Imperfect Coverage Research Articles

Comparison of Availability Between Two Systems with Warm Standby Units and Different Imperfect Coverage

This paper deals with two availability systems with warm standby units and different imperfect coverage. The time-to-failure and the time-to-repair of the active and standby units are assumed to be exponentially and generally distributed, respectively. We assume that the coverage factor of the active-unit failure is different from that of the standby-unit failure. We provide a recursive method, using the supplementary variable technique and treating the supplementary variable as the remaining repair time, to develop the steady-state availability, (or Av), for two systems. An efficient Maple computer program is utilized to calculate the availability of two systems. Comparisons are performed for five different repair time distributions such as exponential, gamma, uniform, deterministic, and normal.

Cyanide-Coordinated Fe(III) Meso-Tetra(4-carboxyphenyl) Porphyrin as a Possible Electrocatalytic Material for Selective H2S Oxidation

Electrodeposited Fe(III) meso-Tetra(4-carboxyphenyl) porphyrin with cyanide ligands coordinated to the central iron ion was investigated as a possible material capable of selectively oxidizing H2S in the presence of CO and NO in physiologically relevant solutions. Preliminary results suggest this to be an attractive material as it oxidized H2S at low potentials (0.15 V vs. Ag/AgCl) while exhibiting only mild activity toward both CO and NO. While this is a step in the right direction, the selectivity is currently inadequate for practical applications. Additionally, the electrocatalyst also exhibited a limited response at elevated sulfide concentrations, presumably due to the deposition of S0 on the electrode surface. However, a cathodic-post run treatment was able to regenerate the electrochemical activity of the porphyrin-modified electrode surface by stripping any S0 that deposited on the electrode surface during the H2S oxidation. The imperfect porphyrin coverage of the electrode surface and incomplete coordination of cyanide are suspected as the primary reasons for these shortcomings. Research is ongoing to remedy these limitations.

Combinatorial Algorithm for Reliability Analysis of Multistate Systems With Propagated Failures and Failure Isolation Effect

This paper considers the reliability analysis of multistate systems (MSSs) subject to propagated failure with global effect (PFGE) and failure isolation effect. The PFGE can be caused by an imperfect fault coverage despite the presence of fault-tolerant mechanism or by a destructive effect of failures that originate from some system components on other components. The failure isolation effect is caused by functional dependence among system components, where the failure of some component can prevent the propagation of failures that originate from other components within the same system. Existing approaches for simultaneously addressing PFGE and failure isolation are limited to binary-state systems in which the system and its components exhibit two and only two states: operation or failure. In practice, however, many systems are MSS in which the system and/or its components may exhibit multiple performance levels corresponding to different states ranging from perfect operation to complete failure. In this paper, a separable and combinatorial methodology is proposed for evaluating the reliability of MSS subject to both PFGE and the failure isolation effect. The proposed method has no limitation on the type of time-to-failure distributions for the system components and is applicable to MSS with any arbitrary system structure. Application and advantages of the proposed method are illustrated through a detailed analysis of an example of a multistate memory system.

Recognition Properties of Cucurbit[7]uril Self-Assembled Monolayers Studied with Force Spectroscopy

Specific (host-guest) and unspecific (substrate-guest) interactions between self-assembled monolayers of cucurbit[7]uril (CB[7]) on gold (Au) substrates and neutral adamantyl (Ad) guests were resolved by studying these interactions at the single molecule level using dynamic force spectroscopy. The dissociation rate constants of the Ad-Au and the Ad-CB[7] interactions were 0.3 s(-1) and 0.03 s(-1), respectively, indicating that the specific binding is more stable. The probability of observing a specific interaction (40 ± 9%) is similar to the reported surface coverages of CB[7] monolayers on Au substrates. The higher strength and stability of the Ad-CB[7] interactions explains why, although presenting an imperfect coverage, CB[n] monolayers can be used successfully as a platform for surface immobilization.

Performability Analysis of Multistate Computing Systems Using Multivalued Decision Diagrams

A distinct characteristic of multistate systems (MSS) is that the systems and/or their components may exhibit multiple performance levels (or states) varying from perfect operation to complete failure. MSS can model behaviors such as shared loads, performance degradation, imperfect fault coverage, standby redundancy, limited repair resources, and limited link capacities. The nonbinary state property of MSS and their components as well as dependencies existing among different states of the same component make the analysis of MSS difficult. This paper proposes efficient algorithms for analyzing MSS using multivalued decision diagrams (MDD). Various reliability, availability, and performability measures based on state probabilities or failure frequencies are considered. The application and advantages of the proposed algorithms are demonstrated through two examples. Furthermore, experimental results on a set of benchmark examples are presented to illustrate the advantages of the proposed MDD-based method for the performability analysis of MSS, as compared to the existing methods.

Identifying candidate causal variants via trans‐population fine‐mapping

Genome-wide association studies have discovered and confirmed a large number of loci that are implicated with disease susceptibility and severity. Polymorphisms that emerged from these studies are mostly indirectly associated to the phenotype, and the natural progression is to identify the causal variants that are functionally responsible for these association signals. Long stretches of high linkage disequilibrium (LD) benefitted the initial discovery phase in a genome-wide scan, allowing commercial genotyping products with imperfect coverage to detect genomic regions genuinely associated with the phenotype. However, regions of high LD confound the fine-mapping phase, as markers that are perfectly correlated to the causal variants display similar evidence of phenotypic association, hampering the process of differentiating the functional polymorphisms from neighboring surrogates. Here, we explore the potential of integrating information across different populations for narrowing the candidate region that a causal variant resides in, and compare the efficacy of this process of trans-population fine-mapping with the extent of variation in patterns of LD between the populations. In addition, we explore two different strategies for pooling data across multiple populations for the purpose of prioritizing the rankings of the causal variants. Our results clearly establish the benefits of trans-population analysis in reducing the number of possible candidates for the causal variants, particularly in genomic regions displaying strong evidence of inter-population LD variation. Directly integrating the statistical evidence by summing the test statistics outperforms the standard meta-analytic procedure. These findings have direct relevance to the design and analysis of ongoing fine-mapping studies.

Delineating the rupture planes of an earthquake doublet using Source-Scanning Algorithm: application to the 2005 March 3 Ilan Doublet, northeast Taiwan

SUMMARY Correct identification of the fault plane(s) associated with an earthquake doublet is a very challenging problem because the pair of events often occurs in close space and time with almost the same magnitude. Most long-period waveforms of an earthquake doublet are severely tangled and thus unsuitable for conventional waveform inversion methods. In this study, we try to resolve this issue by utilizing the recently developed Source-Scanning Algorithm (SSA). The SSA systematically searches the model space for seismic sources whose times and locations are most compatible with the observed arrivals of large amplitudes on seismograms. The identification of a seismic source is based on the brightness function, which is defined as the summation of the normalized waveform amplitudes at the predicted arrival times at all stations. By illuminating the spatiotemporal distribution of asperities during an earthquake's source process, we are able to constrain the orientation of the rupture propagation that, in turn, leads to the identification of the fault plane. A series of synthetic experiments are performed to test SSA's resolution under various scenarios including different directions of rupture propagation, imperfect station coverage and short origin time difference between the two events of a doublet. Because only short-period records are needed in the analysis, the proposed method is best suited for an earthquake doublet with a short time gap between the two events. Using the 2005 Ilan doublet (the origin time difference is only 70 s) that occurred in northeast Taiwan as an example, we show that the trace of the brightest spots moves towards the west and infer the E–W-striking plane to be the actual fault plane.

Simulation inferences for an availability system with general repair distribution and imperfect fault coverage

We study the statistical inferences of an availability system with imperfect coverage. The system consists of two active components and one warm standby. The time-to-failure and time-to-repair of the components are assumed to follow an exponential and a general distribution respectively. The coverage factors for an active-component failure and for a standby-component failure are assumed to be the same. We construct a consistent and asymptotically normal estimator of availability for such repairable system. Based on this estimator, interval estimation and testing hypothesis are performed. To implement the simulation inference for the system availability, we adopt two repair-time distributions, namely, lognormal and Weibull and three types of Weibull distributions characterized by their shape parameters are considered. Finally, all simulation results are displayed in appropriate tables and curves for highlighting the performance of the statistical inference procedures.

Probability of Loss Assessment of Critical $k$-Out-of-$n$:G Systems Having a Mission Abort Policy

The survival of an aircraft is dependent on continuous operation of its flight critical systems until it has safely landed. Such a system may utilize a mission abort policy, requiring termination of typical mission operations, and entering a recovery phase in the event of certain failures. If, subsequent to such a mission abort fault, the system is able to safely complete the recovery phase, it will have survived, but may have failed to complete its mission objectives. This type of system is of interest when 1) the system's survival is dependent on its unattended operational capability; 2) there exist circumstances under which system survival has a greater priority than completing a mission of defined length; or 3) a mission abort policy can be established which results in entering a recovery phase requiring less time than completing the mission. Computing the probability of mission survival for systems using a mission abort policy is not a classical reliability problem, generally defined as the probability of performing its intended mission for a specified period of time, because the assessment does not, in general, involve a mission of a specific period of time. The period of time of interest will be either the specified mission time, or the sum of the time prior to a mission abort plus the time required to complete a recovery phase. This paper addresses determining the probability of survival for redundant systems utilizing a mission abort policy. Systems having both perfect, and imperfect fault coverage are addressed; and numerical, and exact algebraic results are provided for i.i.d. <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> -out-of- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> :G systems.

Dispersibility in organic solvents of nanosized silica particles used in semiconductor package substrates

This study examined the influence of different types of silane coupling agents on the dispersion of nanosilica particles with diameters of ca. 10 nm and 25 nm in two organic solvents, methylisobutylketone (MIBK) and cyclohexanone (CXN). The nanosilicas were prepared by the sol–gel process, and the following coupling agents were used: 3-glycidylpropyltrimethoxysilane (EpoSi), N-phenyl-3-aminopropyltrimethoxysilane (AmiSi), 3-mercaptopropyltrimethoxysilane (MerSi), and alkyltrimethoxysilane (AlkSi). While the 25-nm silica particles displayed dispersibility similar to that of submicron silica particles (ca. 150 nm and 250 nm), the dispersibility of the 10-nm silica particles proved to be dependent on the combinations of the agents and solvents; for example, EpoSi and MerSi were found to be suitable agents for MIBK and CXN, respectively. The slurries were also subjected to heating at 40 °C for 24 h, which resulted in a considerably higher stability of the dispersion state when CXN was employed as the solvent, regardless of which silica coupling agents were used. FTIR measurement revealed the presence of unreacted free Si–OH at 3740 cm −1 for some nanosilicas. Combustion of the organic moieties introduced onto the silica surfaces also proved that the amount of organic groups present in a unit surface area of the nanosilicas were only 1/3 or 1/2 of that found on submicron silicas. From these observations, the low dispersibility of the nanosilicas was inferred to be due to the imperfect coverage of the silanol groups by the silane coupling agents.

An Availability System with General Repair Distribution: Statistical Inference

This article we study the statistical inferences of an availability system with imperfect coverage. The time-to-failure and time-to-repair of the active and standby components are assumed to be exponential and general distribution, respectively. Assume that the coverage factor is the same for an active-component failure as that for a standby-component failure. Firstly, we propose a consistent and asymptotically normal (CAN) estimator of availability for such repairable system. Based on the CAN estimator of the system availability, interval estimation and testing (hypothesis) are performed. To implement the simulation inference for the system availability, we adopt two repair-time distributions, such as lognormal and Weibull distribution, in which three types of Weibull distribution are considered according to the shape parameter β. The component holds the decreasing repair rate (DRR), constant repair rate (CRR), and increasing repair rate (IRR) if β < 1, β = 1, and β > 1, respectively. Finally, all simulation results are displayed by appropriate tables and curves for understanding performance of the statistical inference procedures presented in this article.

A repairable system with imperfect coverage and reboot: Bayesian and asymptotic estimation

System characteristics of a two-unit repairable system are studied from a Bayesian viewpoint with different types of priors assumed for unknown parameters, in which the coverage factor for an operating unit failure is possibly considered. Time to failure and time to repair of the operating units are assumed to follow exponential distributions. In addition, the recovery time and reboot time of the failed units also follow exponential distributions. When time to failure, time to repair, recovery time and reboot time are with uncertain parameters, a Bayesian approach is adopted to evaluate system characteristics. Monte Carlo simulation is used to derive the posterior distribution for the mean time to system failure and the steady-state availability. Some numerical experiments are performed to illustrate the results derived in this paper.

A Logarithmic Binary Decision Diagram-Based Method for Multistate System Analysis

Multistate systems (MSS) are systems in which both the systems, and/or their components may exhibit multiple performance levels or states. MSS can model complex behaviors such as shared loads, performance degradation, imperfect fault coverage, standby redundancy, and limited repair resources. The non-binary state property of MSS, and their components makes the analysis of MSS challenging. In this paper, we propose efficient logarithmically-encoded binary decision diagram (LBDD)-based methods for analysing MSS. The application and advantages of the proposed LBDD-based approaches, as compared to the existing binary decision diagram-based approaches, are demonstrated through the analyses of practical MSS examples, and a set of benchmark examples.

Achievable Limits on the Reliability of $k$-out-of-$n$:G Systems Subject to Imperfect Fault Coverage

Systems which must be designed to achieve very low probabilities of failure often use redundancy to meet these requirements. However, redundant <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> -out-of- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> :G systems which are subject to imperfect fault coverage have an optimum level of redundancy, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">opt</sub> . That is to say, additional redundancy in excess of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nopt</i> will result in an increase, not a decrease, in the probability of system failure. This characteristic severely limits the level of redundancy considered in the design of highly reliable systems to modest values of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> . Correct modeling of imperfect coverage is critical to the design of highly reliable systems. Two distinctly different <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> -out-of- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> :G imperfect fault coverage reliability models are discussed in this paper: Element Level Coverage (ELC), and Fault Level Coverage (FLC). ELC is the appropriate model when each component can be assigned a given coverage level, while FLC is the appropriate model for systems using voting as the primary means of selection among redundant components. It is shown, over a wide range of realistic coverage values and relatively high component reliabilities, that the optimal redundancy level for ELC systems is 2 and 4 for FLC systems. It is also shown that the optimal probability of failure for FLC systems exceeds that of ELC systems by several orders of magnitude. New functions for computing the mean time to failure for i.i.d. <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> -out-of- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> :G ELC, and FLC systems are also presented.

Efficient Reliability Assessment of Redundant Systems Subject to Imperfect Fault Coverage Using Binary Decision Diagrams

Systems requiring very high levels of reliability, such as aircraft controls or spacecraft, often use redundancy to achieve their requirements. This paper provides highly efficient techniques for computing the reliability of redundant systems involving simple k-out-of-n arrangements, and those involving complex structures which may include imbedded k-out-of-n structures. Techniques for modeling systems subject to imperfect fault coverage must be appropriate to the redundancy management architecture utilized by the system. Systems for which coverage can be associated with each of the redundant components, perhaps taking advantage of the component's built-in test capability, are modeled with what we term element level coverage (ELC); while systems which utilize majority voting for the selection from among redundant components are modeled with fault level coverage (FLC). In FLC, systems coverage is a function of the fault sequence, i.e., coverage will be greater for the initial faults which can utilize voting for redundant component selection, but will have a lower coverage value when the system must select from among the last two operational components. Occasionally, FLC systems can be adequately modeled using a simplified version of FLC in which it can be assumed that the initial fault coverage values are unity. This model is called one-on-level coverage (OLC). The FLC algorithms provided in this paper are of particular importance for the correct modeling of systems which utilize voting to select from among their redundant elements. While combinatorial, and recursive techniques for modeling ELC, FLC, and OLC have been previously reported, this paper presents new table-based algorithms, and binary decision diagram-based algorithms for these models which have superior computational complexity. The algorithms presented here provide the ability to analyse large, and complex systems very efficiently, in fact with a computational complexity comparable to the best available techniques for systems with perfect fault coverage.

Random walk search procedures for reliability optimization of systems with fault tolerance

AbstractIn this paper we develop two efficient discrete stochastic search methods based on random walk procedure for maximizing system reliability subjected to imperfect fault coverage where uncovered component failures cause immediate system failure, even in the presence of adequate redundancy. The first search method uses a sequential sampling procedure with fixed boundaries at each iteration. We show that this search process satisfies local balance equations and its equilibrium distribution gives most weight to the optimal solution. We also show that the solution that has been visited most often in the first m iterations converges almost surely to the optimal solution. The second search method uses a sequential sampling procedure with increasing boundaries at each iteration. We show that if the increase occurs slower than a certain rate, this search process will converge to the optimal set with probability 1. We consider the system where reliability cannot be evaluated exactly but must be estimated through Monte Carlo simulation. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Bayesian Analysis for a Redundant Repairable System with Imperfect Coverage

System characteristics of a redundant repairable system with two primary units and one standby are studied from a Bayesian viewpoint with different types of priors assumed for unknown parameters, in which the coverage factor is the same for an operating unit failure as that for a standby unit failure. Times to failure and times to repair of the operating and standby units are assumed to follow exponential distributions. When times to failure and times to repair with uncertain parameters, a Bayesian approach is adopted to evaluate system characteristics. Monte Carlo simulation is used to derive the posterior distribution for the mean time to system failure and the steady-state availability. Some numerical experiments are performed to illustrate the results derived in this paper.

Error repair for broadcast transmissions in DVB‐H systems

AbstractPeople travelling in public transportation vehicles are foreseen to be a relevant use case of future Digital Video Broadcast—Handhelds (DVB‐H) systems. However, it is also one of the critical user cases, since, besides the vehicle penetration loss, the received signal quality is strongly affected by the high mobility of the users, which is accompanied by temporary shadowing and fast fading. On the other hand, mobility can be also beneficial thanks to the bursty character of DVB‐H transmissions, as additional repair data can be sent in the time intervals between original service bursts in order to hide the coverage discontinuities from the users' perception. In this way, users which miss the original transmission have more chances to receive the information correctly. By managing the amount of repair data the network operator can trade system capacity and network latency for improved vehicular user satisfaction in a DVB‐H network with imperfect coverage of the service area (as perceived by the vehicular users). To illustrate the potential of this approach, we evaluate the mentioned trade‐off in a DVB‐H system dimensioned for pedestrian outdoor users, for both streaming and filecasting services. To enable an easy and efficient implementation of the repair mechanisms, we adopt application layer forward error correction (FEC), as it can provide a multi‐burst protection of the transmission. Copyright © 2008 John Wiley &amp; Sons, Ltd.

An Efficient Binary-Decision-Diagram-Based Approach for Network Reliability and Sensitivity Analysis

Reliability and sensitivity analysis is a key component in the design, tuning, and maintenance of network systems. Tremendous research efforts have been expended in this area, but two practical issues, namely, imperfect coverage (IPC) and common-cause failures (CCF), have generally been missed or have not been fully considered in existing methods. In this paper, an efficient approach for fully incorporating both IPC and CCF into network reliability and sensitivity analysis is proposed. The challenges are to allow multiple failure modes introduced by IPC and to cope with multiple dependent faults caused by CCF simultaneously in the analysis. Our methodology for addressing the aforementioned challenges is to separate the consideration of both IPC and CCF from the combinatorics of the solution, which is based on reduced ordered binary decision diagrams (ROBDD). Due to the nature of the ROBDD and the separation of IPC and CCF from the solution combinatorics, our approach has a low computational complexity and is easy to implement. A sample network system is analyzed to illustrate the basics and advantages of our approach. A software tool that we developed for fault-tolerant network reliability and sensitivity analysis is also presented.

On a repairable system with detection, imperfect coverage and reboot: Bayesian approach

System characteristics of a repairable system are studied from a Bayesian viewpoint with different types of priors assumed for unknown parameters, in which the system consists of one active component and one standby component. The detection of standby, the coverage factor and reboot delay of failed components are possibly considered. Time to failure of the components is assumed to follow exponential distribution. Time to repair and time to reboot of the failed components also follow exponential distributions. When time to failure, time to repair and time to reboot have uncertain parameters, a Bayesian approach is adopted to evaluate system characteristics. Monte Carlo simulation is used to derive the posterior distribution for the mean time to system failure and the steady-state availability. Some numerical experiments are performed to illustrate the results derived in this paper.