• Journal of the Korea Institute of Information and Communication Engineering
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• v.19 no.6
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• pp.1333-1340
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• 2015

IEEE 802.11 MAC(Media Access Control) defines DCF(Distributed Coordination Function) for data transmission control. BEB(Binary Exponential Backoff) algorithm of DCF has a problem that if the number of stations connected are over a certain threshold, it degrades network performance because of packet collisions caused from the minimum contention window size. To cope with this problem, we proposed a novel algorithm, named as VBS(Variable Backoff Stage) algorithm, which adjusts the rate of backoff stage increment depending on the number of stations associated with an AP(Access Point). Analytic model of proposed algorithm was derived and simulations on the BEB and the VBS algorithms have been conducted on the OFDM (Orthogonal Frequency Division Multiplexing) method. Simulation results showed that when the rate of backoff state increment was 5 and 10, the number of retransmission were reduced to 1/5 and 1/10 comparing to that of BEB, respectively. Our algorithm showed improvement of 19% and 18% in network utilization, respectively. Packet delay was reduced into 1/12.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.8 no.3
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• pp.541-546
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• 2004

Space-Time Coding(STC) is a technique that utilizes joint correlation of transmitted signals in both time and space domains. Through this approach, diversity and coding gains can be simultaneously obtained. In this paper, we use SPW simulation tool to implement the IEEE 802.11a system. Based on this system, OFDM system with STC and convolutional coder concatenated is implemented. The system performance is analyzed and compared with the performance of the IEEE 802.11a system. The simulation results show that the performance with concatenated codes at a data rate of 6Mbps shows approximately a 5dB gain over the system with the convolutional code only. At a data rate of 12Mbps, the performance with concatenated codes is further improved by approximately 6dB.

• Journal of KIISE:Information Networking
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• v.35 no.2
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• pp.99-111
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• 2008

In recent years, the needs for WLANs(Wireless Local Area Networks) technology which can access to Internet anywhere have been dramatically increased particularly in SOHO(Small Office Home Office) and Hot Spot. However, unlike wired networks, there are some unique characteristics of wireless networks. These characteristics include the burst packet losses due to unreliable wireless channel. Note that burst packet losses, which occur when the distance between the wireless station and the AP(Access Point) increase or when obstacles move temporarily between the station and AP, are very frequent in 802.11 networks. Conversely, due to burst packet losses, the performance of 802.11 networks are not always as sufficient as the current application require, particularly when they use TCP at the transport layer. The high packet loss rate over wireless links can trigger unnecessary execution of TCP congestion control algorithm, resulting in performance degradation. In order to overcome the limitations of WLANs environment, MAC-layer LDA(Loss Differentiation Algorithm)has been proposed. MAC-layer LDA prevents TCP's timeout by increasing CRD(Consecutive Retry Duration) higher than burst packet loss duration. However, in the wireless channel with high packet loss rate, MAC-layer LDA does not work well because of two reason: (a) If the CRD is lower than burst packet loss duration due to the limited increase of retry limit, end-to-end performance is degraded. (b) energy of mobile device and bandwidth utilization in the wireless link are wasted unnecessarily by Reducing the drainage speed of the network buffer due to the increase of CRD. In this paper, we propose a new retransmission module based on Cross-layer approach, called BLD(Burst Loss Detection) module, to solve the limitation of previous link layer retransmission schemes. BLD module's algorithm is retransmission mechanism at IEEE 802.11 networks and performs retransmission based on the interaction between retransmission mechanisms of the MAC layer and TCP. From the simulation by using ns-2(Network Simulator), we could see more improved TCP throughput and energy efficiency with the proposed scheme than previous mechanisms.

• Journal of Korea Society of Digital Industry and Information Management
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• v.9 no.1
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• pp.9-23
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• 2013

In recent days, there are many researches of wireless LAN services as the communication environment of wireless LAN are so improved that a lot of services are available in wireless environments. The frequency of the wireless LAN is a general resource that can be used to everyone without any permission. Many technologies using this ISM (Industry Science Medical) frequency band are developed fast and widely. But, as many devices use the same frequency band at th same time, the service quality is degraded and the speed of the service rate id degraded by the result of the interference. For overcome this problem, we must provide the new technology of the mobile devices and a new cell design scheme for obtaining maximum throughput that considering the wireless environments effectively. In this paper, we explain the main technology at the IEEE 802.11n environments and proposes the optimal cell design and reference model for gaining maximum performance to many mobile devices at the same time by investigating real environment testing results.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.5B
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• pp.463-468
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• 2009

In VANET(Vheicle Ad hoc Network), the researches based on the wireless LAN are conducting and the method based on 802.11a is being adapted in IEEE 802.11p WAVE(Wireless Access in Vehicular Environments). However, wireless LAN which uses a single channel in a competition can cause transmission delays because of a frequent collision and a use of CSMA/CA to avoid competition in VANET requiring a fast access. In this paper, we designed CMMP (Clustering based Multi-channel MAC protocol) adequate to VANET and then confirmed the appropriate channel conditions in a V2V communication on the basis of this protocol. The simulation results showed that a packet collision and a transmission delay by the use of an existing single channel based on the contention decreased more than 60% by CMMP.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.22 no.10
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• pp.1-7
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• 2008

This paper describes the proposal, design, and implementation of a novel WDMX controller which can provide versatile light effects with the increased capability of the mobility and the number of universes for LED lights. Both the algorithm for DMX protocol and how to implement the frame based on a 8bit micro-controller are explained. The conversion algorithm between the DMX frame and wireless LAN 802. 11b protocol stack is introduced and implemented. In final, some experiments are made to verily the interoperability of the developed WDMX controller with a commercial product of LED lights. 최대 9,999 staves with 30 refresh rate per second may have capability to show various light effects.

• Journal of IKEEE
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• v.17 no.2
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• pp.96-103
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• 2013

Since multimedia data takes large part of realtime transmission in wireless communication environments such as IEEE 802.11, QoS issues became one of the important problems with network performance. 802.11e MAC provides differentiated services based on priority schemes to solve existing 802.11 MAC problems. The TXOP is an important factor with the priority to improve network performance and QoS because it defines the time duration in which multiple frames can be transferred at one time for each station. In this paper, therefore frame sizes, TXOP Limit, and Priority values in accordance with the number of stations are experimented and derived for best network performance and QoS. Using 802.11e standard parameters, simulation results show the best throughput when the number of stations is 5 and TXOP Limit value is 6.016ms. For fairness, the best result is achieved at 3.008ms of TXOP Limit value and 15-31 of CW(Contention Window) that is lower priority than CW 7-15.

• The Journal of the Korea Contents Association
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• v.10 no.5
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• pp.45-51
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• 2010

Appearance of a cloned AP(Access Point) causes MS(Mobile Station) to break an association with a normal AP(Access Point). If signal power of the cloned AP is stronger than that of the normal AP, MS associates with the cloned AP. Therefore, MS is easily exposed to attackers who installed the cloned AP. In this paper, we distinguish cloned AP from normal AP by using the association time and frame sequence number between normal AP and MS, then isolates the cloned AP. The simulation by NS-2 shows that our mechanism isolates efficiently a cloned AP and builds safer wireless LAN environment.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.11 no.2
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• pp.95-100
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• 2011

This paper analyzes the interference effect of WLAN(Wireless Local Area Network) service on WiBro(Wireless Broadband) service in adjacent frequency band by using Monte Carlo method. For the analysis, we make scenario that WLAN service operates on frequency band of 2401 MHz ~ 2423 MHz and WiBro service operates on frequency band of 698 MHz ~ 704 MHz. In case of WiBro receiver is close to WLAN AP(Access Point), the interference will be increased. So, we calculate the protection distance between WiBro receiver and WLAN AP to meet interference probability of 5 % below and required interference power of WLAN AP to meet interference probability of 5 % below in the case of fixed protection distance. As a result, the protection distance of 60 m and -9.5 dBm of required interference power of WLAN AP at 10 m of the fixed protection distance are analyzed.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.27 no.3C
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• pp.248-255
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• 2002

An RF module in compliance with the IEEE 802.11a standard has been designed and its performance has been measured. Conventional heterodyne architecture with 580MHz intermediate frequency has been realized. Measurement results show that the receiver has a low Noise Figure of 5dB, the maximum gain of 70dB, and dynamic range as wide as 61dB. Also, the SAW filter used for channel selection in the IF section allowed minimum inter-channel interference. in addition to satisfying the RF output power requirement, the transmitter features its output P1dB as high as 34dBm so that the high peak-to-average ratio of the Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme can be handled with minimum nonlinear distortion. The output P1dB of 34dBm of the transmitter corresponds to back-off powers of 18dB and 11dB with respect to the output power for the low and the middle frequency bands, respectively.