• International Journal of Fluid Machinery and Systems
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• v.4 no.1
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• pp.114-132
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• 2011

We have proposed a new approach based on energy gradient concept for the study of flow instability and turbulent transition in parallel flows in our previous works. It was shown that the disturbance amplitude required for turbulent transition is inversely proportional to Re, which is in agreement with the experiments for imposed transverse disturbance. In present study, the energy gradient theory is extended to the generalized curved flows which have much application in turbomachinery and other fluid delivery devices. Within the frame of the new theory, basic theorems for flow instability in general cases are provided in details. Examples of applications of the theory are given from our previous studies which show comparison of the theory with available experimental data. It is shown that excellent agreement has been achieved for several configurations. Finally, various prediction methods for turbulent transition are reviewed and commented.

• The KIPS Transactions:PartC
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• v.9C no.1
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• pp.89-96
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• 2002

Handoff is one of the most important factors that ma? degrade the performance of TCP connections in wireless data networks. In this paper, we present a lossless and duplication free handoff scheme called LPM (Last Packet Marking) for improving Cellular If semisoft handoff. LPM signals the safe handoff cue by sending a specially marked packet to mobile hosts. SPM (Semisoft rePly Message) is the only newly introduced control packet. Our performance study shows that LPM achieves lossless packet delivery without duplication and increases TCP throughput significantly.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.40 no.6
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• pp.1166-1174
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• 2015

There were the attempts to prevent the spread of harmful materials of the agri-food through the record tracking of the products with the bar code, the partial information tracking of the agri-food storage and the delivery vehicle, or the control of the temperature by intuition. However, there were many problems in the attempts because of the insufficient information, the information distortion and the independent information network of each distribution company. As a result, it is difficult to prevent the spread over the life-cycle of the agri-food using the attempts. To solve the problems, we propose the mechanism mainly to do context awareness, predict, and track the harmful materials of agri-food using big data processing.

• Journal of KIISE:Information Networking
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• v.32 no.3
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• pp.339-349
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• 2005

For recent years, data broadcast technology has been recognized as a very effective data delivery mechanism in mobile computing environment with a large number of cli;ents. Especially, a hybrid broadcast algorithm in real-time environment, which integrates one-way broadcast and on-demand broadcast, has an advantage of adapting the requests of clients to a limited up-link bandwidth and following the change of data access pattern. However, previous hybrid broadcasting algorithms has a problem in the methods to get a grip on the change of data access Pattern. It is caused by the diminution of requests for the data items which are contained in periodic broadcasting schedule because they are already broadcasted. To solve this problem, existing researches may remove data items in periodic broadcasting schedule over a few cycles multiplying cooling factor or find out the requests of data items with extracting them on purpose. Both of them are the artificial methods not considering the property of data. In this paper, we propose a real-time adaptive hybrid broadcasting based on data type(RTAHB-DT) to broadcast considering data property and analysis the performance of our aigorithm through simulation study.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.8A
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• pp.879-888
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• 2004

In this paper, we propose a real time QoS(Quality of Service) guaranteed transmission mechanism for MPEG video traffic at the congested node in IP based networks. Recent spread of the Internet has increased the demands of a real time multimedia service of the quality, Because the type of Internet services can, however, offer the best effort delivery strategies, it is difficult to treat all the types of traffic with differential COS (Class of Service). Most of all, the hierarchical coding method of MPEG data utilizes the reference frame for the motion prediction. The loss of the reference frames makes QoS of the video traffic degraded because the reference frame bit error causes the consecutive packet loss. Therefore we have studied the real time QoS guaranteed mechanism for video traffic by analyzing the previous methods. Computer simulation results show that the proposed scheme has better performance than the previous one.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.8 no.2
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• pp.406-423
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• 2014

The recent advances in wireless communication systems and semiconductor technologies are paving the way for new applications over wireless sensor networks. Health-monitoring application (HMA) is one such emerging technology that is focused on sensing and reporting human vital signs through the communication network comprising sensor devices in the vicinity of the human body. The sensed vital signs can be divided into two categories based on the importance and the frequency of occurrence: occasional emergency signs and regular normal signs. The occasional emergency signs are critical, so they have to be delivered by the specified deadlines, whereas the regular normal signs are non-critical and are only required to be delivered with best effort. Handling the occasional emergency sign is one of the most important attributes in HMA because a human life may depend on correct handling of the situation. That is why the underlying network protocol suite for HMA should ensure that the emergency signs will be reported in a timely manner. However, HMA based on IEEE 802.15.4 might not be able to do so owing to the lack of an appropriate emergency-handling mechanism. Hence, in this paper, we propose a new emergency-handling mechanism to reduce the emergency reporting delay in IEEE 802.15.4 through the modified superframe structure. A fraction of an inactive period is modified into three new periods called the emergency reporting period, emergency beacon period, and emergency transmission period, which are used opportunistically only for immediate emergency reporting and reliable data transmission. Extensive simulation is performed to evaluate the performance of the proposed scheme. The results reveal that the proposed scheme achieves improved latency and higher emergency packets delivery ratio compared with the conventional IEEE 802.15.4 MAC.

• The Journal of Korea Robotics Society
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• v.7 no.4
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• pp.292-298
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• 2012

It is a real-time system that the system correctness depends not only on the correctness of the logical result of the computation but also on the result delivery time. Real-time Operating System (RTOS) is a software that manages the time of a microprocessor to ensure that the most important code runs first so that it is a good building block to design the real-time system. The real-time performance is achieved by using real-time mechanisms through data communication and synchronization of inter-task communication (ITC) between tasks. Therefore, test on the response time of real-time mechanisms is a good measure to predict the performance of real-time systems. This paper aims to analysis the response characteristics of real-time mechanisms in kernel space for real-time embedded Linux: RTAI and Xenomai. The performance evaluations of real-time mechanism depending on the changes of task periods are conducted. Test metrics are jitter of periodic tasks and response time of real-time mechanisms including semaphore, real-time FIFO, Mailbox and Message queue. The periodicity of tasks is relatively consistent for Xenomai but RTAI reveals smaller jitter as an average result. As for real-time mechanisms, semaphore and message transfer mechanism of Xenomai has a superior response to estimate deterministic real-time task execution. But real-time FIFO in RTAI shows faster response. The results are promising to estimate deterministic real-time task execution in implementing real-time systems using real-time embedded Linux.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.2B
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• pp.80-90
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• 2006

WSN(Wireless Sensor Network) performs to detect and collect environmental information for one purpose. As examples, WSN is applicable for home network, patient management of a hospital, logistics management, status detection during the war and so on. The WSN is composed of a sink node and several sensor nodes and has a constraint in an aspect of energy consumption caused by limited battery resource. So many required mechanisms in WSN should consider the remained energy condition. A routing mechanism is requested to deliver the collected information to a sink considering energy efficiency in WSN. There have been many researches to establish (a) route(s) for data delivery to the sink. In this paper, we propose establishment of efficient routes. We proposed a uniform routing mechanism together with considering energy efficiency. For the routing, we define energy probability as routing metrics information and Performs suppression of exchange of control messages. In addition, we derive to uniformly consume the energy of the sensor node when establishing the routes. Also, we evaluate and analyze the energy efficiency for proposed mechanisms through NS-2 simulator.

• The KIPS Transactions:PartD
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• v.10D no.2
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• pp.301-308
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• 2003

In this paper, we propose a prototype system of digital data broadcasting system based on the ATSC data broadcasting standard. This prototype system uses data piping as a mechanism for delivery of arbitrary user-defined data inserted directly into the payload part of the MPEG-2 Transport Stream packets. This data type includes URL or HTML content. After the contents are inserted into the MPEG-2 Transport Stream, they can be delivered through the broadcasting to the DTV set-top receiver. The 75 packets received in real-time during the TV broadcast are used to start display or switch content. This prototype system describes how to achieve common design goals and integrating digital TV and web pages based on the ATSC data broadcasting standard. The prototype system can be used to display digital data contents - HTML, images-on existing TV or digital TV set-tops.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.8 no.2
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• pp.371-388
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• 2014

Cognitive Radio (CR) has been recently proposed as a promising technology to remedy the problems of spectrum scarcity and spectrum underutilization by enabling unlicensed users to opportunistically utilize temporally unused licensed spectrums in a cautious manner. In Cognitive Radio Ad Hoc Networks (CRAHNs), data routing is one of the most challenging tasks since the channel availability and node mobility are unpredictable. Moreover, the network performance is severely degraded due to large numbers of path failures. In this paper, we propose the Fault-Tolerant Cognitive Ad-hoc Routing Protocol (FTCARP) to provide fast and efficient route recovery in presence of path failures during data delivery in CRAHNs. The protocol exploits the joint path and spectrum diversity to offer reliable communication and efficient spectrum usage over the networks. In the proposed protocol, a backup path is utilized in case a failure occurs over a primary transmission route. Different cause of a path failure will be handled by different route recovery mechanism. The protocol performance is compared with that of the Dual Diversity Cognitive Ad-hoc Routing Protocol (D2CARP). The simulation results obviously prove that FTCARP outperforms D2CARP in terms of throughput, packet loss, end-to-end delay and jitter in the high path-failure rate CRAHNs.