• 한국IT서비스학회:학술대회논문집
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• 2006.05a
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• pp.241-248
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• 2006

네트워크 거리 (Network distance : 일반적으로 ping이나 traceroute 등으로 측정 가능한 round trip time 등 네트워크 상에서 패킷 전송 시간) 추정 기법은 인터넷상의 많은 응용프로그램들에서 유용하게 사용된다. 예를 들면 다수의 서버를 인터넷상에 설치하고자 하는 경우 사용자들 간의 네트워크 거리를 알고 있다면 서버와 사용자간의 왕복 전송 시간 (Round Trip Time)등을 최소화할 수 있도록 서버를 분산하여 설치하는 구성을 도출해 낼 수 있을 것이다. Peer to Peer 응용 프로그램들에서도 이 네트워크 거리 정보는 매우 유용하다. 기존에 존재하는 추정 기법들은 대부분 유클리드 공간 좌표 기반 기법들로서 유클리드 좌표 상의 거리가 실제 네트워크 거리와 유사하도록 유클리드 공간 좌표를 지정한다. 그러나 이런 방법들의 문제점은 인터넷 상의 네트워크 거리가 삼각 부등식을 만족하지 않는 경우가 존재하는 등 유클리드 공간의 기본적인 가정을 만족하지 못한다는데 있다. 이런 문제점 때문에 새로운 모델이 필요하고, 이 논문에서는 가상 토폴로지(Virtual Topology) 모델과 지역 조정 항 (Local Adjustment Term) 모델을 제시하고, 기본적인 성능 분석을 시도하였다.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.7 no.8
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• pp.1674-1683
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• 2003

The performance of a packet data multiplexer with stop­and­wait ARQ protocols under Markovian interruption is considered in this work It is assumed that the input process, into the system is Poisson process, and that the output channel is divided into a series of time slots and a data packet can be transmitted in a slot time. In this system the round­trip propagation delay is defined to be the frame time. It is modeled that the output channel can be blocked by some Markevian interruption, whose state change between the blocking and non­-blocking states is given by Markov process. The overall system has been analyzed by constructing a relationship, taking the Markovian interruption into account, about the buffer behavior between the successive frames of slots. The validity of this analytical results has been verified by computer simulation.

• The KIPS Transactions:PartC
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• v.12C no.3 s.99
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• pp.437-442
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• 2005

The measurement of RTT (Round Trip Time) can be used for the analysis of Internet congestion. However, simple measuring of RTT which measures only hun around time of a packet can not infer a packet forward/backward delay variation. In this thesis, we present a new algorithm which can be used for the estimation of forward/backward delay variation of packets. These delay variations are implication of network congestion state. In this algorithm, the reference forward/backward delay can be determined based on the minimum RTT value. The delay variation of each packet can be calculated by comparing reference delay with the packet delay. We verified our proposed algorithm by NS-2 simulation and delay measuring in a real network.

• Microbiology and Biotechnology Letters
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• v.6 no.2
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• pp.75-80
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• 1978

This study was conducted to investigate the condition of enhancing glutathione content of yeast. Rhodotorula glutinis in various kind of yeasts produced high content of glutathione and dry cell by cultivating for 48 hours at 30'C and pH 6.0 on reciprocal shaker. In order to enhance the glutathione content, as 0.7％ of amino acid was applied into the medium. Glutathione was produced high for 36 hours cultivation. When glutamic acid, cysteine and glycine composing the glutathione were added, glutathione content increased to 219 $\mu\textrm{g}$/ml. However the control showed to 73 $\mu\textrm{g}$/ml.

• Journal of Satellite, Information and Communications
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• v.5 no.2
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• pp.92-96
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• 2010

We propose a new satellite OFDMA(Orthogonal Frequency Division Multiple Access) scheme with greatly enhanced tolerance of timing offset among the users. In uplink OFDMA systems, timing misalignment among users destroys subcarrier orthogonality and thus, it degrades the performance. In order to avoid this performance degradation, the accurate processing, so called 'ranging', is required to synchronize among users. However, ranging scheme is not available in the satellite systems due to the very long round trip delay. Exploiting the property that PSW(Propoerly Scrambled Walsh-code) code has zero correlation despite ${\pm}1$ chip timing offset, the proposed OFDMA achieves MAI free performance with the timing offset up to ${\pm}1$ OFDM symbol duration for the satellite systems.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.44 no.8
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• pp.33-41
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• 2007

The IEEE 802.11 DCF channel access function allows single transmission inside two-hop network in order to avoid collisions and eliminate the hidden and exposed terminal problems. Singular transmission capability causes data frames waiting for the entire roundtrip time in the transmitter neighborhood, and results in increased frame latency and lower network throughput. Real-time and pervasive applications are severely affected for the lower medium utilization; especially with high network traffic. This work proposes a new scheme with the help of Frame Aggregation technique in IEEE802.11n and overcomes the single transmission barrier maintaining the basic DCF functionality. Proposed scheme allows parallel transmissions in non-interfering synchronized slots. Parallel transmissions bypass the conventional physical carrier sense and random Backoff time for several cases and reduce the frame latency and increase the medium utilization and network capacity.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• v.20 no.4
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• pp.375-381
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• 2002

Depth of water information is obtained mainly from echo-sounding instrument which observes the round-trip time of signal from water surface to the bottom. Photogrammetry, underwater survey and laser survey etc. are also used as another method of bathymetric surveying. These methods are used specially for making track chart in a shallow water area. On the other hand, aircraft or satellite imagery ara also used in the sea area where the effect of suspended material is low and water quality is good. Presently, general bathymetric surveying has been performed in our country, but the spatial density of surveyed point are relatively low. Therefore, in this study we built a grid water depth chart which measured combing echosounder with GPS-RTK method and the depth accuracy was analyzed by using the data of direct survey water depth. As a results, the bathymatric mapping which use echosounder is more economical method compared to the existing methods.

• Journal of Digital Contents Society
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• v.16 no.3
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• pp.381-388
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• 2015

TCP, which performs congestion control in congestion condition, is able to help a reliable transmission. However, packet loss can be increased because congestion window is increased by the time the packet is dropped in the process of congestion avoidance. In this paper, to solve the above problem, we propose a new congestion estimation based TCP congestion control scheme using the weighted average value of the RTT. After measuring a SRTT, which means the weighted average value of RTTs, at this point of time when a buffer overflow is occurred by an overloaded packet, the proposed scheme estimates the time, when the same SRTT is made in packet transmission, as a congestion time and then decreases the congestion window. The simulation results show that the proposed schem has a good performance in terms of packet loss rate and throughput when the packet loss due to buffer overflow is larger than that due to wireless channel.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.8
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• pp.637-648
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• 2021

This paper aims to propose new reentry guidance for Korean Space plane (KSP). Similar to the Space Shuttle guidance concept, a reference drag profile is first determined to satisfy several flight path constraints and boundary conditions, and the proposed guidance commands are realized in a way to track the predetermined reference drag profile. To this end, the drag dynamics is examined. The investigation uncovers that the dynamics characteristics of the drag and the flight path angle are considerably different. Based on this fact, the proposed guidance commands are determined using the time-scale separation technique and the feedback linearization methodology. The key feature of the proposed guidance lies in its simple structure and a clear working mechanism. Therefore, the proposed method is simple to implement compared to existing methods. Numerical simulations are performed to investigate the performance of the proposed method.

• The KIPS Transactions:PartC
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• v.17C no.4
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• pp.307-314
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• 2010

Recently, it is proved that contactless smart-card based RFID tags, which is used for proximity authentication, are vulnerable to relay attacks with various location-based attacks such as distance fraud, mafia fraud and terrorist fraud attacks. Moreover, distance bounding protocols have been researched to prevent these relay attacks that can measure the message transmitted round-trip time between the reader and the tag. In 2005, Hancke and Kuhn first proposed an RFID distance bounding protocol based on secure hash function. However, the Hancke-Kuhn protocol cannot completely prevent the relay attacks because an adversary has (3/4)$^n$ attack success probability. Thus, this paper proposes a new distance-bounding protocol for light-weight RFID systems that can reduce to (5/8)$^n$ for the adversary's attack success probability. As a result, the proposed protocol not only can provide high-space efficient based on a secure hash function and XOR operation, but also can provide strong security against the relay attacks because the adversary's attack success probability is optimized to (5/8)$^n$.