• 한국경영과학회지
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• 제34권3호
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• pp.137-153
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• 2009

In this paper, we present an analytic model that evaluates the performance of B-MAC protocol under contention transmission conditions of two senders in a single-hop wireless sensor network. Our model considers the impact of several important factors such as sleep cycle, the backoff mechanism and incoming traffic loads. After obtaining the service delay distribution of a sending node and expected delay of a receiving node, an iterative algorithm is presented for calculating the performance measures such as expected energy consumption usage per packet and latency. Simulation results show that the proposed analytic model can accurately estimate the performance measures under different traffic conditions.

• 한국통신학회논문지
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• 제32권9A호
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• pp.905-915
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• 2007

본 논문은 IEEE 802.11 DCF가 잡음과 부하가 고려된 실제적인 환경에서 효과적으로 작동할 수 있도록 환경 변화에 따라 contention window를 동적으로 적응시키는 필터 기반의 메카니즘을 제안한다. 이는 이미지 프로세싱에서의 미디언 필터개념을 적용시킨 것으로 필터링의 결과에 따라 윈도우의 크기를 조정한다. 이를 통하여 잡음과 충돌에 의해 필연적으로 패킷 전달에 문제가 발생할 수밖에 없는 무선 환경에서 불필요한 윈도우의 조정을 제거하고, 성능을 최적화 할 수 있게 된다. 또한 제안하는 방법은 기존에 전송된 결과를 반영하는 히스토리 비트-패턴을 이용함으로써 기존 연구결과들을 거의 포함하는 일반적인 방법으로 쉽게 확장할 수 있는 장점을 가지고 있다. 제안된 방법은 시뮬레이션을 통해 성능과 지연 측면에서 기존의 연구결과보다 확실한 향상을 가져옴을 확인할 수 있었다.

• Journal of Communications and Networks
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• 제6권4호
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• pp.376-386
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• 2004

In ad hoc networks, loss-based congestion window progression by the traditional means of duplicate ACKs and timeouts causes high network buffer utilization due to large bursts of data, thereby degrading network bandwidth utilization. Moreover, network-oriented feedbacks to handle route disconnection events may impair packet forwarding capability by adding to MAC layer congestion and also dissipate considerable network resources at reluctant intermediate nodes. Here, we propose a new TCP scheme that does not require the participation of intermediate nodes. It is a purely end-to-end scheme using TCP timestamps to deduce link conditions. It also eliminates spurious reductions of the transmission window in cases of timeouts and fast retransmits. The scheme incorporates a receiver-oriented rate controller (rater), and a congestion window delimiter for the 802.11 MAC protocol. In addition, the transient nature of medium availability due to medium contention during the connection time is addressed by a freezing timer (freezer) at the receiver, which freezes the sender whenever heavy contention is perceived. Finally, the sender-end is modified to comply with the receiver-end enhancements, as an optional deployment. Simulation studies show that our modification of TCP for ad hoc networks offers outstanding performance in terms of goodput, as well as throughput.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제13권7호
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• pp.3470-3493
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• 2019

IEEE 802.11s-based infrastructure Wireless Mesh Networks (iWMNs) are envisaged as a promising solution to provide ubiquitous wireless Internet access. The limited network capacity is a problem mainly caused by the medium contention between mesh users and the mesh access points (MAPs), which gets worst when the mesh clients employ the Transmission Control Protocol (TCP). To mitigate this problem, we use wireless network coding (WNC) in the MAPs. The aim of this proposal is to take advantage of the network topology around the MAPs, to alleviate the contention and maximize the use of the network capacity. We evaluate WNC when is used in MAPs. We model the formation of coding opportunities and, using computer simulations, we evaluate the formation of such coding opportunities. The results show that as the users density grows, the coding opportunities increase up to 70%; however, at the same time, the coding delay increments significantly. In order to reduce such delay, we propose to adaptively adjust the time that a packet can wait to catch a coding opportunity in an MAP. We assess the performance of moving-average estimation methods to forecast this adaptive sojourn time. We show that using moving-average estimation methods can significantly decrease the coding delay since they consider the traffic density conditions.

• Journal of Communications and Networks
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• 제13권2호
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• pp.132-141
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• 2011

IEEE 802.15.4 networks are a feasible platform candidate for connecting all health-care-related equipment dispersed across a hospital room to collect critical time-sensitive data about patient health state, such as the heart rate and blood pressure. To meet the quality of service requirements of health-care systems, this paper proposes a multi-priority queue system that differentiates between various types of frames. The effect of the proposed system on the average delay and throughput is explored herein. By employing different contention window parameters, as in IEEE 802.11e, this multi-queue system prioritizes frames on the basis of priority classes. Performance under both saturated and unsaturated traffic conditions was evaluated using a novel analytical model that comprehensively integrates two legacy models for 802.15.4 and 802.11e. To improve the accuracy, our model also accommodates the transmission retries and deferment algorithms that significantly affect the performance of IEEE 802.15.4. The multi-queue scheme is predicted to separate the average delay and throughput of two different classes by up to 48.4% and 46%, respectively, without wasting bandwidth. These outcomes imply that the multi-queue system should be employed in health-care systems for prompt allocation of synchronous channels and faster delivery of urgent information. The simulation results validate these model's predictions with a maximum deviation of 7.6%.

• 제어로봇시스템학회논문지
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• 제19권7호
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• pp.653-660
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• 2013

This paper deals with the problem of unfairness among uplink TCP (Transmission Control Protocol) flows associated with frame aggregation employed in IEEE 802.11n WLANs (Wireless Local Area Networks). When multiple stations have uplink TCP flows and transmit TCP data packets to an AP (Access Point), the AP has to compete for channel access with stations for the transmission of TCP ACK (acknowledgement) packets to the stations. Due to this contention-based channel access, TCP ACKs tend to be accumulated in the AP's downlink buffer. We show that the frame aggregation in the MAC (Medium Access Control) layer increases TCP ACK losses in the AP and leads to the serious unfair operation of TCP congestion control. To resolve this problem, we propose the TAC (TCP ACK Compression) mechanism operating at the top of the AP's interface queue. By exploiting the properties of cumulative TCP ACK and frame aggregation, TAC serves only the representative TCP ACK without serving redundant TCP ACKs. Therefore, TAC reduces queue occupancy and prevents ACK losses due to buffer overflow, which significantly contributes to fairness among uplink TCP flows. Also, TAC enhances the channel efficiency by not transmitting unnecessary TCP ACKs. The simulation results show that TAC tightly assures fairness under various network conditions while increasing the aggregate throughput, compared to the existing schemes.

• 한국통신학회논문지
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• 제34권5B호
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• pp.463-468
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• 2009

차량 간 통신에서는 무선 랜을 기반으로 한 연구가 활발히 진행되고 있으며, IEEE 802.11p WAVE(Wireless Access in the Vehicular Environment)에서도 802.11a를 기반으로 한 방식을 채택하고 있다. 하지만 무선 랜 방식은 하나의 채널을 경쟁 모드로 사용하기 때문에 빠른 접속을 요구하는 차량 환경에서 경쟁을 회피하기 위해 CSMA/CA(Carrier Sense multiple Access/Collision Avoidance) 알고리즘을 사용하는 것은 찾은 충돌로 인한 통신 지연을 유발 할 수 있다. 본 논문에서는 차량 간 통신에서 이러한 문제점을 해결하고 신뢰성 있는 데이터 전송을 지원하기 위하여 차량 환경에 적합한 클러스터링 기반의 멀티채널 매체접속제어 프로토콜을 설계하고 설계된 프로토콜을 바탕으로 차량 간 통신에 적합한 채널 조건을 확인하였다 클러스터 헤드를 통해 각 채널을 관리하고 채널을 할당함으로써 기존 경쟁 기반의 싱글 채널을 사용함에 따라 생기는 패킷 충돌 및 통신 지연을 60%이상 줄이는 것을 시뮬레이션을 통해 확인 하였다.

• 전자공학회논문지
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• 제49권11호
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• pp.132-138
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• 2012

VANET(Vehicular Ad-hoc Network)을 지원하기 위해 IEEE 802.11p 규격이 표준화 진행중에 있다. 802.11 MAC 프로토콜은 전송률에 관계없이 모든 노드들에 동일한 확률의 채널 획득 기회를 부여하므로, 전송률이 낮은 노드가 상대적으로 채널 점유시간이 길어져서 전송률이 높은 노드도 전송률이 낮은 노드의 전송률로 하향 평준화되는 문제와, 노드가 많아질 경우 충돌 확률이 높아지는 문제가 있다. 본 논문에서 제안하는 알고리듬은 일단 채널을 획득한 단말기가 다른 노드와의 채널획득 경쟁의 우위를 일정 기간 유지하는 알고리듬에, 현재 전송률에 따라 적절히 연속전송 한계값을 정하는 TXOP 개념을 더했다. CW의 폭을 표준보다 더 넓게 설정여 효과적으로 충돌을 회피하면서도 대기시간을 최소화하면서 또한 채널 점유 시간에 따라 연속전송 한계값을 다르게 정하는 방법을 통해 전체 네트워크의 향상을 보이고 있다.