• Journal of KIISE:Computer Systems and Theory
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• v.33 no.5
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• pp.282-296
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• 2006

In a wireless client server environment, data broadcasting is an efficient data dissemination method; a server broadcasts data, and some of broadcasted data are cached in a mobile client's cache to save the narrow communication bandwidth, limited resources, and data access time. A server also broadcasts invalidation reports to maintain the consistency between server data and a client's cached data. Most of existing works on the cache consistency problems simply purge the entire cache when the disconnection time is long enough to miss the certain amount(window size) of IRs. This paper presents a cache invalidation method to increase mobile clients' cache reusability in case of a long disconnection. Instead of simply dropping the entire cache regardless of its consistency, a client estimates the cost of purging all the data with the cost of selective purge. If the cost of dropping entire cache is higher, a client maintains the cache and selectively purge inconsistent data using uplink bandwidth for validation request. The simulation results show that this scheme increases the cache reusability since it effectively considers the update rates and the broadcast frequencies of cached data in estimating the cost of cache maintenance.

• Proceedings of the Korea Society for Simulation Conference
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• 2003.11a
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• pp.165-171
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• 2003

We have developed Virtual Reality Simulator MMI for operator training. MMI communicates with server model and client PC through OPCServer. We used Dataplant for Soft Panel and Simdiagram which shows more sophisticated and reality colors. We used vector method for MCR images instead of conventional bit-map method to protect images during zoom in and zoom out. VBScript and Components were used for real movements and images of instruments. VRMCR panel is consisted of 12 groups and designed to accommodate user's convenience.

• Journal of Information Processing Systems
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• v.9 no.3
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• pp.489-498
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• 2013

In this paper, we propose a common simulation model that can be reused for different performance evaluations of networked virtual environments. To this end, we analyzed the common features of NVEs, in which multiple regions compose a shared space, and where a user has his/her own interest area. Communication architecture can be client-server or peer-server models. In usual simulations, users move around the world while the number of users varies with the system. Our model provides various simulation parameters to customize the region configuration and user movement pattern. Furthermore, our model introduces a way to mimic a lot of users in a minimal experiment environment. The proposed model is integrated with our network framework, which supports various scalability approaches. We specifically applied our model to the interest management and load distribution schemes to evaluate communication overhead. With the proposed simulation model, a new simulation can be easily designed in a large-scale environment.

• 제어로봇시스템학회:학술대회논문집
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• 2004.08a
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• pp.524-529
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• 2004

In this paper, We described the method which developed the optimal control system for air separation unit to change production rates frequently and rapidly. Control models of the process were developed from actual plant data using subspace identification method which is developed by many researchers in resent years. The model consist of a series connection of linear dynamic block and static nonlinear block (Wiener model). The model is controlled by model based predictive controller. In MPC the input is calculated by on-line optimization of a performance index based on predictions by the model, subject to possible constraints. To calculate the optimal the performance index, conditions are expressed by LMI(Linear Matrix Inequalities).In order to access at the Bailey DCS system, we applied the OPC server and developed the Client program. The OPC sever is a device which can access Bailey DCS system.The Client program is developed based on the Matlab language for easy calculation,data simulation and data logging. Using this program, we can apply the optimal input to the DCS system at real time.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2000.05a
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• pp.952-955
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• 2000

Nowadays, Internet is so popular that we can easily access the remote site to search information and to communicate remote site and users. People who want to make a collaborate working environment can use JAVA, CORBA, and other internet programming tools like a Perl/XML. The mechanist are try to make the environment fur collaboration within design/manufacturing, simulation, remote sensing through TCP/IP And many industries and research institutions are working towards the agile manufacturing. This paper describes an internet-based real-Time remote monitoring system. The system consists of a hardware setup and a software interface. The hardware setup consists of a machine and its data acquisition hardware, while the software interface incorporates the data acquisition software, the server program, and the client program. The server program acts as the main interface between the data acquisition system and the internet technology. The client program is to be distributed to the remote users who want to monitor the machining status. The system has been demonstrated and verified for an industrial High-Speed Machine (HSM) especially measuring cutting force and acoustic emission. To share the signal, we make the WWW server and display its value. The system has been found to be highly efficient, reliable and accurate.

• Journal of KIISE:Computer Systems and Theory
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• v.33 no.8
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• pp.505-513
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• 2006

To satisfy SLA(Service Level Agreement) contract between a client and a service provider, the client requests are classified and processed by priorities. In providing differentiated service, a request from a client who has low priority can be dealt with less important. In this paper, we study static and dynamic partitioning mechanism of web servers and also admission control policy for multiclass request distribution. Through simulation experiments, we analyze web server throughput and response time considering SLA.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.8 no.7
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• pp.2512-2531
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• 2014

We propose a new Distributed Denial of Service (DDoS) defense mechanism that protects http web servers from application-level DDoS attacks based on the two methodologies: whitelist-based admission control and busy period-based attack flow detection. The attack flow detection mechanism detects attach flows based on the symptom or stress at the server, since it is getting more difficult to identify bad flows only based on the incoming traffic patterns. The stress is measured by the time interval during which a given client makes the server busy, referred to as a client-induced server busy period (CSBP). We also need to protect the servers from a sudden surge of attack flows even before the malicious flows are identified by the attack flow detection mechanism. Thus, we use whitelist-based admission control mechanism additionally to control the load on the servers. We evaluate the performance of the proposed scheme via simulation and experiment. The simulation results show that our defense system can mitigate DDoS attacks effectively even under a large number of attack flows, on the order of thousands, and the experiment results show that our defense system deployed on a linux machine is sufficiently lightweight to handle packets arriving at a rate close to the link rate.

• The Journal of the Korea Contents Association
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• v.5 no.6
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• pp.49-56
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• 2005

To reduce server bottlenecks in client-server computing, each client may have its own cache for later reuse. The pessimistic approach for cache management protocol leads to unnecessary waits, because, it can not be commit a transaction until the transaction obtains all requested locks. In addition, optimistic approach tends to make needless aborts. This paper suggests an efficient optimistic protocol that overcomes such shortcomings. In this paper, we present a simulation-based analysis on the performance of our scheme with other well-known protocols. The analysis was executed under the Zipf workload which represents the popularity distribution on the Web. The simulation experiments show that our scheme performs as well as or better than other schemes with low overhead.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.7B
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• pp.598-612
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• 2004

Client requests have increased with the improvement of network resources at client side, whereas network resources at server side could not keep pace with the increased client request. Therefore, it is primary factor of the Qos that efficiently utilize network resources at server side. In this paper, we proposed a new model that peer-to-peer transfer scheme for partial multimedia stream based on CIWP which it decrease server network bandwidth by utilizing client disk resources saves additional server network resources. Especially, adapting Threshold_Based Multicast scheme guarantees to do that data transfer within clients never exceed service time of previous peer by restriction of which data size transferring from previous peer less than data size transferring from server. Peer-to-peer transfer within clients is limited in same group classified as ISPs. Our analytical result shows that proposed scheme reduces appling network resources at server side as utilizing additional client disk resource. furthermore, we perform various simulation study demonstrating the performance gain through comparing delay time and proportion of waiting requesters. As a result, when we compared to Threshold_Based Multicast scheme, the proposed scheme reduces server network bandwidth by 35%.

• Journal of the Korea Computer Industry Society
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• v.5 no.5
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• pp.721-728
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• 2004

OCSP (Online Certificate Status Protocol) server which checks the certificate status provides the real time status verification in the PKI (Public Key Infrastructure) system which is the essential system of certificate. However, OCSP server need the message authentication with the server and client, so it has some shortcomings that has slow response time for the demands of many clients concurrently and has complexity of the mathematical process in the public encryption system. In this research, simulation model of the certificate status vertification server is constructed of the DEVS (Discrete EVent system Specification) formalism. This sever model is constructed to practice the authentication with hash function when certificate is checked. Simulation results shows the results of increase of the certificate status verification speed and decrease of the response time to the client.