• Proceedings of the Korean Information Science Society Conference
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• 2003.10c
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• pp.499-501
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• 2003

무선링크에서 TCP의 성능을 향상시키기 위한 연구들이 많이 수행되었으며, 다양한 기법들이 제안되고 있다. 하지만, 대부분의 연구들이 TCP 송신측이 무선링크에서의 패킷 손실을 혼잡으로 오해하는 문제를 해결하는데 초점을 맞추고 있다. 또한 이러한 연구들은 대부분 시뮬레이션을 통해 측정되고 검증된 결과들이기 때문에 실제 환경을 반영하는 데는 완전하지 않다. 본 논문에서는 SKT의 CDMA2000­1x 상용 망에서 삼성전자의 SPH­A500 단말기를 이용하여 TCP와 UDP의 성능을 측정하였다. 본 연구에서는 CDMA2000­1x의 정확한 특성을 측정하고 분석하여. 무선링크에서의 delay spike에 의한 불필요한 timeout을 방지하고, 병목 링크에서의 큐 길이를 안정되게 유지하는 방법을 고안하고자 한다. 이를 위해 본 논문에서는 측정 과정에서 분석된 CDMA2000­1x의 특성을 정리한다.

• Proceedings of the Korean Information Science Society Conference
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• 2004.04a
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• pp.517-519
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• 2004

Chord(6)는 N개의 노드로 이루어진 P2P(Peer-to-Peer)네트워크에서 검색에 사용되는 메시지를 O(logN)으로 줄인 P2P 검색 알고리즘이다. 하지만 모바일 Ad-hoc 네트워크에 이를 적용할 경우 검색 성공률이 매우 떨어져 (1000개의 노드가 도보속도(2m/s)로 움직이는 경우： 검색 성공률 30%이하)P2P 검색이 거의 이루어지지 않는 문제점이 발생한다. 본 논문에서는 이 같은 문제점을 극복하기 위한 알고리즘인 Backtracking Chord을 제안한다. Backtracking Chord 방식은 O(logN)메시지를 사용하여 순차적으로 t번까지 검색을 요청함으로써(t： Timeout의 횟수(0 < t< logN)) t에 따라 최고 88%(t>4)까지 검색 성공률을 높일 수 있다.

• Clinical and Experimental Pediatrics
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• v.61 no.1
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• pp.1-5
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• 2018

We often overlook the importance of several safety issues such as identification of patients, timeout procedure, hand hygiene, handoff communication, and many others. This ignorance, along with many other issues, leads to medical error being ranked as a third leading cause of death in the U.S. Consequently, quality improvement (QI) has become one of the major subjects in healthcare despite a relatively short history. Improving quality is about making healthcare safe, effective, patient-centered, timely, efficient, and equitable. Understanding the need and methodology of QI as well as participation is now essential for physicians. Although basic QI methodology has not changed, one of the most fascinating changes in recent QI is conducting large-scale QI projects through multicenter networks. Prospective multicenter QI projects utilizing the Korean Neonatal Network are a substantial initiation of pediatric QI in Korea. The Korean Pediatric Society should set ambitious goals for QI activities for every primary care pediatrician and pediatric subspecialist.

• Proceedings of the Korean Information Science Society Conference
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• 2012.06d
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• pp.387-388
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• 2012

최근 무선 멀티미디어 센서 네트워크(WMSN : Wireless Multimedia Sensor Network)에서 비디오 영상을 전송하는데 있어 그 특성을 고려하지 않은 전송 방법으로 인하여 영상 정보에 대한 신뢰도가 낮다는 문제점이 있다. 본 논문은 WMSN에서 비디오 프레임의 구성요소인 I, P, B 픽처의 특성에 따라 타임아웃을 동적으로 설정하여 노드와 노드간의 패킷 수신율을 증가시켜 전체 서비스의 QoS를 제공하고, 이벤트를 감지하여 특정 구간의 영상 정보를 제공받을 수 있는 기법을 제안한다.

• Proceedings of the Korean Information Science Society Conference
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• 2008.06d
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• pp.165-169
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• 2008

본 논문에서는 IEEE 802.16 시스템에서 ARQ(Automatic Repeat reQuest)의 동작이 TCP 성능에 끼치는 영향을 분석한다. IEEE 802.16 표준에서는 MAC 계층에서 패킷 전송의 성공 혹은 실패를 확인하여, 실패 시 패킷을 재전송하는 ARQ 기법을 포함하고 있다. 이러한 무선 링크상의 재전송은 TCP와 맞물려 동작할 때, 적절한 ARQ 재전송 타임아웃(timeout)이 보장되지 않으면 심각한 TCP 성능 저하를 초래할 수 있다. 이에 대하여 IEEE 802.16 시스템의 ARQ는 총 4가지의 타이머(timer)들이 존재하지만 그 구체적인 값들은 표준에서 다루고 있지 않다. 본 연구는 이러한 ARQ 타이머들의 값에 따라 TCP 성능이 어떻게 변하는지를 시뮬레이션을 통해 분석하고 적절한 값을 제시한다.

• Journal of Communications and Networks
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• v.7 no.3
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• pp.385-393
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• 2005

In this paper, we propose a simple algorithm to adaptively adjust the value of dupthresh, the duplicate acknowledgement threshold that triggers the transmission control protocol (TCP) fast retransmission algorithm, to improve the TCP performance in a network environment with persistent packet reordering. Our algorithm uses an exponentially weighted moving average (EWMA) and the mean deviation of the lengths of the reordering events reported by a TCP receiver with the duplicate selective acknowledgement (DSACK) extension to estimate the value of dupthresh. We also apply an adaptive upper bound on dupthresh to avoid the retransmission timeout events. In addition, our algorithm includes a mechanism to exponentially reduce dupthresh when the retransmission timer expires. With these mechanisms, our algorithm is capable of converging to and staying at a near-optimal interval of dupthresh. The simulation results show that our algorithm improves the protocol performance significantly with minimal overheads, achieving a greater throughput and fewer false fast retransmissions.

• International Journal of Contents
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• v.8 no.4
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• pp.1-6
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• 2012

Mutual Exclusion is one of the most studied topics in distributed systems where processes communicate by asynchronous message passing. It is often necessary for multiple processes at different sites to access a shared resource or data called a critical section (CS) in distributed systems. A number of algorithms have been proposed to solve the mutual exclusion problem in distributed systems. In this paper, we propose the new algorithm which is modified from Garg's algorithm[1] thus works properly in a fault-tolerant system. In our algorithm, after electing the token generator, the elected process generates a new token based on the information of the myreqlist which is kept by every process and the reqdone which is received during election. Consequently, proposed algorithm tolerates any number of process failures and also does even when only one process is alive.

• Journal of Communications and Networks
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• v.10 no.1
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• pp.98-107
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• 2008

In mobile ad hoc networks, packet loss is unavoidable due to MAC contention, link failure or the inherent characteristics of wireless link. Since TCP relies on the timely reception of TCP ACK packets to progress the transmission of the TCP DATA packets, ACK loss obviously affects the performance due to two main problems: (a) Frequent occurrence of spurious retransmissions caused by timeout events and (b) impairment of the fast retransmit mechanism caused by the lack of a sufficient number of duplicate ACK packets. In particular, since most reactive routing protocols force the packets buffered over a path to be discarded while performing a route recovery, the performance degradation becomes more serious due to such ACK loss. In this paper, therefore, TCP with two piggybacking schemes (called TCP-pgy) is proposed in order to resolve the above-mentioned problems over reactive routing protocols. Through extensive simulations using the ns-2 simulator, we prove that our proposed schemes contribute to TCP performance improvements.

• Journal of Information Processing Systems
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• v.16 no.5
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• pp.1048-1063
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• 2020

In this study, we propose a reinforcement learning agent to control the data transmission rates of nodes in carrier sensing multiple access with collision avoidance (CSMA/CA)-based wireless networks. We design a reinforcement learning (RL) agent, based on Q-learning. The agent learns the environment using the timeout events of packets, which are locally available in data sending nodes. The agent selects actions to control the data transmission rates of nodes that adjust the modulation and coding scheme (MCS) levels of the data packets to utilize the available bandwidth in dynamically changing channel conditions effectively. We use the ns3-gym framework to simulate RL and investigate the effects of the parameters of Q-learning on the performance of the RL agent. The simulation results indicate that the proposed RL agent adequately adjusts the MCS levels according to the changes in the network, and achieves a high throughput comparable to those of the existing data transmission rate adaptation schemes such as Minstrel.

• IEMEK Journal of Embedded Systems and Applications
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• v.13 no.3
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• pp.117-124
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• 2018

Wearable devices have become widespread. Fitness band is one of common wearable devices, providing useful functions. It helps users to monitor and collect their status such as heart rate and travel distance. Wearable devices, including fitness bands, are designed in small size and it ends up having small battery capacity. In that regard, it is necessary to expand the lifetime of wearable devices. Conventional power management scheme of wearable devices is based on DVFS Ondemand Governor and peripheral control by timeout event, such as turning off the LCD. In this paper, we propose a hybrid governor applying hardware supporting low power mode such as sleep mode to exploit the periodicity of fitness band task. In addition, we show hybrid governor outperforms in power consumption than conventional power management scheme of wearable devices based on Ondemand Governor through experiments.