• Proceedings of the Korean Information Science Society Conference
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• 2002.10e
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• pp.103-105
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• 2002

802.11과 같은 무선 네트워크에서는 전송오류에 의한 패킷손실이 많이 발생한다. 802.11 MAC 프로토콜에서는 에러 복구를 위해 ARQ방식을 통한 재전송을 통하여 에러를 정정하나 채널 에러 율이 증가하면 재전송 방식의 효율은 급격히 저하된다. 또한 재전송을하는데 있어서 다시 RTS와 CTS를 전송하여 데이터를 보낼 수 있는 채널을 확보해야 하므로 상당한 전송부하가 발생한다. 이에 재전송 없이 효율적인 에러 복구를 위해서는 FEC방식이 필요하다. 그러나 정적인 FEC방식은 연속적으로 변화하는 무선 채널의 전송 오류율에알맞은 정정 코드를 채택하지 못해 과도한 대역폭 낭비로 인하여 효율이 떨어지는 문제가 있다. 이러한 문제를 개선하기 위해서는 채널의 상태에 따라 정정 코드를 동적으로 변경하는 것이 필요하다. 본 논문은 FEC방식을 802.11 MAC 프로토콜에 적용할 수 있는 방안에 대해서 기술하고 채널 에러 변화에 따라 능동적으로 정정 코드 양을 조절하여 재 전송하는적응적 FEC 알고리즘을 제안한다. 본 논문에서 제안한 적응적 FEC 알고리즘을 802.11 MAC 프로토콜에 적용하여 성능을 측정한 결과 최대 80%정도 성능이 향상된 것을 확인할 수 있었다.

• Electronics and Telecommunications Trends
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• v.21 no.3 s.99
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• pp.79-90
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• 2006

CDMA 기술을 기반으로 하는 이동통신시스템의 발전과 DSL 기반의 유선통신시스템의 발전은 우리나라를 세계적인 정보통신 강국으로 성장시키고 있다. 세계적으로 높은 수준의 개인용 컴퓨터와 이동통신단말기 보유율을 바탕으로 폭발적인 인터넷서비스사용량의 증가뿐만 아니라 무선인터넷서비스에 대한 요구사항도 증가하고 있다. 이에 맞춰 무선 LAN은 고속의 가입자 전송속도를 지원하기 위해 발전하고 있다. 이미 high throughput을 목표로 하는 국제 표준화가 진행되어 마무리 단계에 있고, 이에 세계적인 칩셋 업체들이 앞다투어 IEEE 802.11n draft 이전의 EWC 버전을 이용한 칩셋을 발표하고 있다. 현재 ETRI에서는 802.11n draft 표준안을 기반으로 하는 칩셋 개발이 완성 단계에 이르렀으며, 이미 그 기능 및 성능에 대해 FPGA를 이용한 시스템 구축으로 확인하였다. 앞으로 네트워크, 오피스 네트워크 및 휴대폰 탑재 칩 등에 대한 대규모 시장 형성이 예상되고 있어 경제적 기대효과를 기대할 수 있다.

• Journal of Communications and Networks
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• v.9 no.3
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• pp.236-246
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• 2007

This paper investigates the performance of the transmission control protocol (TCP) transport protocol over IEEE 802.11 infrastructure based wireless networks. A wireless link is generally characterized by high transmission errors, random interference and a varying latency. The erratic packet losses usually lead to a curbing of the flow of segments on the TCP connection and thus limit TCP's performance. This paper examines the impact of the lossy nature of IEEE 802.11 wireless networks on the TCP performance and proposes a scheme to improve the performance of TCP over wireless links. A negative acknowledgment scheme, selective negative acknowledgment (SNACK), is applied on TCP over wireless networks and a series of ns-2 simulations are performed to compare its performance against that of other TCP schemes. The simulation results confirm that SNACK and its proposed enhancement SNACK-S, which incorporates a bandwidth estimation model at the sender, outperform conventional TCP implementations in 802.11 wireless networks.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.4B
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• pp.192-198
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• 2008

In this paper, we propose a scheme that improves the transmission performance of realtime multimedia data in wireless Local Area Network (LAN) environment, through the dynamical control of Transmission Opportunity (TXOP) period of the IEEE 802.11e HCCA Medium Access Control (MAC). The existing schedulers which determine the frame transmission time and its duration could not appropriately cope with the change of physical transmission rate, since the TXOP period has remained unchanged with the change of transmission rate of the wireless station. Our scheme is devised to keep the transmission performance of real-time multimedia data effectively unchanged by making TXOP period be extended when the transmission rate gets reduced. The proposed scheme is experimented along with IEEE 802.11e reference model using NCTUns simulator, which shows that the multimedia data is effectively delivered with the change of transmission rate as the distance between the wireless station and its access point increases.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39C no.6
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• pp.482-489
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• 2014

This paper suggests the new network scanning algorithm that makes use of measurement pilot of IEEE 802.11k. The purpose of suggesting this algorithm is to improve the existing network scanning schemes. After introducing new algorithm, this paper shows the difference of time property and energy property between former scanning schemes and new scheme with simulation results. Passive scan has a merit of low-power consumption but it takes too long time to fulfill whole scanning. On the contrary, an advantage of active scan is speed but it consumes more battery power than passive scan. By using measurement pilot's smaller interval than beacon interval, the suggested algorithm can consume less power than active scan does, and also make shorter scanning delay than passive scan does.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.9B
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• pp.1390-1398
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• 2010

We present the concept of applying Wireless Mesh Networking (WMN) technology into Smart Grid, which is recently rising as a potential technology in various areas thanks to its advantages such as low installation costs, high scalability, and high flexibility. Smart Grid is an intelligent, next-generation electrical power network that can maximize energy efficiency by monitoring utility information in real-time and controlling the flow of electricity with IT communications technology converged to the existing power grid. WMNs must be designed for Smart Grid communication systems considering not only the high level of reliability, QoS support and mass-data treatment but also the properties of the traditional power grid. In addition, it is essential to design techniques based on international standards to support interoperability and scalability. In this paper, we evaluate the performance of IEEE 802.11s based Smart Grid Mesh Networks by conducting preliminary simulation studies with the ns-3 simulator. We also outline some challenging issues that should be reviewed when considering WMNs as the candidate for Smart Grid communication infrastructure.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.6 no.2
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• pp.683-701
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• 2012

IEEE 802.11 standard has achieved huge success in the past decade and is still under development to provide higher physical data rate and better quality of service (QoS). An important problem for the development and optimization of IEEE 802.11 networks is the modeling of the MAC layer channel access protocol. Although there are already many theoretic analysis for the 802.11 MAC protocol in the literature, most of the models focus on the saturated traffic and assume infinite buffer at the MAC layer. In this paper we develop a unified analytical model for IEEE 802.11 MAC protocol in ad hoc networks. The impacts of channel access parameters, traffic rate and buffer size at the MAC layer are modeled with the assistance of a generalized Markov chain and an M/G/1/K queue model. The performance of throughput, packet delivery delay and dropping probability can be achieved. Extensive simulations show the analytical model is highly accurate. From the analytical model it is shown that for practical buffer configuration (e.g. buffer size larger than one), we can maximize the total throughput and reduce the packet blocking probability (due to limited buffer size) and the average queuing delay to zero by effectively controlling the offered load. The average MAC layer service delay as well as its standard deviation, is also much lower than that in saturated conditions and has an upper bound. It is also observed that the optimal load is very close to the maximum achievable throughput regardless of the number of stations or buffer size. Moreover, the model is scalable for performance analysis of 802.11e in unsaturated conditions and 802.11 ad hoc networks with heterogenous traffic flows.

• Journal of Communications and Networks
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• v.11 no.1
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• pp.36-41
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• 2009

The IEEE 802.16 wireless metropolitan area network (WMAN) standard is designed to correct expensive communication costs in CDMA-based mobile communication systems and limited coverage problems in wireless LAN systems. Thus, the IEEE 802.16e standard can provide mobile high-speed packet access between mobile stations and the Internet service provider through the base station with cheap communication fees. To efficiently accommodate voice over IP (VoIP) services in IEEE 802.16 systems, an uplink quality of service packet-scheduling scheme is proposed, and its performance is evaluated with an NS-2 network simulator in this paper. Numerical results show that this proposed scheme can increase the system capacity by 100% more than in the unsolicited rand service (UGS) scheme and 30% more than the extended real-time polling service (ertPS) scheme, respectively.

• Journal of Communications and Networks
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• v.13 no.6
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• pp.639-654
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• 2011

In this paper, we present a channel access mechanism, referred to as the enhanced mesh coordinated channel access (eMCCA) mechanism, for IEEE 802.11s-based wireless mesh networks. The current draft of IEEE 802.11s includes an optional medium access control (MAC), denoted as MCCA, which is designed to provide collision-free and guaranteed channel access during reserved periods. However, the MCCA mechanism fails to achieve the desired goal in the presence of contending non-MCCA nodes; this is because non-MCCA nodes are not aware of MCCA reservations and have equal access opportunities during reserved periods. We first present a probabilistic analysis that reveals the extent to which the performance of MCCA may be affected by contending non-MCCA nodes. We then propose eMCCA, which allows MCCA-enabled nodes to enjoy collision-free and guaranteed channel access during reserved periods by means of prioritized and preemptive access mechanisms. Finally, we evaluate the performance of eMCCA through extensive simulations under different network scenarios. The simulation results indicate that eMCCA outperforms other mechanisms in terms of success rate, network throughput, end-to-end delay, packet-loss rate, and mesh coordinated channel access opportunity-utilization.

• 한국정보통신설비학회:학술대회논문집
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• 2007.08a
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• pp.73-76
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• 2007

국내의 홈네트워크 서비스는 홈오토메이션 제공 중심에서 엔터테인먼트 요소가 강화된 멀티미디어 기반의 네트워크 서비스로 변모하고 있다. 홈네트워크를 구성하는 요소 중 PC와 TV는 댁내에 멀티미디어 기반의 네트워크 서비스를 구축하는데 있어 가장 중요한 역할을 수행할 것으로 예상된다. 그에 따라 PC와 TV 간 네트워크를 구축하는 것이 중요한 이슈로 떠오르고 있다. 그러나 PC와 TV 간 네트워크 구축을 위해서는 댁내 노출 배선 등의 문제로 인하여 무선 기술을 사용하는 것이 유용하지만, 현재 기존 무선 전송 기술(IEEE 802.11 a/b/g, Bluetooth 등)들은 HD 등의 대용량 컨텐츠 전송 및 QoS 를 제공하기가 어렵다. 따라서 본 논문에서는 PC와 TV 간 무선으로 네트워크 구축시 고려사항 및 무선전송기술개발 요소를 살펴본다. 특히 무선전송기술로는 고속 데이터 전송이 가능한 UWB(Ultra Widebnad)와 IEEE 802.11n 기술을 중심으로 고려한다. 또한 UWB와 IEEE 802.11n을 이용하여 네트워크가 구축되었을 때 제공 가능한 서비스 시나리오에 대해서도 소개한다.