A simulation approach for computing systems reliability

The basis of the concept of reliability is that a given component has a certain stress-resisting capacity; if the stress induced by the operating conditions exceeds this capacity, failure results. Most of the published results in this area are based upon analytical modelling of stress and strength, using various probability distributions, and then trying to find an exact expression for system reliability, which can be very difficult to obtain sometimes. The approach used in this paper is very simple and uses simulation techniques to repeatedly generate stress and strength of a system by the computer, using a random number generator and methods such as the inverse transformation technique. The advantage of this approach is that it can be used for any stress-strength distribution functions. Finally, numerical results obtained from using this approach are compared with results obtained using the analytical methods for various strength-stress distribution functions, such as exponential, normal, log normal, gamma and Weibull. Results show the viability of the simulation approach.     

An unified numerical approach for computing the outage probabilityfor mobile radio systems

A general finite-range integral for the probability of outage in mobile radio systems is derived. The method handles noninteger Nakagami-fading indices, unequal Rice factors, unequal shadowing spreads, and unequal transmitted powers as well as all the common fading distributions (Rayleigh, Rice, Nakagami-m, Nakagami-q, lognormal-Rice, Suzuki, and lognormal-Nakagami-m). The integral expression can also be approximated by a Gauss-Chebychev quadrature (GCQ) formula requiring knowledge of the moment generating function at only a small number of points. An estimate of the remainder term is also derived. This numerical technique allows computing the outage with arbitrary precision and it is extremely easy to program     

An Eulerian PDE approach for computing tissue thickness

We outline an Eulerian framework for computing the thickness of tissues between two simply connected boundaries that does not require landmark points or parameterizations of either boundary. Thickness is defined as the length of correspondence trajectories, which run from one tissue boundary to the other, and which follow a smooth vector field constructed in the region between the boundaries. A pair of partial differential equations (PDEs) that are guided by this vector field are then solved over this region, and the sum of their solutions yields the thickness of the tissue region. Unlike other approaches, this approach does not require explicit construction of any correspondence trajectories. An efficient, stable, and computationally fast solution to these PDEs is found by careful selection of finite differences according to an upwinding condition. The behavior and performance of our method is demonstrated on two simulations and two magnetic resonance imaging data sets in two and three dimensions. These experiments reveal very good performance and show strong potential for application in tissue thickness visualization and quantification.     

An approach for computing tight numerical bounds on renewalfunctions

This method computes tight lower and upper bounds for the renewal function. It is based on Riemann-Stieltjes integration, and provides bounds for solving certain renewal equations used in the study of availability. An error analysis is given for the numerical bounds when inter-renewal time distributions are sufficiently smooth. Three examples are explored that demonstrate the accuracy of these computed numerical bounds     

An efficient anycast scheme for discovering k services in cluster‐based mobile ad hoc networks

Anycast is a one‐to‐any bidirectional transmission scheme that can be used to provide various service‐oriented applications. However, traditional anycast cannot support some distributed applications that need to locate multiple servers such as network time protocols and threshold cryptography. Although there are some extended anycast schemes that can support the discovery of k services, they may cause high control overhead, low satisfaction ratio, or high searching latency when there are more service requesters and providers. The main reasons for these shortcomings are unknown service information and limited hierarchical structure. This work proposes an efficient anycast scheme, called AnyKast, for discovering k services in cluster‐based mobile ad hoc networks. In the AnyKast scheme, an anycast tree based on the clustering and virtual backbone is established to reduce unnecessary message transmission. In the anycast tree, anycast clusterheads disseminate their information to certain specific nodes that take the service information and perform the service selection task instead of blindly searching. Furthermore, to decrease the cost of service information collection and maintenance, scope flooding is used to limit the information transmission, and information piggybacking and periodical inquiry mechanism are used to increase the service information accuracy. The experimental results demonstrate that our scheme can effectively discover services, reduce request and reply message control overhead, and lower the searching latency. Copyright © 2010 John Wiley & Sons, Ltd.     

Data hiding approach for efficient image indexing

A data hiding approach to embed the indexing keys inside the JPEG compressed images for their retrieval, inspired from the spirit of digital watermarking, is proposed. At the stage of database population, the existing approach is to compress all images by standard JPEG in order to save their storage space, but this ignores the compression of their indexing keys under the notion that the size of each indexing key is negligible. When a large image database is established, however, this part of storage space becomes non-trivial. By hiding the indexing keys inside the JPEG compressed codes, significant advantages are gained in that the indexing keys can be compressed naturally by JPEG without any additional cost. Experiments on hiding an indexing key of 256 bytes illustrate that: no noticeable distortion is introduced in comparison with non-watermarked images; and their storage space is reduced into the region of 7-27% of their original size.     

Personal autonomic computing reflex reactions and self-healing

The overall goal of this research is to improve the self-awareness and environment-awareness aspect of personal autonomic computing (PAC) to facilitate self-managing capabilities such as self-healing. Personal computing offers unique challenges for self-management due to its multiequipment, multisituation, and multiuser nature. The aim is to develop a support architecture for multiplatform working, based on autonomic computing concepts and techniques. Of particular interest is collaboration among personal systems to take a shared responsibility for self-awareness and environment awareness. Concepts mirroring human mechanisms, such as reflex reactions and the use of vital signs to assess operational health, are used in designing and implementing the PAC architecture. As proof of concept, this was implemented as a self-healing tool utilizing a pulse monitor and a vital signs health monitor within the autonomic manager. This type of functionality opens new opportunities to provide self-configuring, self-optimizing, and self-protecting, as well as self-healing autonomic capabilities to personal computing.     

Adaptive middleware for autonomic systems

The increasingly dynamic nature of resource discovery and binding in modern large-scale distributed and mobile systems poses significant challenges for existing middleware platforms. Future platforms must provide strong support for adaptive behaviour in order both to maintain and optimise services in the face of changing context. We use a survey of existing middleware systems to develop some core themes that characterise and constrain the ability of these approaches to support the development of adaptive and autonomic systems, and draw some possible trends for developing future platforms more appropriate to these domains.     

An evaluation methodology for multielement emergency medical services systems

The provision of efficient public safety services continues to be one of the high priority efforts to improve the quality of life in our nation. These efforts are too often equipment oriented without realizing the system input of providing better services in terms of benefit to the public as opposed to providers of the services. These services are multielement containing many items which can be quantified and qualitative aspects which are difficult to assess. The methodology presented herein is designed to specifically evaluate such multi-element systems in a logical manner. Its use expanded to other than emergency medical service systems will permit weak areas to be identified, develop comparisions and rank system elements as to their individual contributions to "bottom line" results, and to help establish priorities for allocation of funding resources.     

An efficient optimistic message logging scheme for recoverable mobile computing systems

A number of checkpointing and message logging algorithms have been proposed to support fault tolerance of mobile computing systems. However, little attention has been paid to the optimistic message logging scheme. Optimistic logging has a lower failure-free operation cost compared to other logging schemes. It also has a lower failure recovery cost compared to the checkpointing schemes. This paper presents an efficient scheme to implement optimistic logging for the mobile computing environment. In the proposed scheme, the task of logging is assigned to the mobile support station so that volatile logging can be utilized. In addition, to reduce the message overhead, the mobile support station takes care of dependency tracking and the potential dependency between mobile hosts is inferred from the dependency between mobile support stations. The performance of the proposed scheme is evaluated by an extensive simulation study. The results show that the proposed scheme requires a small failure-free overhead and the cost of unnecessary rollback caused by the imprecise dependency is adjustable by properly selecting the logging frequency.     

An approximate method for computing performance-related reliability of gracefully degrading systems

Using existing methods, the computation of performance-related reliability (PRR) of large-scale gracefully degrading systems is very tedious and time consuming. In this paper, the behaviour of such systems is respectively modeled as two types of diffusion processes according to their reconfiguration coverage. If the coverage is 1 (i.e. the reconfiguration is always successful), it is modeled as a regular diffusion. If, on the other hand, the coverage is less than one, it is modeled as diffusion with killing. Kolmogorov backward equations for regular diffusion processes and for diffusions with killing are then applied to compute the PRR. The methods have been applied in several examples, and the results satisfactorily agree with the accurate results.     

An ESPRIT-based approach for initial ranging in OFDMA systems

In this letter, an initial ranging scheme for orthogonal frequency-division multiple-access systems is proposed by which users that intend to establish a communication link with the base station (BS) perform spreading in both the time and frequency domains and their synchronization parameters are estimated at the BS in closed-form using the ESPRIT algorithm. Compared to existing alternatives, the resulting scheme exhibits increased robustness against residual frequency offsets without involving computationally demanding peak search procedures.     

云环境数据服务的可信安全模型

针对云服务提供商的可信状态和云环境数据服务的安全需求,提出了云环境数据服务的可信重加密安全模型,即在云环境下的数据安全需要云服务提供商满足一定的可信程度,再结合有效的重加密算法才能得以保证。通过对重加密模型进行安全分析,并用密码算法对重加密模型进行验证,得到实现重加密算法的约束条件,同时提出可信评价模型,对云服务提供商的可信状态进行动态评价,为建立云环境数据服务的可信安全提供理论基础和实现依据。     

An enhanced scanning-based MPPT approach for DMPPT systems

In this paper, an improved maximum power point tracking (MPPT) approach being low parameter dependency, simple structure and limited search interval has been presented for distributed MPPT photovoltaic (PV) systems. Basically, this approach is based on scanning of power–voltage (P-V) characteristic curve of PV modules in a limited duty ratio interval which makes tracking operation simple, fast and efficiently available in both uniform irradiance and partial shading conditions (PSCs). By limiting the scanning interval of maximum and minimum values of duty ratio via some analyses related to P-V characteristic for PSCs, global MPPT (GMPPT) is achieved in an efficient way. So as to validate performance of the proposed approach, a single-ended primary inductance converter has been used in both simulation and experimental studies. PV simulator has been used as a PV source to obtain different module characteristics with different number of bypass diodes and PV power levels. Both simulation and experimental results clarify that improved MPPT approach realises GMPPT effectively. Due to the high performance results, this approach can be an alternative technique in module-integrated converters, smart modules and PV power optimisers in which single module is used.     

An efficient fault-tolerant approach for Mobile IP in wireless systems

This paper presents the fault tolerance of Mobile IP in wireless systems. Mobile IP can support wireless users with continuous network connections while changing locations. It is achieved by allocating a number of mobility agents (foreign agents and home agents) in the architecture of a wireless system. If a failure occurs in a mobility agent, the wireless users located in the coverage area of the faulty mobility agent will lose their network connections. To tolerate the failures of mobility agents, this paper proposes an efficient approach to maintaining the network connections of wireless users without being affected by the failures. Once detecting a failure in a mobility agent, failure-free mobility agents are dynamically selected to be organized as a backup set to take over the faulty mobility agent. Compared to the previous approaches, the proposed approach does not take any actions against failures during the failure-free period. Besides, the hardware redundancy technique is also not used in the proposed approach. The overhead of the proposed approach is analyzed using the M/G/c/c queuing model. The results show that the proposed approach can effectively resolve the fault-tolerant problem of Mobile IP in wireless systems.     

Optomechanical systems and quantum computing

A review of optomechanical systems is presented, including their basic components and performance data. Their role in experimental physics is discussed. Hybrid systems, whether proposed or implemented, are described in the context of building a full-scale quantum computer.     

Alternative majority-voting methods for real-time computing systems

Two techniques that provide a compromise between the high time overhead in maintaining synchronous voting and the difficulty of combining results in asynchronous voting are proposed. These techniques are specifically suited for real-time applications with a single-source/single-sink structure that need instantaneous error masking. They provide a compromise between a tightly synchronized system in which the synchronization overhead can be quite high, and an asynchronous system which lacks suitable algorithms for combining the output data. Both quorum-majority voting (QMV) and compare-majority voting (CMV) are most applicable to distributed real-time systems with single-source/single-sink tasks. All real-time systems eventually have to resolve their outputs into a single action at some stage. The development of the advanced information processing system (AIPS) and other similar systems serve to emphasize the importance of these techniques. Time bounds suggest that it is possible to reduce the overhead for quorum-majority voting to below that for synchronous voting. All the bounds assume that the computation phase is nonpreemptive and that there is no multitasking