• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.10B
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• pp.891-899
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• 2006

Generally, a geographical area consists of many regions called cells in wireless networks. Each mobile host(MH) in a cell communicates with a base station(BS) located in the center of the cell. The BSs are connected to each other by a static wired network. Since users are expected to move around during communication sessions, one of the most important QoS factors is related to the handoff. But, the handoff could fail due to unavailability of sufficient bandwidth in the next cell. As the individual cell size gradually shrinks to accommodate increasing the number of MHs, the probability of successful handoffs can be dropped. In this paper, we suppose a bandwidth reservation method to guarantee MHs against connection failure in case of frequent handoffs. This method predicts the mobility based on the geographical information and assigns the bandwidth reservation in proportion to the speed of MHs' motion. As a result, we can expect more exact moving path of MH and to reduce the waste of bandwidth.

• Journal of Internet Computing and Services
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• v.10 no.4
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• pp.199-211
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• 2009

Recent advancements in wireless networks have enabled support for mobile applications to transfer data over heterogeneous wireless paths in parallel using data striping technique [2]. Traditionally, high performance data transfer requires tuning of multiple TCP sockets, at sender's end, based on bandwidth delay product (BDP). Moreover, traditional techniques like Automatic TCP Buffer Tuning (ATBT), which balance memory and fulfill network demand, is designed for wired infrastructure assuming single flow on a single socket. Hence, in this paper we propose a buffer tuning technique at senders end designed to ensure high performance data transfer by striping data at transport layer across heterogeneous wireless paths. Our mechanism has the capability to become a resource management system for transport layer connections running on multi-homed mobile host supporting features for wireless link i.e. mobility, bandwidth fluctuations, link level losses. We show that our proposed mechanism performs better than ATBT, in efficiently utilizing memory and achieving aggregate throughput.

• Journal of the Korea Society of Computer and Information
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• v.12 no.4
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• pp.149-158
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• 2007

A In the rapidly changing media environment, radio needs to be digitized in order to provide regular compatible service. Especially, most medium high frequency Medias have program contents thar are similar to the standard FM bandwidth, so digitizing radio will bring less effect than it is expected. However, the characteristics of radio medias are mobility, individuality, site-to-site, wide service coverage, immediate delivery and publicity, so it is necessary to study the future of radio media to provide continuous services, moreover, the main discussion regarding balanced development between medias will be the trend of the digitization of radio. Also it is easy and will cost less to change compared to other medias. AM was the first media to broadcast in Korea, and its network is spread all over Korea, also the receivers are the most widely distributed which means the signal reaches everywhere in Korea. In this study, the proper service model for AM digital radio is provided in this environment in which all Medias are rapidly digitizing due to the advancement of digital technology. The results of experimental are based on library study and KBS data of sound wave.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.490-490
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• 2011

Graphene, hexagonal network of carbon atoms forming a one-atom thick planar sheet, has been emerged as a fascinating material for future nanoelectronics. Huge attention has been captured by its extraordinary electronic properties, such as bipolar conductance, half integer quantum Hall effect at room temperature, ballistic transport over ${\sim}0.4{\mu}m$ length and extremely high carrier mobility at room temperature. Several approaches have been developed to produce graphene, such as micromechanical cleavage of highly ordered pyrolytic graphite using adhesive tape, chemical reduction of exfoliated graphite oxide, epitaxial growth of graphene on SiC and single crystalline metal substrate, and chemical vapor deposition (CVD) synthesis. In particular, direct synthesis of graphene using metal catalytic substrate in CVD process provides a new way to large-scale production of graphene film for realization of graphene-based electronics. In this method, metal catalytic substrates including Ni and Cu have been used for CVD synthesis of graphene. There are two proposed mechanism of graphene synthesis: carbon diffusion and precipitation for graphene synthesized on Ni, and surface adsorption for graphene synthesized on Cu, namely, self-limiting growth mechanism, which can be divided by difference of carbon solubility of the metals. Here we present that large area, uniform, and layer controllable graphene synthesized on Cu catalytic substrate is achieved by acetylene-assisted CVD. The number of graphene layer can be simply controlled by adjusting acetylene injection time, verified by Raman spectroscopy. Structural features and full details of mechanism for the growth of layer controllable graphene on Cu were systematically explored by transmission electron microscopy, atomic force microscopy, and secondary ion mass spectroscopy.

• Journal of the Korea Society of Computer and Information
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• v.11 no.5 s.43
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• pp.175-181
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• 2006

AMobile-supported spatial database systems have been introduced with development of mobile computing technology, they offer an advantage that we can access the data wherever we are. However, the studies on database system in traditional distributed environments must be reconsidered again, because of characteristics of mobile database system such as mobility and instability of wireless network. A new transaction model should be required for considering the characteristics, since especially a mobile computing environment is location-dependent, compared with the location transparency in a distributed computing environment. In other aspect, normally mobile host may be moving not in all space, but in some limited path. That is, a host is moving along the roads or the geographical features adjacent to the roads. The transaction processing should be considered not on the spatial coordinate but on the geographical features. In this paper, the feature based transaction processing model is proposed to process location-dependent transaction efficiently.

• Journal of Satellite, Information and Communications
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• v.6 no.2
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• pp.51-59
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• 2011

To support seamless IPTV services in IP-based wireless network, delay or interruption time must be minimized because noticeable interruption will make IPTV service users unhappy. A number of protocols have been proposed for solving mobility problem. However effective seamless handover mechanism to support mobile IPTV services yet to be solved. Mobile WiMAX offers a wireless solution in the access networks that can support IPTV services. Goal of this paper is to identify some reasons of delay time during handover process in an IPv6 capable Mobile WiMAX and to perform handover delay of some optimization scenarios given by existing standardization and proposed improvement.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.5 no.9
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• pp.1492-1512
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• 2011

Due to multiple hops, mobility and time-varying channel, supporting delay sensitive real-time traffic in wireless local area network-based (WLAN) mesh networks is a challenging task. In particular for real-time traffic subject to medium access control (MAC) layer control overhead, such as preamble, carrier sense waiting time and the random backoff period, the performance of real-time flows will be degraded greatly. In order to support real-time traffic, an efficient adaptive packet scheduling (APS) scheme is proposed, which aims to improve the system performance by guaranteeing inter-class, intra-class service differentiation and adaptively adjusting the packet length. APS classifies incoming packets by the IEEE 802.11e access class and then queued into a suitable buffer queue. APS employs strict priority service discipline for resource allocation among different service classes to achieve inter-class fairness. By estimating the received signal to interference plus noise ratio (SINR) per bit and current link condition, APS is able to calculate the optimized packet length with bi-dimensional markov MAC model to improve system performance. To achieve the fairness of intra-class, APS also takes maximum tolerable packet delay, transmission requests, and average allocation transmission into consideration to allocate transmission opportunity to the corresponding traffic. Detailed simulation results and comparison with IEEE 802.11e enhanced distributed channel access (EDCA) scheme show that the proposed APS scheme is able to effectively provide inter-class and intra-class differentiate services and improve QoS for real-time traffic in terms of throughput, end-to-end delay, packet loss rate and fairness.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.15 no.6
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• pp.2148-2167
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• 2021

The proliferation of the Internet of Things (IoT) technologies and applications, especially the rapid rise in the use of mobile devices, from individuals to organizations, has led to the fundamental role of secure wireless networks in all aspects of services that presented with many opportunities and challenges. To ensure the CIA (confidentiality, integrity and accessibility) security model of the networks security and high efficiency of performance results in various resource-constrained applications and environments of the IoT platform, DDO-(data-driven operation) based constructions have been introduced as a primitive design that meet the demand of high speed encryption systems. Among of them, the TMN-family ciphers which were proposed by Tuan P.M., Do Thi B., etc., in 2016, are entirely suitable approaches for various communication applications of wireless mobile networks (WMNs) and advanced wireless sensor networks (WSNs) with high flexibility, applicability and mobility shown in two different algorithm selections, TMN64 and TMN128. The two ciphers provide strong security against known cryptanalysis, such as linear attacks and differential attacks. In this study, we demonstrate new probability results on the security of the two TMN construction versions - TMN64 and TMN128, by proposing efficient related-key recovery attacks. The high probability characteristics (DCs) are constructed under the related-key differential properties on a full number of function rounds of TMN64 and TMN128, as 10-rounds and 12-rounds, respectively. Hence, the amplified boomerang attacks can be applied to break these two ciphers with appropriate complexity of data and time consumptions. The work is expected to be extended and improved with the latest BCT technique for better cryptanalytic results in further research.

• Journal of the Korea Institute of Information Security & Cryptology
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• v.15 no.2
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• pp.81-94
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• 2005

Recently, wireless LAN circumstance is being widely deployed in Public spots. Many People use Portable equipments such as PDA and laptop computer for multimedia applications, and also demand of mobility support is increasing. However, handoff latency is inevitably occurred between both APs when clients move from one AP to another. To reduce handoff latency. in this paper, we suggest WFH(Weighted Frequent Handoff) using effective data structure. WFH improves cache hit ratio using a new cache replacement algorithm considering the movement pattern of users. It also reduces unessential duplicate traffics. Our algorithm uses FHR(Frequent Handoff Region) that can change pre-authentication lesion according to QoS based user level, movement Pattern and Neighbor Graph that dynamically captures network movement topology.

• The Journal of Society for e-Business Studies
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• v.27 no.2
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• pp.193-204
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• 2022

Smart card data is representative mobility data and can be used for policy development by analyzing public transportation usage behavior. This paper deals with the problem of classifying metro stations using metro usage patterns as one of these studies. Since the previous papers dealing with clustering of metro stations only considered traffic among usage behaviors, this paper proposes clustering considering traffic time as one of the complementary methods. Passengers at each station were classified into passengers arriving at work time, arriving at quitting time, leaving at work time, and leaving at quitting time, and then the estimated shape parameter was defined as the characteristic value of the station by modeling each transit time to Weibull distribution. And the characteristic vectors were clustered using the K-means clustering technique. As a result of the experiment, it was observed that station clustering considering pass time is not only similar to the clustering results of previous studies, but also enables more granular clustering.