Discrete Event System Simulation Research Articles

On fidelity and model selection for discrete event simulation

In simulation, perhaps the most common use of the term ‘fidelity’ refers to the faithfulness with which model behavior reflects modeled system behavior. While there have been studies of fidelity seeking absolute and quantitative measures, there is not yet a consensus on a workable fidelity metric. We propose a formal modeling framework for comparing discrete event system simulation models in terms of fidelity, using a relative fidelity indicator. Based on the framework, we consider the possibility that the higher fidelity simulation models can also be more productive, even though they are more expensive to develop and use, since they can be used to achieve multiple objectives. First, we propose a formal simulation modeling framework within which the fidelity of simulation models can be discussed. With this framework and a simple example, we then define a relative fidelity indicator that provides a systematic way of comparing the fidelity of two simulation models. The relative fidelity indicator focuses on the most important characteristics in simulation studies: the input and output interfaces and the variables used for specifying a real-world system and simulation models. It does not require any special modeling formalism for model comparison. Based on the relative fidelity indicator and simulation modeling framework, we state the optimum simulation model selection problem to achieve given simulation objectives. Under a practical assumption, we analyze the simulation model selection problem and derive properties related to simulation modeling and the fidelity of simulation models.

Event Scheduling in Simulation of Discrete Event Systems with Application on Queuing Systems.

Event Scheduling in Simulation of Discrete Event Systems with Application on Queuing Systems.

PowerDEVS: a tool for hybrid system modeling and real-time simulation

In this paper we introduce a general-purpose software tool for discrete event system specification (DEVS) modeling and simulation oriented to the simulation of hybrid systems. The environment, called PowerDEVS, allows atomic DEVS models to be defined in C++ language that can then be coupled graphically in hierarchical block diagrams to create more complex systems. The environment automatically translates the graphically coupled models into a C++ code which executes the simulation. A remarkable feature of PowerDEVS is the possibility to perform simulations under a real-time operating system (RTAI) synchronizing with a real-time clock, which permits the design and automatic implementation of synchronous and asynchronous digital controllers. Combined with its continuous system simulation library, PowerDEVS is also an efficient tool for real-time simulation of physical systems. Another feature is the interconnection between PowerDEVS and the numerical package Scilab. PowerDEVS simulations can make use of Scilab workspace variables and functions, and the results can be sent back to Scilab for further processing and data analysis. In addition to describing the main features of the software tool, the article also illustrates its use with some examples which show its simplicity and efficiency.

System Entity Structure Ontological Data Fusion Process Integrated with C2 Systems

With the advent of Network-Centric Warfare (NCW) concepts, Command and Control (C2) systems need efficient methods to produce, exchange, and consume diverse kinds of information within the networks. The System Entity Structure (SES) is an ontology framework that can facilitate information exchange in a network environment. From the perspective of the SES information exchange framework, the Battle Management Language (BML) serves to specify the information desired by a consumer in an unambiguous way. This paper formulates information exchange in the SES ontology via BML and defines novel pruning and transformation processes of the SES to extract and fuse data into higher-level representations. This supports increased interoperability between human users and other sensor systems. To demonstrate the effectiveness of the proposed information exchange framework for data fusion systems, we illustrate the approach in an air defense operation scenario using Discrete Event System Specification (DEVS) modeling and simulation. The demonstration shows that the information requested by a commander is delivered in the right way at the right time so that it can support agile decision making against threats.

Integrated Simulation System for Construction Operation and Project Scheduling

Simulation modeling is important in predicting the productivity of construction operations and the performance of project schedules. It would be desirable if operation and project models are vertically integrated in practice. However, existing discrete event simulation systems do not allow integrating operation and project models. This paper introduces an integrated simulation system named “Construction Operation and Project Scheduling” (COPS). COPS analyzes the productivity of construction operations as well as the performance of a project schedule individually and jointly. It creates operation models, maintains these models in its operation model library, conducts sensitivity analysis with different resource combinations, finds the optimal resource combination that satisfies the user’s requirements relative to hourly production and hourly cost of the operation, feeds this information into a project schedule, and executes stochastic simulation-based scheduling. A case study is presented to demonstrate th...

Operational and strategic supply model redesign for an optical chain company using digital simulation

Decision making inside a company is usually performed by means of decision support tools at three different levels (strategic, tactical and operational) according to the working horizon. Unfortunately, the consequences of any decision at the strategic level are propagated to the tactical and even the operational levels. In order to support highly efficient flexibility while avoiding infrastructure over-sizing, resource saturation and poor coordination of activities and resources, it is important to consider the cause—effect interaction of strategic decisions with operational ones. In this paper, a discrete-event system simulation approach to evaluate the influence of the internal interactions among material, people, information and financial resources on the success of a Spanish optician supply chain is introduced. The emergent dynamics are analyzed and a redesign of the business and distribution network is proposed to foster synergy among supply chain actors.

Modelling an assembly process using a close coupled generative cost model and a discrete event simulation

Static system modelling for a manufacturing organisation does not closely reflect the manufacturing process. A dynamic model more closely simulates the processes involved, and can provide a richer model. This also applies to cost modelling, where some operational details are assumed or obtained from other systems. In order to provide a holistic modelling system, an experimental system has been built closely to couple a generative cost modelling system and a discrete event simulation system. This paper describes the system, its advantages and shortcomings, and the results obtained for the planning of an assembly process.

A Generalized Discrete Event System (G-DEVS) Flattened Simulation Structure: Application to High-Level Architecture (HLA) Compliant Simulation of Workflow

The objective of the paper is to specify a new flattened Generalized Discrete Event System simulation engine structure and the Workflow modeling and simulation environment embedding it. We express first the new flattened simulation structure and give the corresponding transformation functions. We analyze performance tests conducted on this new simulation structure to measure its efficiency. Then, having selected the essential concepts in the elaboration of the Workflow, we present a language of description to define the Workflow processes. Finally, we define a distributed Workflow Reference Model that interfaces components of the Workflow with respect to the High-Level Architecture standard. Today enterprises can take advantage of this platform in the context of networking where interoperability, flexibility, and efficiency are challenging concepts.

Enhanced control of complex production structures by tight coupling of the digital and the physical worlds

Unexpected disturbances and local decisions almost always deteriorate the execution of manufacturing plans. Digital enterprise technologies are hard to use, due to the complexity of production and the frequently changing circumstances. One of the main goals of the research described in the paper is the automatic model building of the discrete-event simulation system, based on intelligent analysis of the huge amount of information incorporated in the production database. The developed solution supports shop-floor dispatching and shop-floor managers in making control decisions.

Digital enterprise solution for integrated production planning and control

Digital enterprise technologies combined with sophisticated optimization algorithms can significantly contribute to the efficiency of production. The paper introduces a novel approach for integrated production planning and control, with the description of the mathematical models and solution algorithms. The deterministic optimization algorithms are complemented by a discrete-event simulation system to assess solution robustness in case of disturbances. The methods are illustrated by describing two prototype systems and by some experimental results obtained in an industry-initiated project.

Modeling energy market dynamics using discrete event system simulation

Abstract This paper proposes the use of Discrete Event System Simulation to study the interactions among fuel and electricity markets and consumers, and the decision-making processes of fuel companies (FUELCOs), generation companies (GENCOs), and consumers in a simple artificial energy market. In reality, since markets can reach a stable equilibrium or fail, it is important to observe how they behave in a dynamic framework. We consider a Nash–Cournot model in which marketers are depicted as Nash–Cournot players that determine supply to meet end-use consumption. Detailed engineering considerations such as transportation network flows are omitted, because the focus is upon the selection and use of appropriate market models to provide answers to policy questions.

A Formal Framework for Stochastic Discrete Event System Specification Modeling and Simulation

We introduce an extension of the classic Discrete Event System Specification (DEVS) formalism that includes stochastic features. Based on the use of the probability spaces theory we define the stochastic DEVS (STDEVS) specification, which provides a formal framework for modeling and simulation of general non-deterministic discrete event systems. The main theoretical properties of the STDEVS framework are treated, including a new definition of legitimacy of models in the stochastic context and a proof of STDEVS closure under coupling. We also illustrate the new stochastic modeling capabilities introduced by STDEVS and their relation with those found in classic DEVS. Practical simulation examples are given involving performance analysis of computer systems and hybrid modeling of networked control systems, applications where the modeling of stochastic components is vital.

Simulation-based optimization vs. mathematical programming: A hybrid approach for optimizing scheduling problems

With the increasing computing power of modern processors, exact solution methods (solvers) for the optimization of scheduling problems become more and more important. Based on the mixed integer programming (MIP) formulation of a scheduling problem, it will be analyzed how powerful the present solvers of this problem class are and up to which complexity real scheduling problems are manageable. For this, initially some common benchmark problems are investigated to find out the boundaries for practical application. Then, the acquired results will be compared with the results of a conventional simulation-based optimization approach under comparable time restrictions. As a next step, the general advantages and disadvantages of both approaches were analyzed. As the result, a coupling of the discrete event simulation system and an MIP solver is presented. This coupling automatically generates an MIP-formulation for the present simulation model which can be solved externally by an MIP solver. After the external optimization process follows a backward transformation of the results into the simulation system. All features of the simulation system (like Gantt-Charts, etc.) could be used to check or to illustrate these results. To perform the coupling for a wide range of simulation models, it has to be defined which general constraints the model has to satisfy.

Simulation-based scheduling of assembly operations

The production of large-sized machines or facilities is fundamentally different from the usual manufacturing processes. Mostly it is a fixed-site production, dominated by typical assembly processes instead of linear operation sequences in the known flow shop or job shop problems. To describe these processes, several graph methods were developed in the past, i.e. AND/OR-graphs or modified Petri nets, but most of them are static methods and they are not suitable for scheduling tasks. This paper demonstrates how Timed AND/OR-graphs can be modelled with a common discrete event simulation (DES) system. The objective is a simulation-based scheduling system for assembly processes which is able to improve the prediction of due date keeping as well as to optimise the workflow of the assembly systems.

A fast computational algorithm for simulating round-robin service

An efficient computation-based algorithm for effecting round-robin (RR) service in discrete-event simulation systems is presented. The new algorithm improves upon an older computational algorithm which itself is an improvement over naive RR scheduling currently in use in process-oriented simulation systems. The naive approach can be prohibitively expensive due to process context switch overhead. The original computational algorithm avoids much of this context switching but offers a run-time complexity of only O(n2), where n is the size of the job pool. This is because the processing of each arrival and departure event requires a traversal of the entire pool. In this paper, we propose an improved algorithm which reduces run-time complexity to O(n log n) by using tree-based data structures. The expected computation to process each arrival or departure event is proportional to O(log n). Empirical evidence suggests that, except for very small size of job pool, this algorithm is considerably faster than other methods.

Tools for Graphical Specification and Visualization of DEVS Models

We introduce advanced graphical modeling and visualization facilities for Discrete Event System Specification (DEVS) modeling and simulation (M&S) in the CD++ environment. The objective is to provide general users with a variety of easy-to-use environments to facilitate the model analysis process and thereby promoting the adoption of M&S by a wider community of practitioners and researchers. CD++Modeler allows users without much experience in software development to construct rather complex DEVS models and to analyze simulation data using 2D graphics. We also introduce a graphical platform called MAPS designed specifically for urban traffic systems, and other advanced 3D animation tools (CD++/VRML, CD++/Maya, CD++DEVSView, and CD++/Blender) based on both commercial and open-source software packages. We elaborate on the design of these toolkits and demonstrate their capabilities as well as relative merits and limitations with realistic applications. Following a highly modular approach, the resulting architecture can be easily extended to incorporate other modeling and visualization techniques in future development. We show that these facilities can reduce the model development cost significantly, lower the learning curve for general users, and improve the comprehension of continuously evolving models, making them suitable for efficient decision making.

Interactive evolutionary modelling and simulation of discrete-event systems using prototypical objects

In this paper, we describe an alternative approach to the traditional class-based approach of building simulation models using object-oriented languages. We use the Discrete EVent Systems specification (DEVS) formalism implemented by prototypical objects. The main reason why we use prototype-based object orientation for simulation modelling is the fact that it offers the level of flexibility that we need for interactive evolutionary modelling and simulation, which is used for simulation-based development of systems without exact specifications. We introduce a reflective framework model that constructs, inspects and edits the models specified using DEVS. The meta-object protocol of this framework allows the models to be constructed from scratch and inspected and edited during run-time. Interactive evolutionary modelling and simulation is supported by a visual tool set which has been highly influenced by the user interface of Self, an object-oriented language based on prototypical objects.

Parallel discrete event simulation on desktop grid computing infrastructures

In contrast to traditional Parallel Discrete Event Simulation (PDES) systems using dedicated machines, an alternative master/worker approach for high throughput execution on shared computing platforms is proposed. Master/worker systems offer potential advantages over traditional PDES systems such as better support for heterogeneous machines, client-driven load balancing, fault tolerance, and execution on non-dedicated machines. We examine feasibility of executing PDES codes on master/worker systems. We characterise application properties that are important for efficient execution and define suitable performance metrics. The Aurora Parallel and Distributed Simulation System utilising the master/worker approach is described along with performance measurements in executing conservative PDES codes.

Stochastic Elements in Transportation System Simulation - Unnecessary?

Abstract Discrete event system simulation is often seen as a genuine tool to investigate the performance of transportation systems. The complexity of real-world systems often prevents us from accurately describing these by a mathematical model that can be evaluated analytically, thus, simulation is often the only realistic alternative. Another advantage of the simulation is the ability to include statistical analysis for different simulation scenarios. Well-known approaches of incorporating uncertainty into models include trace driven simulations and sampling directly from gathered data. Another aspect to be taken into account is the economics of simulation modelling; a more detailed model requires additional building time, and proper treatment of stochastic models requires statistical analysis, which again usually implies several simulation runs. From this outset the following question arises: Should stochastic behaviour be included in transportation simulation models in the first place? We present real case examples including evaluation of the internal logistics of a beverage manufacturer, capacity analysis of an automatic guided vehicle system, cost benefit analysis (CBA) of a railway network investment and evaluation of different multipurpose railway wagons, where stochastic behaviour is dealt with in different ways. Based on the cases we make an initial attempt to formulate framework for deciding how to include stochastic behaviour in the simulation model. For further research topics we suggest formulating explicit guidelines to deal with stochastics to increase the efficiency of model building.

Enhancing the efficiency of railways by multi-use of wagons

Russia is the main source for imported raw material in Finnish pulp and paper production. Transports from Russia are completed mainly using rails, which thus have idle capacity in the eastbound direction. In this article, we evaluate the possibility to use railway timber wagons to serve the eastbound container flow currently on roads. The analysis is based on a discrete-event system simulation model concentrating on a case network. Our research results show that combined use of wagons would enhance the efficiency of railway transports. Furthermore, the eastbound container transit flow on roads could be eliminated to a large extent.