Dong-Won Seo

An ANP approach for R&D project evaluation based on interdependencies between research objectives and evaluation criteria

As countries strive to become more efficient in investing limited resources in various national R&D projects, the evaluation of project has become increasingly important. However, due to the heterogeneity of the objectives of national R&D programs, few studies have been conducted on comparing programs on the basis of performance. This study explores the application of the analytic network process (ANP) approach for the evaluation of R&D projects that are elements of programs with heterogeneous objectives. The ANP produced the final priorities of projects with respect to benefits and costs when there are interdependencies between programs and evaluation criteria. The paper provides value to practitioners by providing a generic model for project evaluation. For researchers, the paper demonstrates a novel application of ANP under specific situation of heterogeneous objectives. The ANP approach is tested against empirical data drawn from R&D projects sponsored by the Korean government.

Stationary Waiting Times in m-Node Tandem Queues With Production Blocking

We consider a stationary waiting time in a Poisson driven single-server m-node tandem queue with either constant or non-overlapping service times. Each node except for the first one has a finite buffer, and is operated under production blocking. By using (max,+ )-algebra, we explicitly express the stationary waiting time at each node as a function of finite buffer capacities. We also address that the explicit expression is separable and monotone decreasing with respect to the buffer sizes. These properties are applied to a buffer allocation problem with probabilistic constraints on stationary waiting times.

Approximations of lead-time distributions in an assemble-to-order system under a base-stock policy

In this paper, we consider an assemble-to-order production system in which a product with multiple components is produced under a base-stock policy. The demand follows a Poisson arrival process, and the production time at each facility is exponentially distributed.Our main results are closed-form formulae for approximating the lead-time distributions in equilibrium. The approximation formulae for the lead-time distributions can be obtained with a linear combination of bounds, in which certain constants are obtained from auxiliary properties and simulation experiments. To determine the quality of the approximations, we obtained sample lead-time distributions from many simulations. Kolmogorov-Smirnov tests show that our formulae are very good fits of the sample distributions. The approximations are good over the entire range of traffic intensities and are even more accurate in light or heavy traffic.

Performance evaluation of WIP-controlled line production systems with constant processing times

We consider WIP-controlled line production systems with constant processing times.For a given QoS, both CONWIP and DBR outperform a Kanban.A DBR system needs less buffer capacity than or at most equal a CONWIP system.In general, a CONWIP system outperforms a DBR system under the same QoS.Under the certain condition a DBR system outperforms a CONWIP system. We compared three types of WIP-controlled line production systems with constant processing times such as Kanban, CONWIP (constant work-in-process) and DBR (drum-buffer-rope). Based on the observation that such WIP-controlled line production systems are equivalent to m-node tandem queues with finite buffers under communication blocking policy, we applied a max-plus algebra based solution method for the tandem queue to evaluate their performance. Within our knowledge, this research is the first attempt to apply an exact solution method for comparing all three WIP-controlled line production systems at a time. Six-node numerical examples were also used to demonstrate the proposed analysis. The numerical results can be generalized and also provide some insights in designing production systems under certain limited condition.

An evaluation of federated cloud computing effect with service level

In this paper, we consider a federated cloud computing effect. Cloud computing provides software, platform, and infrastructure as services. Customers pay for the services in a pay-as-you-go model. As an effort of quantifying the effect of cloud computing, this paper evaluates a federated cloud computing network. For the evaluation we studied the simple deterministic demand model and the simple probabilistic demand model. In both cases, the effects of online migration of virtual machines can be clearly identified. Specially, for the probabilistic model we define service level as the proportion of time that the capacity of a data center satisfies the amount of the demand. With the service level concept, the performance of the federation and non-federation can be compared. The models in the paper are basic quantitative ones which can be extended to several other complex models with financial effects.

Explicit expressions for moments of waiting times in Poisson driven deterministic two-node tandem queues with blocking

There has been no analytic expression for a multi-node queue even with constant processing times because of a correlation caused by blocking between nodes. This study introduces explicit expressions for moments and tail probability of stationary waiting times in a Poisson-driven deterministic 2-node tandem queue with blocking. Unlike the normal queueing theory, we derive these expressions from the previous results based on the max-plus algebraic approach. Two blocking policies are considered: blocking before service (BBS) and blocking after service (BAS).

An Approximation Method for Blocking Probabilities in M/D/1/K1 → ⋅/D/1/K2 Queues

Obtaining exact blocking probabilities for tandem queues with finite capacities is not a trivial problem. In this paper, we propose a computational approximation method using max-plus algebra for computing blocking probability in a Poisson-driven 2-node tandem queue with finite capacities and constant service times. The blocking probability of a finite-capacity queueing system can be obtained from either the tail probability of stationary waiting time or the difference between two expected stationary waiting times at the first node of the corresponding extended 3-node tandem queue. The computational results in this study show that the proposed approach provides a good approximation of the blocking probability, and in particular, it works well under moderately to heavily loaded situations. The proposed approach is not limited to a particular blocking policy, system structure, or service time; hence, it is applicable to general queues with finite buffer capacities and various blocking policies.

Closed-form formulae for moment, tail probability, and blocking probability of waiting time in a buffer-sharing deterministic system

Abstract Obtaining analytic expressions for characteristics in probabilistic systems with finite buffer capacities such as (higher) moments and tail probabilities of stationary waiting times, and blocking probabilities is by no means trivial. This is also true even for a system with deterministic processing times. By using the max–plus algebraic approach in this study, we introduce closed-form formulae for characteristics of stationary waiting time in a complete buffer-sharing m-node tandem system with constant processing times. Numerical examples are also provided.

A Mathematical Programming Approach for Cloud Service Brokerage

Cloud computing is fast becoming the wave of the future for both home and business computing. Because of this growing acceptance, we can expect an explosion of diverse cloud service providers in the coming years. However, the cloud is not a single entity, rather it is a set of many disconnected islands of application (SaaS), infrastructure (IaaS), and different platform (PaaS) services. Cloud brokering mechanisms are essential to transform the heterogeneous cloud market into a commodity-like service. Cloud service brokers (CSBs) are the new form of business entities to help to aggregate the scattered set of cloud services and make conveniently available to their diverse users. CSBs can reserve a certain percentage of their clients’ (users’) demand and satisfy the remaining portion in an on-demand basis. In doing so, they need to minimize cost of both reserved and on-demand instances as well as the distance of a link between the cloud service provider (CSP) and the user. The study proposes a reservation approach with a mixed integer model to optimizes the cloud service cost and quality.