Robust optimization models for the dicrete time/cost trade-off problem

Cataloged from PDF version of article.Developing models and algorithms to generate robust project schedules that are less sensitive to disturbances are essential in today’s highly competitive uncertain project environments. This paper addresses robust scheduling in project environments; specifically, we address the discrete time/cost trade-off problem (DTCTP). We formulate the robust DTCTP with three alternative optimization models in which interval uncertainty is assumed for the unknown cost parameters. We develop exact and heuristic algorithms to solve these robust optimization models. Furthermore, we compare the schedules that have been generated with these models on the basis of schedule robustness. & 2010 Elsevier B.V. All rights reserved

Dimensioning Shared-per-Node Recirculating Fiber Delay Line Buffers in an Optical Packet Switch

Cataloged from PDF version of article.Optical buffering based on fiber delay lines (FDLs) has been proposed as a means for contention resolution in an optical packet switch. In this article, we propose a queuing model for feedback-type shared-per-node recirculating FDL optical buffers in asynchronous optical switching nodes. In this model, optical packets are allowed to recirculate over FDLs as long as the total number of recirculations is less than a pre-determined limit to meet signal loss requirements. Markov Modulated Poisson Process (MMPP)-based overflow traffic models and fixed-point iterations are employed to provide an approximate analysis procedure to obtain blocking probabilities as a function of various buffer parameters in the system when the packet arrival process at the optical switch is Poisson. The proposed algorithm is numerically efficient and accurate especially in a certain regime identified with relatively long and variably-sized FDLs, making it possible to dimension optical buffers in next-generation optical packet switching systems. © 2013 Elsevier B.V. All rights reserved

Stochastic analysis of finite population bufferless multiplexing in optical packet/burst switching systems

In this letter, we study the blocking probabilities in an asynchronous optical packet/burst switching system with full wavelength conversion. Most of the existing work use Poisson traffic models that is well-suited for an infinite population of users. In this letter, the optical packet traffic arriving at the switching system is modeled through a superposition of a finite number of identical on-off sources. We propose a block tridiagonal LU factorization algorithm to efficiently solve the two dimensional Markov chain that arises in the modeling of the switching system. Copyright © 2007 The Institute of Electronics, Information and Communication Engineers

Comparative study of limited-range wavelength conversion policies for asynchronous optical packet switching

We study an asynchronous optical packet (OP)-switching node equipped with a number of limited range (LR) wavelength converters shared per output link. We study both circular and noncircular LR-wavelength-conversion schemes. A wavelength conversion policy governs the selection of the outgoing wavelength for an OP if the incoming wavelength is in use. Through simulations, we show that the so-called far-conversion policy for which the OP is switched onto the farthest available wavelength in the tuning range, outperforms the other policies we studied. We point out the clustering effect in the use of wavelengths to explain this phenomenon. We also provide an approximate analytical method to find the packet-blocking probability in circular-type LR-wavelength-conversion systems. Based on the simulation results, the approximate method appears to lead to a lower bound for blocking probabilities for all the conversion policies we study. © 2007 Optical Society of America

Resource recovery management using inventory models and supply contracts: An application to leaded waste recovery

Several recent studies have shown that significant quantities of leaded wastes recovered from the disposal of television cathode ray tubes (CRTs) will be entering the waste stream over the next 50 years in a form that is ideal for post-consumer remanufacturing. Furthermore, numerous countries have recently enacted legislation requiring industrial systems to engage in the practice of industrial ecology by having all discarded, returned, or otherwise spent products from manufacturing processes become raw material inputs in subsequent manufacturing operations. Therefore legislation banning CRT disposal together with mandated remanufacturing requirements could lead to numerous potentially attractive business ventures for reprocessing and recycling the high lead content found in this waste. This paper examines inventory issues related to the effective management of these leaded CRT wastes and the nature of the waste flows is considered from the perspective of different management options for inventory control through the use of supply contracts. An effective inventory management policy is extremely important when there is great uncertainty and variability in the year-to-year or within-year quantity of product available - as is the case with the supply of CRT wastes. If it is anticipated that a high percentage of the waste stream will be utilized, then an effective inventory policy proves absolutely essential - but also proves to be an extremely complex process. Consequently, the supply contract approach can be employed to reconcile different pricing preferences with the varying delivery time horizons of different customers. © 2005 ISEIS - International Society for Environmental Information Sciences

On the relationship between preventive maintenance and manufacturing system performance

Cataloged from PDF version of article.A common lament of the preventive maintenance (PM) crusaders is that production supervisors are often unwilling to lose valuable machine time when there are job waiting to be processed and do not assign high enough priority to PM. Maintenance activities that depend dynamically on system state are too complicated to implement and their overall impact on system performance. measured in terms of average tardiness or work-in-process (WIP) inventory, is difficult to predict, In this article, we present some easy to implement state-dependent PM policies that are consistent with the realities of production environment. We also develop polling models based analyses that could be used to obtain system performance metrics when such policies are implemented, We show that there ape situations in which increased PM activity can lower total expected WIP (and overall tardiness) on its own, i.e., without accounting for the lower unplanned downtime. We also include examples that explain the interaction between duration of PM activity and switchover times. We identify cases in which a simple state-independent PM policy outperforms the more sophisticated state-dependent policies, (C) 2001 Elsevier Science B.V. All rights reserved