• Journal of Communications and Networks
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• v.7 no.3
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• pp.313-324
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• 2005

The success of the IEEE 802.11 standard has prompted research into efficiency of the different medium access methods and their support for different traffic types. A modified version of the point coordination function (PCF) called modified PCF has been introduced as a way to improve the efficiency over the standard method. It has been shown through a simulation study and a mathematical analysis that channel utilization can be much improved compared to the standard, in case there is no so-called hidden station problem. However, under the hidden station problem, the efficiency of the modified PCF would obviously decrease. In this paper, some enhancements of the modified PCF are introduced. Firstly, we propose a retransmission process to allow frames involved in collisions to be retransmitted. Then, we propose a collision resolution mechanism to reduce the frame collision probability due to the hidden station problem. In addition, we propose a priority scheme to support prioritization for different traffic types such as interactive voice and video, and real-time data traffic in the modified PCF. To prevent the starvation of one low priority traffic, minimum transmission period is also guaranteed to each traffic type via an admission control algorithm. We study the performance of the modified PCF under the hidden station problem and the performance of the modified PCF with priority scheme through simulations. To illustrate the efficiency of the priority scheme, we therefore compare its simulation results with those of some standardized protocols: The distributed coordination function (DCF), the enhanced distributed channel access (EDCA), the PCF, and our previously proposed protocol: The modified PCF without priority scheme. The simulation results show that the increment of delay in the network due to the hidden station problem can be reduced using the proposed collision resolution mechanism. In addition, in a given scenario the modified PCF with priority scheme can provide better quality of service (QoS) support to different traffic types and also support a higher number of data stations than the previous proposals.

• Journal of IKEEE
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• v.13 no.3
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• pp.7-17
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• 2009

WiMedia UWB technology is a fully distributed data communication technology developed for the application demanding a high data transmission rate in the wireless PAN area. In general, devices can send data either by reserving time slots or by using prioritized CSMA/CA. If the PCA protocol of prioritized CSMA/CA is used, they are suffered congestion as the number of devices increases. In this paper, we propose a Deterministic Access Protocol(DAP) in WiMedia WPANs. A DAP is a method to transmit data in the non-reserved DRP period without competition as each device informs the beacon order information in the beacon period and the queue information. In addition, the problem that the devices with a lower beacon slot number have more transmission opportunities is addressed by introducing the reference point. Simulation results are given to demonstrate that a DAP can improve the throughput and reduce the packet loss rate.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2007.10a
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• pp.223-228
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• 2007

In this paper, Wideband antenna using E-shaped stacked patches has been designed and fabricated for 4th generation mobile communication(IMT-Advanced) AccessPoint application. E-shaped patch was miniature and brodband by made the movement route of current long. And inductive of coaxial probe compensates capacitive by slot. Therefore we fabricated to improve the bandwidth of proposed antenna. The E-shaped single patch antenna has an impedance bandwidth of about 13%(510[MHz]), By adding a second patch at the top of the first patch a bandwidth of 56%(2060[MHz]). The final fabricated antenna could have a good return loss(Return loss ${\leq}-10dB$) and a high gain(over 9.6dBi) at the range of 3.23 ${\sim}$ 5.29 [GHz].

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.9A
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• pp.892-899
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• 2006

In this paper, we propose an automatic switching technology between the ad-hoc and the infrastructure modes without user intervention in the IEEE 802.11b wireless LAN. Also, we design our proposed technology and implemented on the Linux machine. For this operation, the area within an Access Point (AP) coverage is defined as a switching area, and a node without any transmission in this area is assumed to be able to relay frames between the AP and nodes in the shaded area that is outside the coverage and cannot reach the AP. By using the proposed technology, it is possible to provide the seamless Internet access service to nodes at the ad-hoc mode in the shaded area. In this paper, we explains the operation of the detection method of the switching area, presents the flowchart and implementation environment. To prove the operation of our technology, we obtain the results of captured packets transmitted between nodes and throughput results through ftp transmission experiment. Hence, we can see that our proposed scheme can be improve the wireless access service in wireless and mobile networks.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.9 no.2
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• pp.388-394
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• 2008

In this paper, MAC(Medium Access Control) algorithm for the IEEE 802.11 DCF(Distributed Coordination Function) improving the performance is proposed and analyzed by simulation. The MAC of IEEE 802.11 WLAN to control data transmission uses two control methods called DCF and PCF(Point Coordination function). The DCF controls the transmission based on CSMA/CA(Carrier Sense Multiple Access With Collision Avoidance). The DCF shows excellent performance relatively in situation that competition station is less but has a problem that performance is fallen from throughput and delay viewpoint in situation that competition station is increased. This paper proposes an enhanced DCF algorithm that increases the CW to maximal CW after collision and decreases the CW smoothly after successful transmission in order to reduce the collision probability by utilizing the current status information of WLAN. To prove efficiency of proposed algorithm, a lots of simulations are conducted and analyzed.

• Journal of Information Management
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• v.38 no.1
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• pp.21-36
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• 2007

This study aimed to investigate present status of online reference shelf and analyze access point in the home page and types of online reference sources in the university library. Data were collected by accessing home pages of 41 university libraries in the region of Seoul and Kyonggi-Do. 30 libraries had prepared reference shelf out of 41 libraries, and "User Service", "Electronic Reference Sources", or "Reference & Information Sources" were mostly used as home page menu. Types of references sources varied among libraries, and "Directories", "Dictionaries", "Statistics" or "Encyclopedias" were mostly loaded types in the reference shelf. For the enhancement of reference services through reference shelf, establishing and applying standards for evaluation, ease of access, access by subject, and continuous updates and additions of reference sources were suggested.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.5
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• pp.3440-3446
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• 2015

One of technologies that make it possible for users to enjoy always-on Internet service using potable devices does be Wifi. However, users frequently feel its connection is not stable or its speed becomes slow. On the other hand, users see multiple available Wifi access points exist often. In this paper, we present a smartphone application, called to WiSMax, that periodically changes its Wifi connection to the best one among available Wifi access points to maximize its speed. Experimental results show that WiSMax presents faster Internet connection than IEEE802.11 as much as about fifteen percent and higher signal strength than IEEE802.11 as much as about five dBm.

• The Journal of The Korea Institute of Intelligent Transport Systems
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• v.13 no.4
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• pp.57-62
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• 2014

Recently, research activities to develop services for providing safety information to the drivers in fast moving vehicles based on various wireless network technologies such as DSRC (Dedicated Short Range Communication), IEEE 802.11p WAVE (Wireless Access for Vehicular Environment) are widely being carried out. This paper presents a proof-of-concept model based on a robot-car for Dilemma Zone Decision Assistant Service using the wireless LAN technology. The proposed model system consists of a robot-car based on an embedded Linux OS equipped with a WiFi interface and an on-board unit emulator, an Android-based remote controller to model a human driver interface, a laptop computer to run a model traffic signal controller and signal lights, and a WiFi access point to model a road-side unit.

• Journal of the Korean Society for Library and Information Science
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• v.44 no.2
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• pp.51-72
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• 2010

The purpose of this study is to explore usefulness of the Library of Congress Subject Headings(LCSH)for librarians as an information access point. Sixty four librarians at the University of Washington(UW)and 67 librarians from the Council on East Asian Libraries(CEAL) participated through web surveys about LCSH and 9 librarians from UW and 10 librarians from CEAL joined in-depth interviews and e-mail correspondences. It was concluded that LCSH was very useful for finding other related materials as a subject access point. However, there were two necessary improvements recommended for the LCSH. First of all, in aspect of terminology, subject headings should be more familiar to the public with currency and show clear definitions and relationships. Second, with regard to the structure of LCSH, it should have simplicity, ease of use, and consistency when using combinations of subject headings. Since many other countries are using or adapting LCSH, its effect can be very strong. Therefore, the meaning of "Tok Island (Korea)" as well as diplomatic and scholarly efforts with sensing movements of other countries.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.12 no.5
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• pp.267-273
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• 2012

The short-range communication module was developed in this paper which is suitable for the point-to-multipoint circumstances of 40 terminals communicating frequently in about 50 meters. The media access control layer of this communication module using asynchronous time-division multiplexing provides a fast and robust performance even in the worst case of simultaneous transmission events, and low packet error rate was measured a in LOS (Line-of-sight) circumstance by adding the function of acknowledge response to media access control layer. The difference test was carried out in order to measure the performance of point-to-multipoint communication. Two communication modules are respectively measured and graphed in 10 branches to 40 branches. The communication module developed in this paper showed a faster performance than the commercial Zigbee module in the specific case presented in this paper. Especially, in over 20 branches showed wide differences of the transmission speed. This results is caused by more network overhead of Zigbee whose wider applications needs the network layer and applicaiton layer besides media access control layer. Also, the asynchronous time-division multiplexing proposed in this paper are more suitable than CSMA-CA of Zigbee module when a lot of module ought to be frequently communicated in small area.