairable systems is presented, where failure and repair rates of components can be varied with time. One type of special vehicle with such mechanical systems illustrates system reliability and availability solutions     

Existence and Construction of Capacity-Achieving Network Codes for Distributed Storage

In a distributed storage system based on erasure coding, when a storage node fails, repairing the erasure code incurs some network traffic. Previous work has characterized the fundamental tradeoff between storage efficiency and repair network bandwidth. This was done via a cut-based analysis on a graph that models the evolution of information flow in the storage system subject to arbitrary sequences of node failures/repairs. This paper presents techniques for constructing network codes that achieve the optimal tradeoff between storage efficiency and repair network bandwidth. The challenge here is that network coding is applied over an unbounded graph with an unbounded number of receivers. It is shown in this paper that optimal codes can be constructed over a finite field whose size depends only on the maximum number of nodes at any instant, but independent of how many failures/repairs can happen. Key to the code construction is a "path-weaving" procedure that leads to inductive existence proof and code construction.     

基于MSP430飞机空调车人机界面的设计与研究

针对军用飞机空调车人机界面耐温性差与性价比低的问题,以及对数据的实时采集和存储要求比较高的特点,文章以超低功耗单片机MSP430F147为核心,研究开发了具有海量存储与超宽温功能的人机界面系统,使得技术人员维修设备时间缩短了40%左右,大大降低了维修成本。利用直接嵌入仿真技术,通过对软、硬件的在线仿真调试,采用军用环境可靠性设计,实现了该人机界面在极限军用环境中连续无故障运行超过一年,普通技术人员维修复杂设备的目标。这一设计已成功应用于飞机空调车系统中,且人机界面具有稳定性强、实时性好、性价比高等优点。     

Cost optimal, parallel reliability systems with fixed repair sanction

Optimisation methods under varied criteria for different parameters in stochastic reliability systems are being increasingly developed and have been reported in recent literature. The large interest evinced in this fascinating area is primarily due to its applicational value and operational role in the decision making process. Recently a parallel system has been considered and the optimal number of units discussed, as well as optimal replacement times for the system based on acquisition and replacement costs.In this paper we consider an improved version of the model formulation, by bringing in additionally the maintenance and per unit repair time costs, and develop a procedure to obtain the optimal number of components in the system with the condition that the system is allowed to undergo a prefixed maximum number of repairs, after which the system is to be replaced.The applicational use of the results is illustrated through numerical work, specialising to some known laws governing the system parameters and corresponding to different fixed number of repair sanctions.     

Stochastic behaviour of a two-unit cold standby redundant electronic equipment under human failure

This paper presents a mathematical model to evaluate availability and M.T.T.F. of a two-unit cold standby system with three possible states of each unit; viz. good, partially failed and failed, incorporating the concept of human failure. The model has been developed for exponential failures and general repairs. Single service facility is available in each state during the operational stage of the electronic equipment. Laplace transforms of the various state probabilities have been obtained. Steady-state probabilities, steady-state availability and mean time to failure have been derived.     

Expert system development methodology and the transition from prototyping to operations: Fiesta, a case study

A major barrier in taking expert systems from prototype to operational status involves instilling end user confidence in the operational system. End users want assurances that their systems have been thoroughly tested, meet all their specifications and requirements, and are built based on designs which are reliable and maintainable. For most software systems, the waterfall life cycle model can provide those assurances. However, this model is inappropriate for expert system development, where an iterative refinement approach is commonly employed. This paper will look at different life cycle models and explore the advantages and disadvantages of each when applied to expert system development. The Fault Isolation Expert System for TDRSS Applications (FIESTA) is presented as a case study example of development of an expert system. FIESTA is planned for use in the Network Control Center (NCC) at Goddard Space Flight Center in Greenbelt, Maryland. The end user confidence necessary for operational use of this system is accentuated by the fact that it will handle real-time data in a secure environment, allowing little tolerance for errors. The paper discusses how FIESTA is dealing with transition problems as it moves from an off-line standalone prototype to an on-line real-time system.     

Optimal replacement policy (N, n) for a parallel system with common cause failure and geometric repair time

The purpose of this article is to present an improved replacement model for a parallel system of N identical units, by bringing in common cause failure (CCF), maintenance cost and repair cost per unit time additionally, and to develop a procedure to obtain the optimal redundant units N^* and optimal number of repairs n^* with the conditions that the system is allowed to undergo at most a prefixed number of repairs before to be replaced and the successive reapir times after failures constitute a non-decreasing Geometric process. Several conditions for the existence of the optimal N^* and n^* is stated and the results are illustrated by a numerical example.