Optimum allocation of computer spares for three-level stocks

This paper presents a model for determining the optimum stock in a three level depots system. The method for determining stock levels uses “incremental performance per price algorithm” (IPPA) and the “new Lawler-Bell algorithm” (NLB). The practical calculation is carried out on a computer, the results being presented in the form of a list of necessary spare units from the first to the third level.     

Generic qualification of semi-custom IC product families

The problem of verifying the integrity, and quantifying the reliability of semi-custom IC product families is addressed, and one possible approach to efficient “generic qualification” is outlined. The approach used relies on qualification via a set of chips specifically designed for this purpose, rather than via one or more “product” chips. Key features integrated into these test vehicles are that the entire “cell library” is represented, as well as that the “worst case” features are maximized.     

RAM analysis of a navigational system by Markov technique: A case study

An analysis is made for a typical repairable Dual-VHF Omni Range (Dual-VOR), for prediction of its reliability, availability and maintainability by adopting Markov technique. Dual-VOR is a ground based azimuth navigational system for aircraft. The failure rates of various functional blocks for the Dual-VOR have been derived from the MIL-HDBK. The repair rates have been based on maintenance practices and repair policies adopted for the system. Markov modelling for reliability involves solving a set of differential equations. Markov modelling for availability involves solving a set of linear simultaneous equations. A simple technique which involves only the availability model has been adopted for determining the mean time to repair (MTTR). RAM analysis of “hot-standby” configuration of Dual-VOR has been discussed.     

Reliability of networks of three-state devices

A three-state device is a device that can exhibit two different types of failure mode, an “open” failure and a “shorted” failure. This paper treats networks whose arcs may experience these two failure modes in addition to the normal “success” state. The network is undirected and has two designated nodes as source and sink. Such a network is itself subject to each of the two failure modes, and the reliability problem considered is computation of the probability of each of the three states of the network. Our observation is that such problems are easily reduced to the usual two-state network reliability problem for which common techniques such as the factoring theorem are readily applicable.     

Stochastic optimization of discrete event systems simulation

The Score Function (SF) method has been proposed to estimate the gradient of a performance measure with respect to some continuous parameters in a stochastic system. In this paper we experiment with the use of this estimate in a stochastic approximation algorithm to perform a single-run optimization. The experiment is done on a simple M/M/1 queue. The performance measure involves the average system time per customer at steady-state, and the decision variable is the service rate. The optimal solution is easy to compute analytically, which facilitates the evaluation of the algorithm. Combined with appropriate variance reduction techniques, the method has been shown to be effective in the test problem.We study the algorithm's properties and examine the validity of the estimates based on this single run procedure by performing some experimental studies. Implementation “details” necessary for packaging this method with existing simulation software are provided. Finally, there is a set of recommended directions for future research.     

A flexible bathtub hazard model for non-repairable systems with uncensored data

This research effort has developed a mathematical model for bathtub shaped hazards (failure rates) for operating systems with uncensored data. The model will be used to predict the reliability of systems with such hazards. Early in the life-time of a system, there may be a relatively large number of failures due to initial weaknesses or defects in materials and manufacturing processes. This period is called the “infant mortality” period. During the middle period of an operating system fewer failures occur and are caused when the environmental stresses exceed the design strength of the system. It is difficult to predict the environmental stress amplitudes or the system strengths as deterministic functions of time, thus the middle-life failures are often called “random failures.” As the system ages, it deterioates and more failures occur. This region of failure is called the “wearout” period. Graphing these failure rates simultaneously will result in a bathtub shaped curve. The model developed for this bathtub pattern of failure takes into account all three failure regions simultaneously. The model has been validated for accuracy by using Halley's mortality table and is used to predict the reliability with both least squares and maximum likelihood estimators.     

Capacity Planning of Survivable Mesh-based Transport Networks under Demand Uncertainty

Almost all existing work on the design of survivable networks is based on a specific demand forecast to which one optimizes routing and transport capacity assignment for a single target planning view. In practice these single-forecast models may be used repetitively by a planner to consider a range of different scenarios individually, hoping to develop intuition about how to proceed. But this is not the same as having a planning method that can inherently and quantitatively consider a range of possible futures all at once. Our approach considers both the cost of initial design construction and the expected cost of possible augmentations or “recourse” actions required in the future, adapting the network to accommodate different actual future demands. In practice, these recourse actions might include lighting up a new DWDM channel on an existing fiber or pulling-in additional cables, or leasing additional capacity from third party network operators, and so on. A stochastic linear programming approach is used to achieve designs for which the total cost of current outlays plus the expected future recourse costs is minimized. Realistic aspects of optical networking such as network survivability based on shared spare capacity and the modularity and economy-of-scale effects are considered. These are not only important practical details to reflect in planning, but they give the “future-proof” design problem for such networks some unique aspects. For instance, what is the working capacity under one future scenario that may not waste capacity if that demand scenario does not materialize, because the same channels may be used as shared spare capacity under other future scenarios. Similarly economy-of-scale effects bear uniquely on the future-proof planning problem, as the least-cost strategy on a life-cycle basis may actually be to place more capacity today than current requirements would suggest. This is of obvious relevance to planners given the recent hard times in the telecommunications industry, causing a tendency to minimize costs now regardless of the consequences.     

Function process and reliability analysis of a two-dependent-unit system

In this paper, a two-dependent-unit system is discussed. We assume that the life distribution of the two units is a two-dimensional exponential distribution, and the repair distributions are general distributions, respectively, and the failure unit cannot be repaired “as good as new”. By using a function process, some reliability indices are derived.     

Unreliable queuing with repeated orders

The study of queuing models with repeated orders is motivated by new developments in telecommunication technology as the use of “repeat-last-number”, “auto-repeat”, “ring-back-when- free” and other facilities [31,38,40]. In this paper we focalise our attention in an important aspect: the influence of the reliability of the communication line on the distribution of the number A(t) of customers in the system.First we give a short review of studies in this area. Next, we consider a version of the unreliable M/G/1/1 queue with repeated orders. The distribution of the non-markovian jump-process A(t), t≥0 is studed by an adequate “markovization”. We obtain a generalization of the Pollaczek-Khintchin-Alexandrov formula     

Comparison between state probabilities and availabilities of four-state and five-state models using Laplace transform and matrix multiplication methods

When it is required to study the behaviour of any system after start-up or repair, transient solutions are particularly important. Also, these solutions become necessary when the behaviour need to be studied over a fixed period of time during which the system is required to perform a given mission. There are a number of methods available to solve the first order differential equations of any system, including: the Laplace Transform method, and the Matrix Multiplication method. Both methods are used in this paper to calculate the state-probabilities for four-state and five-state models, respectively, and the results are compared. A comparison is also made between the system “availability” and “unavailability” for both four-state and five-state models using the Laplace Transform and the Matrix Multiplication methods.     

A survey of dynamic bandwidth allocation algorithms for Ethernet Passive Optical Networks

Ethernet Passive Optical Network (EPON) has been widely considered as a promising technology for implementing the FTTx solutions to the “last mile” bandwidth bottleneck problem. Bandwidth allocation is one of the critical issues in the design of EPON systems. In an EPON system, multiple optical network units (ONUs) share a common upstream channel for data transmission. To efficiently utilize the limited bandwidth of the upstream channel, an EPON system must dynamically allocate the upstream bandwidth among multiple ONUs based on the instantaneous bandwidth demands and quality of service requirements of end users. This paper introduces the fundamental concepts on EPONs, discusses the major issues related to bandwidth allocation in EPON systems, and presents a survey of the state-of-the-art dynamic bandwidth allocation (DBA) algorithms for EPONs.     

Optimal planned maintenance with salvage cost for a two-unit standby redundant system

This paper considers the optimal spare ordering policies for a cold standby redundant system with two dissimilar units. Especially, the planned maintenance schedule with salvage cost is discussed. The failure time distributions for respective units are assumed to be arbitrary. By applying the expected cost per unit time in the steady-state and the stationary availability as criteria of optimality, the optimal ordering policy minimizing or maximizing each criterion is obtained under some economical and/or physical assumptions. Finally, numerical examples are presented, and the effect of the failure time distributions for the optimal ordering policy is examined in detail.     

Analysis of a repairable redundant system with delayed replacement

The paper deals with a redundant system with two types of spare units—a warm standby unit for instantaneous replacement at the time of failure of the active unit and a cold standby (stock) unit which can be replaced after a random amount of time. Failure time distributions of operative and standby units are exponential whereas all repair times follow arbitrary distributions. The system has been studied in detail by applying the results from the theory of semi-Markov process and mean-time-to-system-failure, steady-state availability, expected number of visits to a state, second moment of time in an up-state and expected profit of the system have been obtained.     

Fuzzy measure of complex system outages

In recent years, the planning and operation of complex systems has involved many uncertainties. More probabilistic concepts are available for complex modelling problems, where all uncertainties are treated in a probabilistic way. There are some statements in complex systems modelling problems which are not treated or viewed in a probabilistic way. A power system example is considered because of its high complexity, owing to the uncertain operation of the system. For example, a statement such as “the available reserve capacity is very large and the present demand is average”, used in the operation of power systems, is very imprecise. Subjective terms are used to characterize the above statement. The terms “large” and “average” are called linguistic variables. These subjective terms are well modelled by fuzzy sets. As an illustration, fuzzy concepts are applied to measure power generation system outages.     

Optimum path planning using convex hull and local search heuristic algorithms

Whether in improving quality or productivity the impact of mechatronic systems such as robots in industry is unquestionable. One aspect of interest in robotics is planning the optimum path for a mobile robot or the optimum trajectory for link movements of a stationary robot in order to increase their efficiency. However, for a given set of points complete enumeration of all the possible paths to establish an optimal one is not feasible as the search space increases exponentially (explodes combinatorially) as the number of points increases. This problem, traditionally known as the “Traveling Salesman Problem” (TSP) has attracted a great deal of attention for a long time. Proven enumerative techniques such as “nearest neighbour algorithm”, “branch and bound”, “cutting planes”, and “dynamic programming” as well as approximation methods such as “tabu search”, “greedy algorithm”, “simulated annealing” and “genetic algorithm”, have had only a limited success in solving this problem. Recently “convex hull”, a minimum area and perimeter shape, has been used as an initial sub-tour along with enumerative techniques such as minimising insertion costs to solve the TSP problem. We present a system which uses heuristic rules to augment the convex hull initial sub-tour created by the Graham scan algorithm. The system is able to provide a solution in a polynomial time.     

Fixed-width confidence interval estimation of the inverse coefficient of variation in a normal population

A sequential procedure is developed in order to construct a confidence interval of “fixed-width and preassigned coverage probability” for the inverse of the coefficient of variation of a normal population. The proposed sequential procedure is proved to be “asymptotically efficient and consistent” in the sense of Chow and Robbins ([1]: Ann. Math. Statist. 36, 457–462 (1965)). Asymptotic distribution of the stopping time is derived.     

Proper Framework for the Application of Power Management Algorithms

A power aware system can reduce its energy dissipation by dynamically powering off during idle periods and powering on again upon a new service request arrival. We minimize the dissipated energy, by selecting the optimal waiting interval before powering off, under consideration of the expected time of the next arrival. This approach has been already proposed in the past, using the idle times distribution, rather than the interarrival periods captured at the moment of service completion. Algorithms proposed in the literature utilize the history of idle periods or assume a vanishing service time. There has been no clear proposition on how service time affects the time instance of our power off decision; rather, whenever service time has been significant, a “blurred” image of the system’s characteristic and a corresponding approximated optimal policy occurred. We clearly show analytically and experimentally that the idle times distribution should not be used as a primary design input, since it is the product of two separate inputs; the interarrival times and the service times. We give insight to our problem, using a mechanical equivalent established at the moment of service completion of all pending requests and show through analytical examples how service time affects our power-off decision. We explain the paradox of being advantageous to wait for intervals more than the shutdown threshold (which is a system characteristic) and show how the introduction of idle period lengths instead of interarrival periods “blurs” the input distribution, leading to non-optimal decisions. Our contribution is to define and solve the proper problem, solely relying on the interarrival distribution. Further, we examine the problem under the framework of competitive analysis. We show how the interarrival distribution that maximizes the competitive ratio, being an exponential distribution, intervenes with power management; it renders the optimization procedure worthless through its “memoryless property”. Exponential interarrivals, irrespective of the service time pattern, are the marginal case where we cannot obtain energy gains. In all other cases the framework we promote ensures considerable advantages compared to other approaches in the literature. Moreover, it leads to a self contained module, implementable in software or hardware, which is based on an iterative formula and thus reduces power management calculations significantly. Here we exploit all operational features of the problem in proposing an implementation which spreads computations over the whole of the waiting period. We extensively compare our results numerically both against claimed expectations and against previous proposals. The outcome fully supports our framework as the one most appropriate for the application of power management.Part of this work has been supported by the EU IST-2001-34157 project PACKWOMAN (Power Aware Communication for Wireless Optimised Personal Area Networks).     

A note on the confidence interval for the availability ratio

If one is interested in determining the worth of equipment to perform a given task, often the primary concerns are reliability, maintainability, and availability. Availability consider both reliability and maintainability since it is a measure of the ratio of the operating time of the system to the operating time plus the downtime or time to repair. One method commonly used to establish a confidence interval for availability is to establish a confidence interval for the meantime between failure of the equipment and then determine a confidence interval for the meantime to repair the equipment. These two confidence intervals may then be utilized to obtain an “at least” type confidence interval for availability. This approach is not entirely satisfactory since points outside of both of the above mentioned intervals can give points inside the confidence interval for availability. This paper proposes a method for establishing an exact confidence interval for availability under the assumption that the time between failures and the time to repair are independent gamma and lognormal random variables respectively. The method requires computations of the distribution of , where Ut X²(q) and . In order to make the results usuable, tables of the cumulative distribution of W are included.     

Generalized formulation of yield, variability, minimax and Taguchi circuit optimization problems

A generalized formulation of several circuit design problems, such as manufacturing yield optimization, circuit performance variability minimization, deterministic and statistical minimax design, Income Index maximization, Taguchi approach, etc., is developed. Several other “intermediate” problems can be defined in a sense similar to the one used in Zadeh's fuzzy set theory. A specific problem is identified by the selection of a generalized membership function ω(·) of the acceptability region, and a sequence of the values of the “smoothing” parameter β. Generalized gradient formulas are developed, and various possible algorithmic implementations discussed. As a result, trade-offs between different design strategies can be investigated by circuit designers, within one coherent methodology.     

Ultra Wide Band WLANs: A Self-Configuring Resource Control Scheme for Accessing UWB Hot-Spots with QoS Guarantees

In the framework of wireless access networks the Hot-Spot concept is attracting several operators. In a Hot-Spot near stationary terminals may reach one or more Radio Access Points (RAP) offering wireless access to the fixed network. Mobile terminals should be able to register to the network, associate to a RAP and activate a wireless communication supporting given bit rates and Quality of Service (QoS) features. Several mobile users, requiring different services, enter and exit the Hot-Spot. In this scenario network operators should have the opportunity to configure quickly radio resources to serve the mobile terminals and to handle efficiently the network resources in order to maximize the income. Among the different technologies emerging in this field, we investigate the feasibility of a wireless access based on Ultra Wide Band (UWB) radio, combined with a flexible admission control scheme based on transmission power selection. We employ UWB in unlicensed mode, i.e., we operate in accordance to the limits imposed by the regulatory bodies (e.g., US Federal Communications Commission). The flexibility of the admission control depends mainly on the capability of a mobile terminal of “measuring” the environment it is entering and thus supporting the RAP in the selection of the appropriate transmission parameters. The proposed approach provides an admission policy based on the Maximum Extra Interference (MEI) and selects the power level through a simple interaction among the involved mobile terminals. The information for basing the decision on is collected through measurements and signaling. In order to increase the system efficiency, transmission parameters are selected in accordance to a “balancing” criterion (thus Balanced-MEI, B-MEI). The B-MEI approach keeps quite simple the admission of new mobile terminals in a RAP’s area but contemporarily satisfies the trade-off between fair resource assignment and system efficiency. This is a key trade-off in wireless access systems where interference effects determine the upper limit of the number of users that can be admitted in the network.Francesca Cuomo received her “Laurea” degree in Electrical and Electronic Engineering in 1993, magna cum laude, from the University of Rome “La Sapienza”, Italy. She earned the Ph.D. degree in Information and Communications Engineering in 1998, also from the University of Rome “La Sapienza.” Since 1996 she is an Assistant Professor at the INFOCOM Department of this University. Her main research interests focus on broadband integrated networks, Intelligent Networks, architectures and protocol for wireless networks, mobile and personal communications, Quality of Service guarantees and real time service support in the wired and wireless Internet.She participated in: (I) the European ACTS INSIGNIA project dedicated to the definition of an Integrated IN and B-ISDN network; (III) IST WHYLESS.COM project focusing on adoption of the Ultra Wide Band radio technology for the definition of an Open Mobile Access Network; (III) RAMON project, funded by the Italian Public Education Ministry, focused on the definition of a reconfigurable access module for mobile computing applications. She is now participating to the European IST ePerSpace Project focusing on the support of personalized audio/video services at home and everywhere. She is also involved in FIRB project VIRTUAL IMMERSIVE COMMUNICATIONS (VICOM) where she is responsible of the research activities on the BAN and PAN networks.Dr. Cuomo is in the editorial board of the Elsevier Computer Networks journal and she has served on technical program committees and reviewer in several international conferences and journals including ACM Wireless Mobile Internet Workshop, IEEE ICC and GLOBECOM, IEEE Trans. on Wireless Communications and IEEE Journal on Selected Areas on Communications.Cristina Martello received her Laurea degree in Electronic Engineering (magna cum laude) in July 2000 from Università di Roma “La Sapienza”. She earned the PhD degree in Information and Communications Engineering in February 2004 (Università di Roma “La Sapienza”).Since January 2001 she has been working in the IST European HYPERLINK “/” “_blank” Whyless.com project on an open mobile access network based on the Ultra Wide Band radio technology. She collaborated with HYPERLINK “http://www.coritel.it/” “_blank” Co.Ri.Tel. (a research consortium on Telecommunications) as a fellowship holder in 2000/2001 for the project SWAP on the feasibility of a re-configurable software module for the dynamic Radio Resource Control in the 3G of mobile wireless systems, and in 2002 for the project PRESTO.Her main research interests regard the developing of flexible and distributed Radio Resource Control techniques for “ad-hoc like” networking paradigms.