• Journal of Digital Contents Society
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• v.9 no.1
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• pp.61-67
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• 2008

In this paper, MAC algorithm for the IEEE 802.11 DCF 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(Distributed Coordination Function) and PCF(Point Coordination Function). The DCF controls the transmission based on CSMA/CA(Carrier Sense Multiple Access with Collision Avoidance), that decides a random backoff time with the range of CW(Contention Window) for each station. Normally, each station increase the CW to double after collision, and reduces the CW to the minimum after successful transmission. 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 Korean Society of Industrial and Systems Engineering
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• v.32 no.3
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• pp.10-19
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• 2009

Wireless LAN systems have been widely implemented for supporting the wireless internet services especially in the hotspot areas such as hospitals, homes, conference rooms, and so on. Compared with wired LAN systems, wireless LAN systems have the advantages of the users' mobility support and low implementation and maintenance costs. IEEE 802.11 wireless LAN systems employ the backoff algorithm to avoid contentions among STAs when two or more STAs attempt to transmit their data frames simultaneously. The MAC efficiency can be improved if the CW values are adaptively changed according to the channel state of IEEE 802.11 wireless LANs. In this paper, a dynamic contention window control algorithm is proposed using the genetic algorithm to improve the MAC throughput of IEEE 802.11 wireless LANs.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.4 no.5
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• pp.896-909
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• 2010

IEEE 802.11 medium access control (MAC) employs the distributed coordination function (DCF) as the fundamental medium access function. DCF operates with binary exponential backoff (BEB) in order to avoid frame collisions. However it may waste wireless resources because collisions occur when multiple stations are contending for frame transmissions. In order to solve this problem, a binary negative-exponential backoff (BNEB) algorithm has been proposed that uses the maximum contention window size whenever a collision occurs. However, when the number of contending stations is small, the performance of BNEB is degraded due to the unnecessarily long backoff time. In this paper, we propose the adaptive BNEB (A-BNEB) algorithm to maximize the throughput regardless of the number of contending stations. A-BNEB estimates the number of contending stations and uses this value to adjust the maximum contention window size. Simulation results show that A-BNEB significantly improves the performance of IEEE 802.11 DCF and can maintain a high throughput irrespective of the number of contending stations.

• Proceedings of the Korean Information Science Society Conference
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• 2004.04a
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• pp.760-762
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• 2004

본 논문에서는 IEEE 802.11b WLAN 종단 간(end to end) 의 DATA 전송에 있어서 효율성 향상을 위한 방법을 충돌 횟수의 감소와 throughput 값의 증가를 나타낼 수 있는 방법을 통해서 제안한다. CW(contention window) 값이 충돌 발생할 시에 증가하는 것이 다음 프레임 전송 시 충돌 발생 확률을 줄이기 위한 것임에 착안하여 CW 값의 변화를 현재 방식에서 변화를 주어 전송 효율성 향상을 위한 알고리즘을 제안한다. 현재의 CW 값이 증가하는 방식은 유지하고 프레임 전송 성공 시 CW 값이 초기화되는 방식을 수정하고자 한다. 만일 CW 값이 크게 증가하였을 시에는 충돌이 많이 발생하고 있는 것으로 간주한다. 그래서 현재의 값 변화 방식인 CW 값이 최소값으로 초기화하는 것이 아니라 최소값 보다는 큰 CW 값을 유지하여 CW 값이 크게 변화되는 것을 막아 충돌 발생 확률을 줄일 수 있음을 확인한다. 본 논문에서는 3가지 방식의 알고리즘에 대하여 시뮬레이션 하였다. original 방식과 다른 알고리즘의 효율성을 시뮬레이션을 통해 비교하여 CW의 값이 전송 성능에 미치는 영향에 대한 결과를 보여준다.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.42 no.10 s.340
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• pp.39-50
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• 2005

In this paper, we propose a performance improving MAC algerian for the IEEE 802.11 DCF. WLAN based IEEE 802.11 uses two control methods called 'Distributed Coordination Function(UF)' and 'Point Coordination Function(PCF)'. The nF controls the Urnsmission based on carrier sense multiple access with collision detection(CSMA/CA), that decides a random backoff time with the range of contention window for each terminal. Normally, each terminal the CW double after collision, and reduces the CW to the minimum after successful transmission. This paper proposes an enhanced DCF algorithm that decreases the CW smoothly after successful transmission in order to reduce the collision Probability by utilizing the current status information of WLAN. We also analyze the throughput and delay performance for the unsaturated case mathematically. Simulation results show that our algorithm enhances the saturation throughput of WLAN. They also coincide well with the analytical results.

• The Journal of Korean Institute of Next Generation Computing
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• v.13 no.5
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• pp.51-70
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• 2017

The reliability of broadcasting over vehicular ad hoc networks (VANETs) is one of the most critical factors for driving safety applications. There exists limitations to improve the reliability of broadcast transmissions in saturated VANETs where previous proposals in literature tackle the problem by heuristically adapting the size of the contention window (CW). This paper considers improving the reliability by proposing a new probability model based on the counting methods of permutations and combinations, which counts all the possible cases of broadcast failures in a single-hop broadcast transmission for a given CW. From the model, we calculate the best CW size given the number of contention nodes, which significantly improves the reliability and satisfying the timely dissemination of emergency broadcasting messages. Through extensive VANET simulations with varying densities, we show that our model maintains near 100 percent success rate for single-hop broadcast as well as multi-hop broadcast (e.g. 40 hops) and achieves minimal broadcast delay.

• Journal of Internet of Things and Convergence
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• v.6 no.4
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• pp.15-19
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• 2020

In this paper, we propose a optimal CW(Conention Window) synchronization scheme in IEEE 802.11 WLANs. IEEE 802.11 WLANs support DCF(Distributed Coordination Function) mode for the MAC(Medium Access Control) operation. In DCF, the CW increases exponentially according to the collisions and becomes minimum CW according to the success of data transmissions. However, since the base minimum CW value is hardware or standard specific, the number of active stations and network status are not considered to determine the CW value. Even though the researches on optimal CW have beend conducted, they do not consider the optimal CW synchronization among mobile stations which occur network performance degradation. Therefore, this paper calculates the optimal CW value and shares it with mobile stations in the network.

• Proceedings of the Korean Society of Computer Information Conference
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• 2009.01a
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• pp.289-292
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• 2009

IEEE 802.11 WLAN(Wireless LAN)은 효율성과 편리함으로 시장에 급격히 보급되었다. 이 IEEE 802.11 WLAN은 CSMA/CA를 기반으로 하는 DCF방식을 주로 사용하고 있으며, Contention Window, Backoff Size를 이용한 각 단말기들간의 자유 경쟁에 의한 전송 방식을 택하고 있다. 그 주된 방법으로 단말기는 CW를 설정하고 CW범위안에서 랜덤한 값을 정해 Backoff Counter를 줄여 채널을 점유하는 방식이다. DCF방식은 채널을 두고 여러 대의 단말기가 서로 경쟁을 함으로써 채널을 할당 받아 사용하게 된다. 본 논문에서는 이러한 채널 할당시 CW값을 조절하여 다른 단말기보다 채널을 빨리 할당받고 높은 처리량을 나타내는 탐욕적(Greedy) 노드문제에 대해서 분석하는 방법에 대해서 설명하려고 한다.

• Journal of the Korea Society of Computer and Information
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• v.12 no.6
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• pp.229-234
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• 2007

In the IEEE 802.11 Wireless Local Area Networks (WLANs), network nodes experiencing collisions on the shared channel need to backoff for a random period of time. which is uniformly selected from the Contention Window (CW) This contention window is dynamically controlled by the Binary Exponential Backoff (BEB) algorithm. The BEB scheme suffers from a unfairness problem and low throughput under high traffic load. In this paper, I propose a new backoff algorithm for use with the IEEE 802.11 Distributed Coordination Function.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2007.10a
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• pp.91-94
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• 2007

In the IEEE 802.11 Wireless Local Area Networks (WLANs), network nodes experiencing collisions on the shared channel need to backoff for a random period of time, which is uniformly selected from the Contention Window (CW). This contention window is dynamically controlled by the Binary Exponential Backoff (BEB) algorithm. The BEB scheme suffers from a fairness problem and low throughput under high traffic load. In this paper, I propose a new backoff algorithm for use with the IEEE 802.11 Distributed Coordination Function.