• 한국통신학회논문지
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• 제31권1B호
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• pp.9-18
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• 2006

무선 통신의 발전으로 인하여 PDA, 휴대전화 등의 이동기기를 위한 멀티미디어 서비스의 개발 또한 빠르게 발전하고 있다. 이러한 모바일 멀티미디어 서비스의 개발에는 모바일 컴퓨팅의 요구조건을 효율적으로 제어할 수 있는 플랫폼이 필요하다. 본 논문에서는 무선 통신의 잦은 통신단절과 모바일 디바이스의 시스템 자원부족에 대응하기 위하여 모바일 에이전트 기반의 그리드 시스템을 사용하는 적응형 서비스 플랫폼을 제시하고, 서비스 플랫폼의 활용예로 모바일 원격 교육 또는 화상 회의에 사용되는 RtoA (Ready-to-Attend) 프레임워크를 구성하였다. RtoA는 사용자들에게 원활한 유비쿼터스 서비스를 제공하기 위하여 그리드 시스템을 이용한 핸드오프, 스피커, 리스너 서비스를 제공한다. ns-2를 이용한 성능평가의 결과 RtoA는 모바일 디바이스의 에너지 소모량을 줄이고, 응답시간을 향상시키는데 효율적임이 증명되었다.

• 한국통신학회논문지
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• 제29권2A호
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• pp.140-146
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• 2004

본 논문은 다중사용자를 위한 OFDMA/FDD(Orthogonal Frequency Division Multiple Access/Frequency Division Duplex) 시스템에서 가변적인 전송율로 전송하기위한 무선 채널의 피드백 정보를 감소하는 방안을 제시한다. 제한된 상향링크 제어채널을 통해 기지국으로 무선 채널 정보를 전송하기 위해서, 제안된 알고리즘은 인근 클러스터간의 무선 채널의 유사 정도를 나타내는 정보로 채널 변화 수준(channel variation level)을 사용하며, 모든 클러스터에 대해서 하나의 MCS(Modulation & Coding Scheme)을 사용한다. 시뮬레이션은 하나의 셀 내에서 수행되었으며, 기존의 알고리즘의 피드백 정보와 유사한 오버헤드로 대역 효율 및 outage probability에서 더 좋은 성능을 나타내었다.

• Journal of Communications and Networks
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• 제5권3호
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• pp.249-257
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• 2003

Ring networks are very commonly exploited among local area and metropolitan area networks (LAN/MAN), whereas cells or small fixed-size packets are widely used in synchronized ring networks. In this paper, we present an analytical method for evaluating the delay-throughput performance of a MAC protocol with multiple self-tokens in a slotted ring network under uniform traffic. In our analysis, we introduce the stationary probability, which indicates the number of packets in a node. Also, it is assumed that each node has a sufficiently large amount of self-tokens, and a slotted ring has the symmetry. The analytical results with respect to delay-throughput performance have similar values to computer simulated ones. Furthermore, in order to achieve fair access under non-uniform traffic, we propose an adaptive MAC protocol, where the number of self-tokens in a node dynamically varies, based on the number of packets transmitted within a specified period. In the proposed protocol, when the number of packets transmitted by a node within a specified period is larger than a specified threshold, the node decreases the number of self-tokens in a per-node distributed method. That results in creating free slots in the ring, thus all nodes can obtain an equal opportunity to transmit into the ring. Performance results obtained by computer simulation show that our proposed protocol can maintain throughput fairness under non-uniform traffic.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제10권4호
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• pp.1768-1789
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• 2016

In contemporary video streaming systems based on HTTP protocol, video players at the client side are responsible for adjusting video quality to network conditions and user expectations. However, when multiple video clips are streamed simultaneously, an intricate application logic implemented in the video players overlays the TCP mechanism which is responsible for a balanced access to a shared network link. As a result, some video players may not obtain a fair share of network throughput and may be vulnerable to an unstable video bit-rate. Therefore, we propose to simplify the algorithms implemented in the video players, which are responsible for the adjustment of video quality and constrain their functionality only to sending feedback to a server about a state of the player buffer. The main logic of the system is shifted to the server, which is now responsible for bit-rate selection and prioritisation of the video streams transmitted to multiple clients. To verify our proposition, we performed several experiments in a laboratory environment which show that when the server cooperates with the clients, the video players experience fewer quality switches and the system achieves better fairness when allocating network throughput among the video players. However, this comes at the cost of worse utilisation of network bandwidth.

• 한국통신학회논문지
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• 제29권9C호
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• pp.1219-1226
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• 2004

본 논문에서는 다중처리이득 기법을 사용한 비동기 다중 전송률 DS-CDMA 시스템에서 발생하는 다중접속 간성을 제거하기 위한 새로운 하이프리도 수신기를 제안한다. 효과적인 간섭제거를 위해, 서로 다른 전송률을 갖는 사용자들을 그룹화 하여 간섭을 제거할 수 있는 새로운 하이브리드 간섭제거 방식을 제시하였다. 즉, 그룹화를 통해 그룹 간에는 병렬 간섭제거방식을 사용하여 그룹간의 간섭을 일차적으로 제거하며 그룹 내에는 직렬 간섭제거방식을 적용하여 그룹 내 사용자간의 간성을 제거하였다. 제안된 시스템을 가우시안 잡음환경 하에서 오류확률을 구했으며, 기존 직렬 간섭제거 방식이나 적응 다단 병렬 간섭제거방식과의 성능비교를 통해 제안된 시스템의 성능이 우수한 것을 알 수 있었다.

• 한국컴퓨터정보학회논문지
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• 제13권4호
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• pp.151-159
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• 2008

기술의 발전과 유선망에서와 동일한 성능을 요구하는 사용자의 기대치 증대로 인해 광대역 무선 이동망은 중요한 연구 분야로 떠오르고 있다. 본 논문에서는 IEEE 802.16e 기반시스템의 매체 접속제어 계층에서 핸드오버 성능 증가를 위한 기법을 제안한다. 이를 위해 본 논문에서는 셀 재설정 과정이라 불리는 스캔 시작과정의 문제점을 식별하고 이를 해결하기 위한 수신 신호 강도 기반 동적 스캔 시작 기법을 제안한다. 모의실험을 통해 제안 기법은 필요한 스캔 지속 시간을 고려한 수신 신호 강도 예측을 통해 적시에 스캔 과정을 시작하여 불필요한 자원의 낭비와 늦은 핸드오버를 야기시키지 않는다는 것을 보였다.

• 한국컴퓨터정보학회논문지
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• 제22권10호
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• pp.83-92
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• 2017

Reconnaissance is performed gathering information from a series of scanning probes where the objective is to identify attributes of target hosts. Network reconnaissance of IP addresses and ports is prerequisite to various cyber attacks. In order to increase the attacker's workload and to break the attack kill chain, a few proactive techniques based on the network-based moving target defense (NMTD) paradigm, referred to as IP address mutation/randomization, have been presented. However, there are no commercial or trial systems deployed in real networks. In this paper, we propose a threat model and the request for requirements for developing NMTD techniques. For this purpose, we first examine the challenging problems in the NMTD mechanisms that were proposed for the legacy TCP/IP network. Secondly, we present a threat model in terms of attacker's intelligence, the intended information scope, and the attacker's location. Lastly, we provide seven basic requirements to develop an NMTD mechanism for the legacy TCP/IP network: 1) end-host address mutation, 2) post tracking, 3) address mutation unit, 4) service transparency, 5) name and address access, 6) adaptive defense, and 7) controller operation. We believe that this paper gives some insight into how to design and implement a new NMTD mechanism that would be deployable in real network.

• ETRI Journal
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• 제38권6호
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• pp.1052-1063
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• 2016

We propose a millimeter wave (MMW)-based mobile hotspot network (MHN) system for application in high-speed railways that is capable of supporting a peak backhaul link throughput of 1 Gbps per train at 400 km/h. The MHN system can be implemented in subways and high-speed trains to support passengers with smart devices and provide access to the Internet. The proposed system can overcome the inherent high path loss in MMW through system designs and high antenna gains. We present a simulation of the system performance that shows that a fixed beamforming strategy can provide high signal-to-interference-plus-noise-ratio similar to those of an adaptive beamforming strategy, with the exception of 15% of the train path in which the network can use link adaptation with low-order modulation formats or trigger a handover to maintain the connection. We also demonstrate the feasibility of the MHN system using a test bed deployed in Seoul subway line 8. The backhaul link throughput varies instantaneously between 200 Mbps and 500 Mbps depending on the SNR variations while the train is running. During the field trial, the smartphones used could make connections through offloading.

• Journal of Communications and Networks
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• 제11권4호
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• pp.375-383
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• 2009

The WiMedia alliance has specified a distributed medium access control (WiMedia MAC) protocol based on ultra wideband (UWB) for high data rate WPANs (HR-WPANs). The merits of WiMedia MAC such as distributed nature and high data rate make it a favorite candidate in HR-WPAN. Although QoS parameters such as the range of service rates are provided to a traffic stream, the WiMedia MAC is not able to use the QoS parameters and to determine or adjust a service rate using the QoS parameters for the traffic stream. In this paper, we propose a fair and adaptive resource allocation method that allocates time slots to isochronous streams according to QoS parameters and the current traffic load condition in a fully distributed manner. Although the traffic load condition changes, each device independently recognizes the changes and calculates fair and maximum allowable service rates for traffic streams. From the numerical and simulation results, it is proved that the proposed method achieves high capacity of traffic streams and fair QoS provisioning under various traffic load condition.

• Journal of Communications and Networks
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• 제14권1호
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• pp.51-62
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• 2012

The exponential growth in wireless services has resulted in an overly crowded spectrum. The current state of spectrum allocation indicates that most usable frequencies have already been occupied. This makes one pessimistic about the feasibility of integrating emerging wireless services such as large-scale sensor networks into the existing communication infrastructure. Cognitive radio is an emerging dynamic spectrum access technology that can be used for flexibly and efficiently achieving open spectrum sharing. Cognitive radio is an intelligent wireless communication system that is aware of its radio environment and that is capable of adapting its operation to statistical variations of the radio frequency. In ad hoc cognitive radio networks, a common control channel (CCC) is usually used for supporting transmission coordination and spectrum-related information exchange. Determining a CCC in distributed networks is a challenging research issue because the spectrum availability at each ad hoc node is quite different and dynamic due to the interference between and coexistence of primary users. In this paper, we propose a novel CCC selection protocol that is implemented in a distributed way according to the appearance patterns of primary systems and connectivity among nodes. The proposed protocol minimizes the possibility of CCC disruption by primary user activities and maximizes node connectivity when the control channel is set up. It also facilitates adaptive recovery of the control channel when the primary user is detected on that channel.