• Journal of KIISE:Information Networking
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• v.34 no.6
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• pp.505-513
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• 2007

Vertical handoff is a new type of handoff in wireless networks. It is issued when a mobile node moves over overlapping wireless networks with each proving a different access bandwidth, transmission latency, and coverage. By issuing the vertical handoff, the mobile node can obtain better network bandwidth. In the sense of TCP throughput, however, the vertical handoff does not always produce positive performance gain, so sometimes it is better for the mobile node to stay at lower bandwidth providing network rather than to select and move to higher bandwidth providing network. In this paper, we analyze TCP throughput for vertical handoff, and propose a new handoff decision scheme which can estimate TCP throughput at the moment of vertical handoff. Based on the estimation, a mobile node can decide to issue vertical handoff to produce better TCP throughput, and we verify the results by simulations.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.7A
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• pp.700-709
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• 2007

The heterogeneous wireless access networks has been envisioned to characterize the future wireless networks. In such environments, TCP(Transmission Control Protocol) has to experience poor end-to-end performance because bandwidth and link delay change suddenly when a mobile node moves over different types of wireless networks, which is called vertical handoff. In this paper, we propose a new TCP which maintains each set of congestion control variables, which we call TCP context, for each type of wireless network. The proposed TCP can switch the TCP context against vertical handoff in order to adjust quickly to a newly arrived network. In simulations, the proposed TCP has higher throughput than TCP SACK(Selective Acknowledgment Options) due to its great features to vertical handoff situations.

• Proceedings of the Korean Information Science Society Conference
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• 2004.10c
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• pp.58-60
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• 2004

차세대 네트워크의 출현으로 다양한 형태의 통신망 들 간의 융합이 중요해졌다. 무선 망의 경우 이질적인 망들 간의 통합 및 융합이 이루어지기 위해서는 끊김 없는 이동성 지원이 매우 중요하다. 끊김 없는 이동성 지원은 이질적인 무선망 간의 이동시에도 지속적으로 세션을 유지하는 기능을 말한다. 이러한 기능을 핸드오프 기법이라 하며 현재 다양한 연구가 진행 중이다. 본 논문에서는 현재 제안되어 있는 핸드오프 기법 중 USHA(Universal Seamless Handoff Architecture)를 기반으로 하여 적용하기 쉬우며 확장성과 신뢰성을 보장하는 핸드오프 모델을 제안한다.

• Proceedings of the Korea Information Processing Society Conference
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• 2006.05a
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• pp.1049-1052
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• 2006

차세대의 무선 네트워크 기술은 기존의 망을 통합하고 IP화 하는 방향으로 발전중이며, WWAN과 WLAN의 통합망에 관한 연구가 활발히 진행 중이다. 본 연구는 최근 상용화 서비스가 이루어지고 있는 WDMB 기술을 WWAN, WLAN과 강결합(Tight Coupling) 방식으로 통합한 네트워크 환경을 제안한다. 본 연구에서 제안하는 수직적 핸드오프(Vertical Handoff) 기법은 WLAN 인터페이스를 필요한 상황에만 구동하여 에너지 소모율을 낮춘다. 또한 WDMB 인터페이스를 상시 구동하여 데이터 트래픽이 발생할 때에 곧바로 WDMB 인터페이스를 통해 데이터 전송을 시작하여 전송률을 높인다. 또한 성능 평가를 통하여 단말에 장착된 모든 인터페이스를 구동하는 방식과 비교하여 에너지 소모율이 경감되었으며, 단말에 장착된 한 가지 인터페이스만을 구동하는 방식과의 비교를 통해 데이터 전송률이 크게 향상되었음을 보인다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.38B no.8
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• pp.597-606
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• 2013

The performance of TCP depends on the degree of traffic congestion between the sender and the receiver. The traffic could increase, and this causes congestion which may cause trouble in data transfer. Then, TCP tries to eliminate the trouble by reducing the transfer speed with slowstart scheme. When a mobile node moves over heterogeneous wireless networks, TCP experiences dramatic change of the amount of traffic, and it performs slowstart. In this paper, we propose the efficient scheme of TCP slowstart that should performs after vertical handoff. In this scheme, TCP receiver forces slowstart, which is different form normal schemes. Its performance is better than the normal schemes in that TCP sender experiences traffic congestion and performs slowstart. We perform simulation to measure and to verify the improved performance.

• The KIPS Transactions:PartB
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• v.12B no.6 s.102
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• pp.679-684
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• 2005

There are a few representative wireless network access technologies used widely. WWAN is celluar based telecommunication networks supporting high mobility, WLAN ensures high data rate within hotspot coverage, and WDMB support both data and broadcasting services correspondingly. However, these technologies include some limitations especially on the mobility, data rate, transmission direction, and so on. In order to overvome these limitations, there are various studies have been proposed in terms of 'Vortical Handoff' that offers seamless connectivity by switching active connection to the appropriate interface which installed in the mobile devices. In this paper, we propose the interface selection algorithm and network architecture to maximize the life time of entire system by minimizing the unnecessary energy consumption of another interfaces such as WLAN, WDMB that are taken in the user equipment. In addition, by using the results of analyzing multiple types of traffic and managing user buffer as a metric for vertical handoff, we show that the energy efficiency of our scheme is $75\%$ and $34\%$ than typical WLAN for WDMB and WLAN preferred schemes, correspondingly.

• Proceedings of the Korean Information Science Society Conference
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• 2005.11a
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• pp.433-435
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• 2005

본 연구에서는 차세대 인터넷 환경의 주요 특징 중 하나인 이기종 네트워크가 혼재되어 있는 상황하에서 핸드오프가 일어나는 경우 여러 TCP 버전들(Newreno, Sack, Westwood, Vegas)이 겪게 되는 성능 변화를 분석하였다. 큰 대역폭의 네트워크와 작은 대역폭의 네트워크를 구성하여 실험해 본 결과 하부 계층의 핸드오프 지연이 없는 이상적인 상황에서도 대부분의 TCP들이 성능 저하를 겪었다. TCP 버전들 중 Vegas가 대역폭이 커지는 경우와 작아지는 경우 모두에서 가장 좋은 성능을 보였다.

• Journal of KIISE:Information Networking
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• v.36 no.1
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• pp.43-49
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• 2009

The Mobile IPv6(MIPv6) has a well-known time gap of packet loss between link down and binding update. To minimize such packet loss, this paper proposes a MIPv6-based dual tunneling mechanism, which keeps exploiting the old tunnel while creating a new tunnel. Superiority of the proposed mechanism is evaluated by quantitative analysis on the lost packets and computer simulation, based on two overlay heterogeneous access networks, such as the UMTS and a WLAN.

• Journal of KIISE:Information Networking
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• v.32 no.1
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• pp.20-28
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• 2005

TCP(Transmission Control Protocol) is one of the most important Internet protocols, which is widely used in wireless networks as well as wired networks. However, when TCP is deployed for wireless networks, it takes severe performance degradation because TCP was designed for wired network. To overcome this drawback, a number of TCP variants have been proposed in the literature. However, most previous schemes did not consider TCP enhancement over heterogeneous networks. In heterogeneous networks, an mobile node (MN) may move from one access network to another(i.e., vertical handover). In the case of vertical handover, an MN experiences a TCP performance degradation caused by the packet loss and the sudden change of link characteristics between two different access networks. In this work, we investigate the TCP performance degradation occurred in vortical handover across heterogeneous networks. First, we have conducted the measurement study over GPRS-WLAN testbed. In the measurement study. we observed the TCP performance degradation in the case of handover from WLAN to GPRS. In order to study more different TCP behaviors during vertical handover, we performed comprehensive simulations using a network simulator 2(ns-2). Based on measurement and simulation results, we investigated how to improve TCP performance in vertical handover and we concluded that the existing mechanisms cannot be perfect solutions and new mechanisms are strongly required.

• Journal of KIISE:Information Networking
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• v.34 no.4
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• pp.239-245
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• 2007

New multimedia services over the cellular/WLAN overlay networks require different Quality of Service (QoS). In the cellular/WLAN overlay structure, the main issue of load balancing is to balance the available bandwidth among different networks. In this paper, we propose a new online network management scheme. Our proposed scheme is designed to improve bandwidth utilization by balancing the traffic load between the cellular network and WLANs. Simulation results indicate the superior performance of our proposed framework to strike the appropriate performance balance between contradictory QoS requirements under widely varying diverse traffic loads.