In this issue, we once again bring 10 regular papers to our readers covering several interesting problems in performability engineering. To start with, there are two papers from Professors Hausken of Stavanger University, and Gregory Levitin of Israel. The first paper from them highlights the how resources should optimally be allocated by a defender between a passive defence by employing false targets and to use strikes against the attacker preventively. There has been an exponential growth of papers in the area of attack and defence strategies of systems considered important by the national interest and a variety of models have been presented in the recent past on the subject of system’s defence and attack strategies. In fact, the authors of the first paper together and individually have made tremendous contributions to the literature in this area. Therefore we felt, it is time that we invited them to contribute a review paper on the topic which can be used as a bench mark by the new researchers to the area in order to apprise them of what has been already published and in which direction the future research can be oriented fruitfully. We do hope the paper on Review of Systems Defence and Attack Models will be found very useful by the future researchers in the area. The authors of this paper have done a good job of review by including some 129 papers and categorizing them according to the models, system configuration and various strategies.
The third paper is again a review paper on the subject of signatures and D-spectra of coherent systems. The approach using signature and D-spectra helps in computing system reliability, particularly, the characteristics such as probabilistic resilience, component importance, the system failure probability as a function of component failure probability q, and approximations to system reliability, and so also the bounds on system reliability, if we know only the bounds on q. However, the computations become unwieldy with large number of components. So authors suggest the use of Monte Carlo methods with this approach.
The fourth paper of the issue presents failure risk analysis of a water distribution system which must be treated as continuous operating system and tries to identify the factors responsible for the failure of a water distribution system. The author claims the originality of the approach and may provide authorities in decision making support with regard to the performance of water distribution system.
In the fifth paper of this issue, the authors present a data processing method, fitted to the actual measurements using the Generalized Weibull-FR function. This, they claim, is likely to remove the external noise from the data before it is used as input to the PHM. Two case studies using real-world vibration monitoring data are used to demonstrate the approach. The approach is validated to be effective and likely to save the total average maintenance cost by increasing the average replacement interval and by making better use of remaining useful life.
The authors of the sixth paper present a model to predict the number of residual defects before testing phase of a software. They claim that this information would help the developers if this information is available a priori for optimal testing resource planning and quality assessment of the software being developed. In the early stages of software development life cycle (SDLC), software residual defects are affected by both product and process characteristics of a project. The paper uses software size metric and three metrics of requirement analysis phase for predicting the residual defects during testing or operational phase using fuzzy logic. The predictive capability is demonstrated using data of twenty software projects.
The seventh paper of the issue presents an inspection strategy for a multi-state system. The system being considered can be in any of three states: nominal operating state, degraded state or failure state. A system state is known only after inspection. A maintenance action is undertaken if at a predetermined instant an inspection reveals that the system is in degraded or failure state. The maintenance action is likely to restore the system to its nominal operating state with a certain probability. The paper considers a periodic inspection policy.
In the eighth paper, the author presents a methodology that helps computing reliability of a cold standby system with constituent components having time dependent hazard rates. To keep generality in mind, the author computes reliability of k-out-of-n cold standby system with components having Erlang life time distribution. The paper also considers the failure of decision switch. The use of Erlang distribution allows constant as well as increasing hazard rates for components besides permitting approximate life time distributions for components. This also allows one to have a closed form expression. The author demonstrates efficiency of his approach using some illustrative examples.
The ninth paper of the issue presents an analytical method for evaluating the throughput of a production line composed of two reconfigurable machines separated by a finite capacity buffer. Each machine is assumed to be composed of essential and non essential equipment. The failure of any essential equipment induces the shutdown of the entire machine. The failure of the non essential equipment implies the continuity of the machine service with a reduced level of functionality. Simulation and numerical experiments have been conducted to demonstrate the efficacy of the approach. The proposed model can be used as the building block for the performance evaluation of a long production line using aggregation techniques.
The tenth paper of this issue proposes a Software Reliability Qualification Model (SRQM) model to predict the reliability of a frozen software packages based on actual usage testing. Also the paper presents the cell based modeling of an application. Using this cell based modeling, the developer can point out which part of the application needs reliability improvement to meet the user requirements and also set criteria to qualify a product for the reliability requirements.
The next three papers are short communications which are also refereed and revised just as the regular papers are processed. In the first short communication, the authors propose a novel importance measure for multi-state systems with the consideration of external factors, such as external environment, common cause failures and human factors etc. The external environment can be either contamination of fluid systems, or corrosion, and electrical noise, dirt, dust, humidity and temperature. The proposed importance analysis allows quantifying the effect of the external factor on the component and system performance. Importance measures as such have been widely used for identifying the weakest component and supporting system reliability and performance improvement activities.
The next short communication, reliability analysis of tracking, telemetry and command (TT&C) and communication systems is discussed. Needless to say that its reliability is essential for the success of launching, motioning and landing of spacecraft. In this paper, a formal description of TT&C and communication task is given to facilitate the reliability modeling of such systems. A continuous-time Markov chain (CTMC) model is built for an idle task arc and a model for TT&C and communication tasks in consecutive flight cycles is proposed.
In the last short communication, the authors present the concept of an unconstrained, economically designed, theoretically appropriate monitor for the Poisson process. The authors have presented in this short communication is an alternative approach which is theoretically appropriate in that its assumptions do not deviate from the true distribution of what is being monitored.
We do hope that the papers published in this issue will invigorate further research in the respective areas.