Justification for the next generation of maintenance modelling techniques

Variant forms of reliability centred maintenance (RCM) have been the maintenance improving tools of choice for the last 20 years. In this case study paper, justification is made for implementing, and a path is laid out to implement, Operations Research in the form of statistical modelling as the next step forward after RCM. The lack of failure data issue has been addressed by using elicitation protocols to provide component lifetime distributions. Graphical analysis and Crowe/AMSAA (Army Materials Systems Analysis Activity) methodologies are developed as a basis for justifying expenditures on maintenance improvement initiatives. A review of historical empirical—inferential techniques dating back to WWII is presented as well as discussions of the current applicability of the same. The deregulation of the Electrical Generation Industry has produced severe restraints on the publishing of failure data. Owing to a fortuitous set of circumstances, a limited amount of data became releasable, which allowed the promise of the method to be demonstrated.     

Model-based assessment of military aircraft engine maintenance systems

The paper describes the development and use of a modular system dynamics model for analysing the dynamics and for assessing the long-term performance of military aircraft engine maintenance systems. Life-cycle engine maintenance is considered as a dynamic system whose behaviour is influenced by parameters specific to the engine's reliability and maintenance characteristics, to the operational requirements, and to the maintenance infrastructure. We show how the model can be used for supporting the aircraft procurement decision through the assessment of its engine's maintenance system, as well as for understanding and determining maintenance system behaviours, towards which operational adjustments of the maintenance infrastructure can be made in a more informed way.     

The next generation CIP as a conservative semi-Lagrangian solver for solid,liquid and gas

We present a review of the CIP method, which is a kind of semi-Lagrangian scheme and has been extended to treat incompressible flow in the framework of compressible fluid. Since it uses primitive Euler representation, it is suitable for multi-phase analysis. The recent version of this method guarantees the exact mass conservation even in the framework of semi-Lagrangian scheme. Comprehensive review is given for the strategy of the CIP method that has a compact support and subcell resolution including front capturing algorithm with functional transformation.     

On the complexity of decomposing matrices arising in satellite communication

Decomposing a square matrix into a weighted sum of permutation matrices, such that the sum of the weights becomes minimal, is NP-hard. This result justifies the heuristic approach to this problem proposed by several authors. An application of this problem arises from intercity communication via transmission satellites.     

Reliability analysis of complex communication systems

This paper deals with a first-come, first-served (FCFS) queueing model to analyze the asymptotic behavior of a heterogeneous finite-source communication system with a single processor. Each source and the processor are assumed to operate in independent random environments, allowing the arrival and service processes to be Markov-modulated ones. Each message is characterized by its own exponentially distributed source and processing time with parameter, depending on the state of the corresponding environment, that is, the arrival and service rates are subject to random fluctuations. Assuming that the arrival rates of the messages are many times greater than their service rates (“fast” arrival), it is shown that the time to the first system failure converges in distribution, under appropriate norming, to an exponentially distributed random variable. Some simple examples are considered to illustrate the effectiveness of the method proposed by comparing the approximate results to the exact ones. Supported by the Hungarian National Foundation for Scientific Research (grant No. OTKA T14974/95). Proceedings of the Seminar on Stability Problems for Stochastic Models, Vologda, Russia, 1998, Part II.     

A heuristic for decomposing traffic matrices in TDMA satellite communication

A heuristic for decomposing traffic matrices in TDMA satellite communication. With the time-division multiple access (TDMA) technique in satellite communication the problem arises to decompose a givenn×n traffic matrix into a weighted sum of a small number of permutation matrices such that the sum of the weights becomes minimal. There are polynomial algorithms when the number of permutation matrices in a decomposition is allowed to be as large asn ². When the number of matrices is restricted ton, the problem is NP-hard. In this paper we propose a heuristic based on a scaling technique which for each number of allowed matrices in the range fromn ton ² allows to give a performance guarantee with respect to the total weight of the solution. As a subroutine we use new heuristic methods for decomposing a matrix of small integers into as few matrices as possible without exceeding the lower bound on the total weight. Computational results indicate that the method might also be practical.This work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung, Project S32/01.     

Chaos-based communication systems in non-ideal channels

Recently, many chaos-based communication systems have been proposed. They can present the many interesting properties of spread spectrum modulations. Besides, they can represent a low-cost increase in security. However, their major drawback is to have a Bit Error Rate (BER) general performance worse than their conventional counterparts. In this paper, we review some innovative techniques that can be used to make chaos-based communication systems attain lower levels of BER in non-ideal environments. In particular, we succinctly describe techniques to counter the effects of finite bandwidth, additive noise and delay in the communication channel. Although much research is necessary for chaos-based communication competing with conventional techniques, the presented results are auspicious.     

Solving a kind of boundary-value problem for ordinary differential equations using Fermi—The next generation CUDA computing architecture

The aim of this paper is to show that a special kind of boundary value problem for solving second-order ordinary differential equations can be efficiently solved on modern heterogeneous computer architectures based on CPU and GPU Fermi processors. Such a problem reduces to the problem of solving a large tridiagonal system of linear equations with an almost Toeplitz structure. The considered algorithm is based on the recently developed divide and conquer method for solving linear recurrence systems with constant coefficients.     

Frequency assignment in a SDMA satellite communication system with beam decentring feature

In satellite communication, Spatial Division Multiple Access (SDMA) has become one of the most promising techniques that can accommodate continuing increase in the number of users and traffic demands. The technology is based on radio resource sharing that separates communication channels in space. It relies on adaptive and dynamic beam-forming technology and well-designed algorithms for resource allocation among which frequency assignment is considered. This paper studies static Frequency Assignment Problem (FAP) in a satellite communication system involving a satellite and a number of users located in a service area. The objective is to maximize the number of users that the system can serve while maintaining the signal to interference plus noise ratio of each user under a predefined threshold. Traditionally, interference is treated as fixed (binary interferences or fixed minimal required separation between frequencies) . In this paper, the interference is cumulative and variable. To solve the problem, we work on both discrete and continuous optimizations. Integer linear programming formulations and greedy algorithms are proposed for solving the discrete frequency assignment problem. The solution is further improved by beam decentring algorithm which involves continuous adjustment of satellite beams and deals with non-linear change of interference.     

Robust synchronization of parametrized nonautonomous discrete systems with applications to communication systems

We study synchronization of a coupled discrete system consisting of a Master System and a Slave System. The Master System usually exhibits chaotic or complicated behavior and transmits a signal with a chaotic component to the Slave System. The Slave System then recovers the original signal and removes the chaotic component. To ensure secured communication, the Master and the Slave systems must synchronize independent of the variation of the systems parameters and initial conditions. Here we develop a general approach and obtain some general results for synchronization of such coupled systems naturally arising from discretization of well-know continuous systems, and we illustrate general results with two specific examples: the discretized Lorenz system and a discretized nonlinear oscillator. We also present some simulations using MatLab to illustrate our discussions.     

On the design of chaos-based secure communication systems

This paper discusses the topic of using chaotic models for constructing secure communication systems. It investigates three different case studies that use encryption/decryption functions with varying degrees of complexity and performance. The first case study explores synchronization of identical chaotic systems, which is considered the most crucial step when developing chaos-based secure communication systems. It proposes a fast mechanism for synchronizing the transmitter and the receiver that is based on the drive-response approach. The superiority and causality of this mechanism is demonstrated via contrasting its performance and practical implementation against that of the traditional method of Pecora and Carroll. The second case study explores the use of an improved cryptography method for improving the scrambling of the transmitted signals. The improvement is based on using both the transmitter states and parameters for performing the encryption. The security analysis of this method is analyzed, highlighting its advantages and limitation, via simulating intruder attacks to the communication channel. Finally, the third case study augments a parameter update law to the previous two designs such that the encryption method is more robust. It uses a decoupling technique for which the synchronization process is completely isolated from the parameter identification algorithm. The Lorenz system was used to exemplify all the suggested techniques, and the transmission of both analog and digital signals was explored, while investigating various techniques to optimize the performance of the proposed systems.     

Queueing models for the analysis of communication systems

Queueing models can be used to model and analyze the performance of various subsystems in telecommunication networks; for instance, to estimate the packet loss and packet delay in network routers. Since time is usually synchronized, discrete-time models come natural. We start this paper with a review of suitable discrete-time queueing models for communication systems. We pay special attention to two important characteristics of communication systems. First, traffic usually arrives in bursts, making the classic modeling of the arrival streams by Poisson processes inadequate and requiring the use of more advanced correlated arrival models. Second, different applications have different quality-of-service requirements (packet loss, packet delay, jitter, etc.). Consequently, the common first-come-first-served (FCFS) scheduling is not satisfactory and more elaborate scheduling disciplines are required. Both properties make common memoryless queueing models (M/M/1-type models) inadequate. After the review, we therefore concentrate on a discrete-time queueing analysis with two traffic classes, heterogeneous train arrivals and a priority scheduling discipline, as an example analysis where both time correlation and heterogeneity in the arrival process as well as non-FCFS scheduling are taken into account. Focus is on delay performance measures, such as the mean delay experienced by both types of packets and probability tails of these delays.     

A storage model for data communication systems

We consider a storage model where the input and demand are modulated by an underlying Markov chain. Such models arise in data communication systems. The input is a Markov-compound Poisson process and the demand is a Markov linear process. The demand is satisfied if physically possible. We study the properties of the demand and its inverse, which may be viewed as transformed time clocks. We show that the unsatisfied demand is related to the infimum of the net input and that, under suitable conditions, it is an additive functional of the input process. The study of the storage level is based on a detailed analysis of the busy period, using techniques based on infinitesimal generators. The transform of the busy period is the unique solution of a certain matrix-functional equation. Steady state results are also obtained; these are not obvious generalizations of the results for simple storage models. In particular, a generalization of the Pollaczek-Khinchin formula brings new insight.Research supported by Grant BD/645/90-RM from Junta Nacional de Investigação Cientifica e Tecnológica.     

Optimal level of diagnostics in distributed communication systems

A unit (multiplexor, switch, terminal, processor, etc.) at a remote site is inspected occasionally from a central site to see if it is functioning properly. Two stochastic models are proposed for monitoring remote units whose operational status can be described by an alternating 0–1 process. Optimal inspection schedules are derived based on the objective of maximizing the overall steady state system availability while limiting inspections per-unit-time costs and customer inconvenience.     

On wear pattern generation in elastic systems

In systems with moving contacts spatially periodic wear patterns may appear on the contact partners surfaces. There is agreement that the patterns are related to structural resonances. Using a simple model with a moving point contact and an idealized wear model the present work reviews some of the present understanding of wear-pattern generation. Then the effect of randomly specified relative velocities on the wavelength selection process is investigated. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)