• Journal of Information Processing Systems
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• 제10권1호
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• pp.119-131
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• 2014

In recent literature on traffic scheduling, the combination of the two-dimensional discrete-time Markov chain (DTMC) and the Markov modulated Poisson process (MMPP) is used to analyze the capacity of VoIP traffic in the cognitive radio system. The performance of the cognitive radio system solely depends on the accuracy of spectrum sensing techniques, the minimization of false alarms, and the scheduling of traffic channels. In this paper, we only emphasize the scheduling of traffic channels (i.e., traffic handling techniques for the primary user [PU] and the secondary user [SU]). We consider the following three different traffic models: the cross-layer analytical model, M/G/1(m) traffic, and the IEEE 802.16e/m scheduling approach to evaluate the performance of the VoIP services of the cognitive radio system from the context of blocking probability and throughput.

• 전자공학회논문지CI
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• 제45권5호
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• pp.125-133
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• 2008

본 논문은 다양한 서비스 클래스가 제공되는 BcN (Broadband convergence Network) 환경에서 상위 우선순위 서비스의 품질을 보장하기 위한 능동 큐 관리 메커니즘 (Active-WRED)을 제안한다. 네트워크 혼잡 상황에서 제안된 메커니즘은 상대적으로 낮은 우선순위를 갖는 서비스의 drop 확률을 증가시키고 이에 따라 상위 우선순위 서비스의 drop 확률을 감소시킴으로써 상위 우선순위 서비스에 대해 패킷 손실로부터 품질을 보장할 수 있다. 제안된 능동 큐 관리 메커니즘의 성능을 분석하기 위해 이산 시간 큐잉 시스템의 통계적 분석 방법을 도입하였다. 두 서비스 클래스 (GS: Guaranteed Service, BS: Best Effort Service)만을 고려하여 제안 방안의 성능 분석을 위해 MMBP-2 (two-state Markov-Modulated Bernoulli arrival Process) 트래픽 소스 모델과 2차 이산시간 Markov 체인을 도입하였다.

• Journal of information and communication convergence engineering
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• 제20권2호
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• pp.79-89
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• 2022

To compute the mean and variance of component-based reliability software, we focused on path-based reliability analysis. System reliability depends on the transition probabilities of components within a system and reliability of the individual components as basic input parameters. The uncertainty in these parameters is estimated from the test data of the corresponding components and arises from the software architecture, failure behaviors, software growth models etc. Typically, researchers perform Monte Carlo simulations to study uncertainty. Thus, we considered a Markov chain Monte Carlo (MCMC) simulation to calculate uncertainty, as it generates random samples through sequential methods. The MCMC approach determines the input parameters from the probability distribution, and then calculates the average approximate expectations for a reliability estimation. The comparison of different techniques for uncertainty analysis helps in selecting the most suitable technique based on data requirements and reliability measures related to the number of components.

• Journal of Communications and Networks
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• 제15권1호
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• pp.77-86
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• 2013

Cognitive radio (CR) has emerged as one of effective methods to enhance the utilization of existing radio spectrum. Main principle of CR is that secondary users (SUs) are allowed to use the spectrum unused by primary users (PUs) without interfering PU's transmissions. In this paper, PUs operate on a slot-by-slot basis and SUs try to exploit the slots unused by PUs. We propose OSA protocols in the single channel and we propose an opportunistic spectrum access (OSA) protocols in the multi-channel cognitive radio networks with one control channel and several licensed channels where a slot is divided into contention phase and transmission phase. A slot is divided into reporting phase, contention phase and transmission phase. The reporting phase plays a role of finding idle channels unused by PUs and the contention phase plays a role of selecting a SU who will send packets in the data transmission phase. One SU is selected by carrier sense multiple access / collision avoidance (CSMA/CA) with request to send / clear to send (RTS/CTS) mechanism on control channel and the SU is allowed to occupy all remaining part of all idle channels during the current slot. For mathematical analysis, first we deal with the single-channel case and we model the proposed OSA media access control (MAC) protocol by three-dimensional discrete time Markov chain (DTMC) whose one-step transition probability matrix has a special structure so as to apply the censored Markov chain method to obtain the steady state distribution.We obtain the throughput and the distribution of access delay. Next we deal with the multi-channel case and obtain the throughput and the distribution of access delay by using results of single-channel case. In numerical results, our mathematical analysis is verified by simulations and we give numerical results on throughput and access delay of the proposed MAC protocol. Finally, we find the maximum allowable number of SUs satisfying the requirements on throughput and access delay.

• Earthquakes and Structures
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• 제24권4호
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• pp.303-316
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• 2023

Rapid post-earthquake damage estimation of subway stations is particularly necessary to improve short-term crisis management and safety measures of urban subway systems after a destructive earthquake. The conventional Performance-Based Earthquake Engineering (PBEE) framework with constant earthquake occurrence rate is invalid to estimate the aftershock risk because of the time-varying rate of aftershocks and the uncertainty of mainshock-damaged state before the occurrence of aftershocks. This study presents a time-varying probabilistic seismic risk assessment framework for underground structures considering mainshock and aftershock hazards. A discrete non-omogeneous Markov process is adopted to quantify the time-varying nature of aftershock hazard and the uncertainties of structural damage states following mainshock. The time-varying seismic risk of a typical rectangular frame subway station is assessed under mainshock-only (MS) hazard and mainshock-aftershock (MSAS) hazard. The results show that the probabilities of exceeding same limit states over the service life under MSAS hazard are larger than the values under MS hazard. For the same probability of exceedance, the higher response demands are found when aftershocks are considered. As the severity of damage state for the station structure increases, the difference of the probability of exceedance increases when aftershocks are considered. PSDR=1.0% is used as the collapse prevention performance criteria for the subway station is reasonable for both the MS hazard and MSAS hazard. However, if the effect of aftershock hazard is neglected, it can significantly underestimate the response demands and the uncertainties of potential damage states for the subway station over the service life.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제7권4호
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• pp.726-747
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• 2013

In order to support a large number of mobile stations (MSs) with statistical multiplexing in cellular networks, a random access scheme is widely used for uplink (UL) bandwidth request (BR). In the design of a random access based BR scheme, there are two important requirements: short connection delay and diverse Quality of Services (QoSs) support. Such requirements are crucial for IMT-Advanced systems like IEEE 802.16m to provide various types of fourth generation (4G) data services. IEEE 802.16m provides advanced UL BR schemes for non-real time polling service (nrtPS) and best-effort (BE) service to meet the requirements of short connection time and multiple QoS level support. In order to provide short connection time and multiple QoS support, three-step and differentiated BR procedures are adopted. In this paper, a novel modelling of IEEE 802.16m contention based BR scheme is proposed that uses a 2-dimensional discrete time Markov chain. Both the short access delay three-step BR procedures and normal five-step BR procedure are considered in the model. Our proposed model also incorporates the IEEE 802.16m differentiated BR procedure. With the proposed model, we extensively evaluate the performance of IEEE 802.16m BR for two different service classes by changing QoS parameters, such as backoff window size and BR timer. Computer simulations are performed to corroborate the accuracy of the proposed model for various operation scenarios. With the proposed model, accurate QoS parameter values can be derived for the IEEE 802.16m contention-based BR scheme.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2016년도 학술발표회
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• pp.197-197
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• 2016

Forecasting future drought events in a region plays a major role in water management and risk assessment of drought occurrences. The creeping characteristics of drought make it possible to mitigate drought's effects with accurate forecasting models. Drought forecasts are inevitably plagued by uncertainties, making it necessary to derive forecasts in a probabilistic framework. In this study, a new probabilistic scheme is proposed to forecast droughts, in which a discrete-time finite state-space hidden Markov model (HMM) is used aggregated with the Representative Concentration Pathway 8.5 (RCP) precipitation projection (HMM-RCP). The 3-month standardized precipitation index (SPI) is employed to assess the drought severity over the selected five stations in South Kore. A reversible jump Markov chain Monte Carlo algorithm is used for inference on the model parameters which includes several hidden states and the state specific parameters. We perform an RCP precipitation projection transformed SPI (RCP-SPI) weight-corrected post-processing for the HMM-based drought forecasting to derive a probabilistic forecast that considers uncertainties. Results showed that the HMM-RCP forecast mean values, as measured by forecasting skill scores, are much more accurate than those from conventional models and a climatology reference model at various lead times over the study sites. In addition, the probabilistic forecast verification technique, which includes the ranked probability skill score and the relative operating characteristic, is performed on the proposed model to check the performance. It is found that the HMM-RCP provides a probabilistic forecast with satisfactory evaluation for different drought severity categories, even with a long lead time. The overall results indicate that the proposed HMM-RCP shows a powerful skill for probabilistic drought forecasting.

• 한국ITS학회 논문지
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• 제11권4호
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• pp.104-111
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• 2012

차량 이동환경에서 빈번하게 발생하는 핸드오버는 서비스 품질 저하의 주된 원인이다. 본 논문에서는 규칙적인 이동경로를 갖는 차량의 이동 패턴을 학습하여 핸드오버 제어를 수행함으로서 핸드오버 빈도를 줄이고, 체류 시간이 매우 짧거나 이동 가능성이 낮은 셀로의 핸드오버를 방지하는 연구를 수행하였다. 이를 위해, 차량의 이동 패턴을 Markov Chain으로 모델링하였으며, 각 셀에서의 체류시간과 평균 신호의 세기를 관찰하여 핸드오버 제어에 이용하였다. 제안한 이동성 관리 기법의 검증을 위해 4개의 서울 시내버스 노선에 대해 적용하였으며, 실험 결과 제안 기법이 기존의 핸드오버 기법보다 핸드오버 빈도를 줄일 수 있으며, 버스 왕복 구간 동안의 평균 처리율이 더 높음을 확인할 수 있었다.

• 한국통신학회논문지
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• 제11권1호
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• pp.25-39
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• 1986

본 논문에서는 packet switching network의 국제 표준 접속 protocol인 CCITT X.25 protocol의 성능을 분석하며 특히 X.25 protocol의 유통 제어 방식에 대하여 분석한다. Protocol의 성능 분석은 normalized channel throughput, mean transmission time과 transmission efficiency를 사용하면 이들은 window 크기, $T_1$ 및 $T_2$ 값 그리고 message길이 등과 같은 주어진 protocol parameter의 함수로 표시된다. 먼저 protocol 서비스에 따른 입력 데이타의 서비스 특성과 piggybacked acknowledgment를 하는 sliding window flow control 방식에 대하여 discrete-time Markov chain을 사용하여 연구한다. Protocol의 성능은 link layer 및 packet layer 에 대하여 각기 독립적으로 분석하며 분석결과를 통하여 각 protocol parameter의 영향을 조사한다. 수치적인 분석 결과로 부터 채널 서비스 환경에 따른 protocol parameter의 최적치를 찾을 수 있는데 window크기는 고속채널의 경우 7이상이 되는 것이 바람직 하며, $T_1$ timer 값은 채널의 전송 유실이 많은 경우 신중히 선택되어져야 하며 보통의 경우에는 1초 정도가 타당하다. $T_2$ parameter는 trnasmission efficiency의 개선에 있어 약간의 효과를 미치나 그리 크지는 않다.

• 융합보안논문지
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• 제8권1호
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• pp.91-100
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• 2008

The previous works in sensor networks security have focused on the aspect of confidentiality, authentication and integrity based on cryptographic primitives. There has been no prior work to assess the survivability in systematic way. Accordingly, this paper presents a survivability model of wireless sensor networks using software rejuvenation for dual adaptive cluster head. The survivability model has state transition to reflect status of real wireless sensor networks. In this paper, we only focus on a survivability model which is capable of describing cluster head compromise in the networks and able to switch over the redundant cluster head in order to increase the survivability of that cluster. Second, this paper presents how to enhance the survivability of sensor networks using software rejuvenation methodology for dual cluster head in wireless sensor network. We model and analyze each cluster as a stochastic process based on Semi Markov Process (SMP) and Discrete Time Markov Chain (DTMC). The proof of example scenarios and numerical analysis shows the feasibility of our approach.