§ Contact Information

• e-mail: jhryu AT mmlab.snu.ac.kr
• Phone: +82-2-880-9147
• Fax: +82-2-872-2045
• Address: Building 138, Room 414, Seoul National University, 599 Gwanangno, Kwanak-gu, Seoul 151-742, Korea

§ Research Interests

• Wireless (MESH) networks
• IEEE 802.11 Wireless LANs
• QoS issues
• Enhancing MAC performance
• Real-time systems
• Overlay networks
• RFID systems

§ Education

• Ph.D. in Dept. of Computer science and Engineering, Seoul National University, Korea, 2005. 3 - current
   (Advisor : Taekyoung Kwon, Yanghee Choi)
• B.S. in Computer Science Division at Department of EECS, Korea Advanced Institute of Science and Technology, Korea, 2001. 3 - 2005. 2, Graduated with honors. (Cum laude)
• High School in Hansung Science High School, Korea, 1999. 3 - 2001. 2

§ Publications

International Journal

• Rajiv Gandhi, Yoo-Ah Kim, Seungjoon Lee, Jiho Ryu, and Peng-Jun Wan, "Approximation Algorithms for Data Broadcast in Wireless Networks," accepted to IEEE Transactions on Mobile Computing (TMC). [PDF]
• Jeongkeun Lee, Jiho Ryu, Sung-Ju Lee, and Ted Taekyoung Kwon, "Improved modeling of IEEE 802.11a PHY through fine-grained measurements," Elsevier Computer Networks (ComNet), Vol. 54, No. 4, pp. 641-657, March 2010. [PDF]
  In wireless networks, modeling of the physical layer behavior is an important yet difficult task. Modeling and estimating wireless interference is receiving great attention, and is crucial in a wireless network performance study. The physical layer capture, preamble detection, and carrier sense threshold are three key components that play important roles in successful frame reception in the presence of interference. Using our IEEE 802.11a wireless network testbed, we carry out a measurement study that reveals the detailed operation of each component and in particular we show the terms and conditions (interference timing, signal power difference, bitrate) under which a frame survives interference according to the preamble detection and capture logic. Based on the measurement study, we show that the operations of the three components in real IEEE 802.11a systems differ from those of popular simulators and present our modifications of the IEEE 802.11a PHY models to the NS-2 and QualNet network simulators. The modifications can be summarized as follows. (i) The current simulators’ frame reception is based only on the received signal strength. However, real 802.11 systems can start frame reception only when the Signal-to-Interference Ratio (SIR) is high enough to detect the preamble. (ii) Different chipset vendors implement the frame reception and capture algorithms differently, resulting in different operations for the same event. We provide different simulation models for several popular chipset vendors and show the performance differences between the models. (iii) Based on the 802.11a standard setting and our testbed observation, we revise the simulator to set the carrier sense threshold higher than the receiver sensitivity rather than equal to the receiver sensitivity. We implement our modifications to the QualNet simulator and evaluate the impact of PHY model implementations on the wireless network performance; these result in an up to six times increase of net throughput.
• Jiho Ryu, Chang-Gun Lee, Taekyoung Kwon, and Junghee Han, "Combined Scheduling and Routing for Deterministic Guarantee of End-to-End Deadlines in Cell Structured Sensor Networks," IEEE Sensors Journal, Vol. 9, No. 10, pp. 1291-1301, October 2009. [PDF]
  Wireless sensor networks have great potentials to be applied to mission critical real-time applications such as target tracking, emergency detection, and disaster management. For such real-time applications, sensory data should be delivered within the end-to-end deadlines; otherwise, the data may become useless or misleading. In order to provide such deterministic guarantee of end-to-end deadlines, not only packet scheduling within a single hop but also routing across multiple hops have to be jointly considered. In this paper, we propose a combined scheduling and routing mechanism that provides a deterministic guarantee of end-to-end deadlines leveraging real-time scheduling and analysis techniques. Our experimental study shows that the proposed mechanism can accommodate a larger workload meeting all the end-to-end deadlines than a pretty comprehensive set of other possible solutions of scheduling and routing that are known to deterministically guarantee end-to-end deadlines.
• Nakjung Choi, Jiho Ryu, Eunkyoung Paik, Taekyoung Kwon, and Yanghee Choi, "A Transparent Failover Mechanism for a Mobile Network with Multiple Mobile Routers," IEEE Communications Letters, (CL), Vol. 11, No. 7, pp. 604-606, July 2007. [PDF]
  In a mobile network that is multihomed by multiple mobile routers, a mobile router that loses link connectivity can be replaced by the other mobile routers. We propose a transparent failover mechanism (TFM) to provide seamless Internet services to nodes in the mobile network, which is validated by implementing a real test-bed. Compared to the network mobility basic support protocol, TFM does not require the nodes attached to the failed mobile router to change their addresses, and hence has two advantages: (a) IP connectivity is maintained transparently, and (b) failover is quickly accomplished by avoiding the address re-configuration process in each node.
• Jiho Ryu, Yongho Seok, Nakjung Choi, Taekyoung Kwon, and Yanghee Choi, "S-EDCA: EDCA based on SuperSlot and Pseudo Collision," ACM SIGMOBILE Mobile Computing and Communications Review (MC2R), Vol. 10, No. 4, pp. 15-16, October 2006.(invited) [PDF]
  The IEEE 802.11e EDCA mechanism cannot guarantee the QoS of high-priority traffic as the bandwidth consumption of low-priority traffic increases. Also, the existence of high priority traffic undermines link utilization of low priority traffic. To solve these problems, we propose the S-EDCA scheme that extends IEEE 802.11e EDCA by introducing a SuperSlot and Pseudo Collision. Compared to IEEE 802.11e EDCA, S-EDCA has two advantages: (a) Higher priority traffic achieves QoS regardless of the amount of low priority traffic, and (b) Low priority traffic experiences a higher throughput with the same amount of high priority traffic.

International Conference

• Jiho Ryu, Changhee Joo, Ted "Taekyoung" Kwon, Ness B. Shroff, and Yanghee Choi "Distributed SINR based Scheduling Algorithm for Multi-hop Wireless Networks," in Proc. of the 13th ACM Intl. Symposium on Modeling, Analysis and Simulation of Wireless and Mobile Systems (MSWIM'2010), Bodrum, Turkey, Oct. 2010. (Short paper, Acceptance Ratio: 39.4 = 52/132, Regular: 32.6 = 43/132). [PDF]
  The problem of developing high-performance distributed scheduling algorithms for multi-hop wireless networks has seen enormous interest in recent years. The problem is especially challenging when studied under a physical interference model, which requires the SINR at the receiver to be above a certain threshold for decoding success. Under such an SINR model, transmission failure may be caused by interference due to simultaneous transmissions from far away nodes, which exacerbates the difficulty in developing a distributed algorithm. In this paper, we propose a scheduling algorithm that exploits carrier sensing and show that the algorithm is not only amenable to distributed implementation, but also results in throughput optimality. Our algorithm has a feature called the ``dual-state'' approach, which separates the transmission schedules from the system state and can be shown to improve delay performance.
• Rajiv Gandhi, Yoo-Ah Kim, Seungjoon Lee, Jiho Ryu, and Peng-Jun Wan, "Approximation Algorithms for Data Broadcast in Wireless Networks," in Proc. of IEEE INFOCOM Mini-conference 2009, Rio de Janeiro, Brazil, Apr. 2009. [PDF]
  Broadcasting is a fundamental operation in wireless networks and plays an important role in the communication protocol design. In multihop wireless networks, however, interference at a node due to simultaneous transmissions from its neighbors makes it non-trivial to design a minimum-latency broadcast algorithm, which is known to be NP-complete. We present a simple 12-approximation algorithm for the one-to-all broadcast problem that improves all previously known guarantees for this problem. We then consider the all-to-all broadcast problem where each node sends its own message to all other nodes. For the all-to-all broadcast problem, we present two algorithms with approximation ratios of 20 and 34, improving the best result available in the literature. Finally, we report experimental evaluation of our algorithms. Our studies indicate that our algorithms perform much better in practice than the worst-case guarantees provided in the theoretical analysis and achieve up to 37% performance improvement over existing schemes.
• Jiho Ryu, Jeongkeun Lee, Sung-Ju Lee, and Taekyoung Kwon, "Revamping the IEEE 802.11a PHY Simulation Models," in Proc. of the 11th ACM/IEEE Intl. Symposium on Modeling, Analysis and Simulation of Wireless and Mobile Systems (MSWIM'08), one of three best paper candidates, Vancouver, British Columbia, Canada, Oct. 2008. (Acceptance Ratio: 29.3 = 55/188, Regular: 21.3 = 40/188). [PDF]
  In simulating wireless networks, modeling of the physical layer behavior is an important yet difficult task. Modeling and estimating wireless interference is receiving great research attention, and is crucial in a wireless network performance study. The implementation of physical layer capture, preamble detection, and carrier sense threshold plays an important role in successful frame reception in the presence of interference. We showed in our previous testbed study that the operations of the frame reception and the capture effect in real IEEE 802.11a systems differ from those of popular research simulators. We present our modifications of the IEEE 802.11a PHY models to the current simulators. The modifications can be summarized as follows. (i) The current simulators' frame reception is only based on the received signal strength. However, the real 802.11 systems can start the frame reception only when the Signal-to-Interference Ratio (SIR) is high enough to detect the preamble. (ii) Different chipset vendors implement the frame reception and capture algorithms differently, resulting in different operations for the same event. We provide different simulation models for several popular chipset vendors and show the performance differences between the chipsets. (iii) The current simulators set the carrier sense threshold equal to the receiver sensitivity. The standard however states that it should be 20 dB higher than the receiver sensitivity. We implement our modifications to the QualNet simulator and conduct a wireless network performance study to evaluate the impact of PHY model implementation.
• Jeongkeun Lee, Wonho Kim, Sung-Ju Lee, Daehyung Jo, Jiho Ryu, Taekyoung Kwon, and Yanghee Choi, "An Experimental Study on the Capture Effect in 802.11a Networks," in Proc. of the Second ACM International Workshop on Wireless Network Testbeds, Experimental evaluation and CHaracterization (WinTECH) 2007, Montreal, QC, Canada, September 2007. [PDF]
  • The revised version with the Atheros driver bug fix: [PDF]
  In wireless networks, a frame collision does not necessarily result in all the simultaneously transmitted frames being lost. Depending on the relative signal power and the arrival timing of the involved frames, one frame can survive the collision and be successfully received by the receiver. Using our IEEE 802.11a wireless network testbed, we carry out a measurement study that shows the terms and conditions (timing, power difference, bit rate) under which this capture effect takes place. Recent measurement work on the capture effect in 802.11 networks [10] argues that the stronger frame can be successfully decoded only in two cases: (1) The stronger frame arrives earlier than the weaker frame, or (2) the stronger frame arrives later than the weaker frame but within the preamble time of the weaker frame. However, our measurement shows that the stronger frame can be decoded correctly regardless of the timing relation with the weaker frame. In addition, when the stronger frame arrives later than the weaker frame's arrival, the physical layer capture exhibits two very distinct patterns based on whether the receiver has been successfully synchronized to the previous weak frame or not. In explaining the distinct cases we observe that the successful capture of a frame involved in a collision is determined through two stages: preamble detection and the frame body FCS check.
• Nakjung Choi, Jiho Ryu, Yongho Seok, Taekyoung Kwon, and Yanghee Choi, "Optimizing Aggregate Throughput of Upstream TCP Flows over IEEE 802.11 Wireless LANs," in Proc. of IEEE International symposium on Personal, Indoor and Mobile Radio Communications (PIMRC) 2007, Athens, Greece, September 2007. [PDF][PPT]
  This paper via analysis and simulation revisits the interaction between MAC contention and TCP congestion control over IEEE 802.11 WLANs, misled in the previous efforts. The results reveal that the effective number of contending wireless stations is not proportional to the number of wireless stations with an upstream TCP flow in IEEE 802.11 wireless LANs. Thus, we propose a new scheme called TCP ACK Priority (TAP) in which, by allowing an access point to transmit TCP ACKs at the highest priority, the optimal number of competing stations are allowed to contend for media access to utilize link bandwidth efficiently. We use an ns-2 simulator to evaluate the performance of TAP with the IEEE 802.11 DCF. The results show that there is an improvement in network performance without the loss of fairness between upstream TCP flows. The extensions for IEEE 802.11e/n are also considered.
• Jiho Ryu, Hojin Lee, Yongho Seok, Taekyoung Kwon, and Yanghee Choi, "A Hybrid Query Tree Protocol for Tag Collision Arbitration in RFID systems," in Proc. of the IEEE International Conference on Communications (ICC) 2007, Glasgow, Scotland, June 2007. [PDF][PPT]
  In this paper, we propose a hybrid query tree protocol that combines a tree based query protocol with a slotted backoff mechanism. The proposed protocol decreases the average identification delay by reducing collisions and idle time. To reduce collisions, we use a 4-ary query tree instead of a binary query tree. To reduce idle time, we introduce a slotted backoff mechanism to reduce the number of unnecessary query commands. For static scenarios of tags, we extended the proposed protocol by adopting two phases. First, in leaf query phase for existing tags, the interrogator queries leaf-nodes directly to reuse query strings in the previous session. Second, in root query phase for new arriving tags, the interrogator starts the query process from the root-node. Simulation reveals that the proposed protocol achieves lower identification delay than existing tag collision arbitration protocols regardless of whether tags are mobile or not.
• Yongsub Nam, Jiho Ryu, Nakjung Choi, Taekyoung Kwon, and Yanghee Choi, "Block-Poll Coordination Function for Improving Throughput Performance in Wireless LANs," in Proc. of IEEE International Conference on Communications Systems (ICCS) 2006, Singapore, October/November 2006. [PDF] [PPT]
  IEEE 802.11 DCF shows the inefficient throughput performance, due to the large backoff and collision overheads. We propose the Block-poll coordination function (BCF) to reduce the backoff and collision overheads, thus increasing the aggregate throughput. By introducing a new Block-poll frame which includes a polling bitmap, each station can decide its transmission order, so that the channel access emulates TDMA. In addition, by dividing the polling bitmap into several chunks and transmitting only the changed chunks, the polling overhead can be reduced. The simulation and analysis results show the proposed protocol achieves higher throughput than IEEE 802.11 DCF, while reducing the average delay and the delay jitter values.
• Nakjung Choi, Jiho Ryu, Yongho Seok, Yanghee Choi, and Taekyoung Kwon, "Unicast-Friendly Multicast in IEEE 802.11 Wireless LANs," the best student paper award(3rd place), in Proc. of IEEE Consumer Communications & Networking Conference (CCNC) 2006, Las Vegas, USA, January 2006. [PDF]
  The IEEE 802.11 protocol has become the de facto standard in wireless LANs. However, it reveals the unfairness problem between unicast flows and multicast flows since multicast packets are not subject to binary exponential backoff. To prevent a multicast flow from overwhelming the wireless link bandwidth is a crucial issue. This paper seeks to achieve fairness between unicast and multicast flows by introducing Unicast-Friendly Multicast (UFM). The central idea behind the UFM algorithm is to dynamically change the contention window size for multicast packets to limit the bandwidth share of a multicast flow equal to that of a unicast flow. The proposed UFM algorithm adjusts the multicast contention window size depending on the number of competing stations. We present two versions of UFM: the first one calculates the multicast window size by inferring the average contention window size of unicast flows, while the second one maintains the mapping table between the number of competing stations and the corresponding multicast window size given by related analysis. Simulation reveals that both versions of UFM achieve the fairness by providing almost the fair share of bandwidth to each flow regardless of unicast or multicast under the saturated network conditions.
• Nakjung Choi, Jiho Ryu, Yongho Seok, Yanghee Choi, and Taekyoung Kwon, "Throughput Enhancement for Uploading TCP Flows in IEEE 802.11 Wireless LANs," in Proc. of IEEE International Conference on Consumer Electronics (ICCE) 2006, Las Vegas, USA, January 2006. [PDF][PPT]
  This paper re-analyzes the relationship between MAC contention and TCP congestion control misled in previous efforts. By introducing a prioritized access of an access point, we seek to allow the optimal number of competing stations to contend for media access.
• Jiho Ryu, Yongho Seok, Yanghee Choi, Taekyoung Kwon, and Jean-Marie BONNIN, "S-EDCF: EDCF based on SuperSlot and Pseudo Collision," ACM International Conference on Mobile Computing and Networking (MobiCom) 2005 Student Poster Session, Cologne, Germany, August/September, 2005. [PPT]
  The IEEE 802.11e EDCF mechanism cannot guarantee the QoS of high-priority traffic as the bandwidth consumption of the low-priority traffic increases. Also, the existence of high priority traffic undermines link utilization of low priority traffic. To solve these problems, we propose the S-EDCF scheme that extends IEEE 802.11e EDCF by introducing a SuperSlot and Pseudo Collision. Compared to EDCF, S-EDCF has two advantages: (a) Higher priority traffic achieves QoS regardless of the amount of low priority traffic, and (b) Low priority traffic experiences a higher throughput with the same amount of high priority traffic.

Domestic Journal

• Dukhyun Chang, Yoonbo Shim, Gilsoo Kim, Nakjung Choi, Jiho Ryu, Taekyoung Kwon, and Yanghee Choi, "MIR: Mobility Information based Routing for Delay and Disruption Tolerant Network," Journal of KIISE: , Vol. 36 No. 2, pp. 130-136, April 2009. [PDF]
  Delay and Disruption Tolerant Network (DTN) employs message delivery based on a store-and-forward method to conquer no guarantee of continuous end-to-end connectivity. Different from general networks, it is hard for the existing TCP/IP-based routing protocols to correctly work, due to the characteristics such as large latency and unstable link connectivity. Thus, many recent studies focus on routing protocols for DTN. In this paper, we propose a novel routing protocol for DTN with efficient message delivery utilizing mobility information such as direction or destination of mobile nodes. And this protocol is enhanced in terms of delivery ratio, decreases latency and overhead compared to the previously proposed solutions. Simulation results prove its superiority.
  ※ The main body of paper is written in Korean.
• Jinyoung Han, Heedong Jung, Youngbin Im, Jeongkeun Lee, Jiho Ryu, Taekyoung Kwon, and Yanghee Choi, "Performance Evaluation in Combination of Wired and Wireless Mesh Networks," Journal of KIISE: Computing Practices and Letters, Vol. 14, No .8, pp. 778-782, November 2008. [PDF]
  The performance in combination of wired and wireless mesh network which will be a main access network in future internet is directly linked with service quality to users. In this paper, we evaluate variant TCP performance and analyze traffic characteristics on Wireless (Seoul National University Mesh testbed) - Wired (KOREN/Abilene) - Wireless (HP Mesh testbed in the USA) Network. We found that RTT is proper in this network to serve a VoIP service. Also we propose an enhanced TCP protocol which can perform well in both wireless network which has many packet losses and wired network which has high bandwidth and long delay.
  ※ The main body of paper is written in Korean.
• Jiho Ryu, Hojin Lee, Yongho Seok, Taekyoung Kwon, and Yanghee Choi, "A Hybrid Approach to Arbitrate Tag collisions in RFID systems," Journal of KIISE: Information Networking, Vol. 34, No. 6, pp. 483-492, December 2007. [PDF]
  본 논문에서는 RFID 시스템에서 쿼리 트리 기반의 태그 충돌 중재를 위한 새로운 프로토콜을 제안한다. 제안한 하이브리드 쿼리 트리 (Hybrid Query Tree) 기법은 이진 쿼리 트리 대신에 4-ary 쿼리 트리를 이용하여 태그 충돌 수를 줄였으며 추가적으로 발생하는 유휴 시간을 줄이기 위해 slotted 백오프 기법을 활용하였다. 실험결과 및 수학적 분석은 제안한 하이브리드 쿼리 트리 프로토콜이 기존에 제시된 기법보다 우수한 성능을 보임을 입증한다.
  ※ The original paper was written in Korean.
• Nakjung Choi, Hakyoung Jung, Jiho Ryu, Yongho Seok, Yanghee Choi, and Taekyoung Kwon, "TAS: TCP-Aware Sub-layer over IEEE 802.11-based Wireless LANs," Journal of KISS: Information Networking, Vol. 33, No. 5, pp. 355-368, October 2006. [PDF]
  본 논문은 IEEE 802.11 기반 무선랜 환경에서 TCP 성능을 향상시키기 위한 새로운 분할TCP 기법을 소개한다. IEEE 802.11 기반 무선랜 환경에서는 유선 환경과는 달리 TCP 데이타 흐름(flow)이 많은 데이타를 보내려고 시도하지 않기 때문에 TCP 데이타 흐름 성능 저하의 주요한 원인이 된다. 본 논문에서는 이런 문제를 완화하여 TCP 데이타 흐름의 성능을 향상시키기 위한 TAS(TCP-Aware Sub-layer) 기법을 제안한다. TAS 기법은 하나의 TCP 데이타 흐름을 AP 등의 분할 지점을 기점으로 두 개의 TCP 데이타 흐름으로 나누는 기존의 분할 연결(split-connection) 기법을 확장한 개념이다. TAS를 기반으로 동작하는 무선 노드는 실제로 TCP ACK을 수신하는 것이 아니라, 수신된MAC ACK을 이용하여 TCP ACK을 에뮬레이션한다. NS2 모의 실험을 통하여 제안된 기법인 TAS 기법의 성능을 기존 TCP 기법, I-TCP (Indirect TCP) 기법의 성능과 비교하였으며 모의 실험 결과는 TAS 기법이 다른 기법들에 비하여 시간당 처리량과 자원 할당의 형평성 측면에서 더 좋은 성능을 보인다는 것을 증명한다. 또한 절전 모드의 경우, 전송 지연 시간도 줄일 수 있다.
  ※ The original paper was written in Korean.

Domestic Conference

• Youngbin Im, Jiho Ryu, Hojin Lee, Taekyoung Kwon, and Yanghee Choi, "A Study on Resource Allocation in Heterogeneous Wireless Networks," in Proc. of Korea Information and Communication Society (KICS) Conference Winter 2011, YongPyong, Korea, February 2011. [PDF]
• Jiho Ryu, Youngbin Im, Hojin Lee, Taekyoung Kwon, and Yanghee Choi, "MCM: Multiple Connection Management for Exploiting Heterogeneous Wireless Networks," in Proc. of The 14th Conference on Next Generation Communication Software (NCS) 2010, Pyeongchang, Korea, December 2010. [PDF]
• Jiho Ryu, Taekyoung Kwon, and Yanghee Choi, "FPA: Frame Power Adjustment for MAC performance enhancements," in Proc. of KICS Conference Summer 2009, Jeju Island, June 2009. [PDF]
• Jinyoung Han, Heedong Jung, Youngbin Im, Jeongkeun Lee, Jiho Ryu, Taekyoung Kwon, and Yanghee Choi, "Performance Evaluation in Combination of Wired and Wireless Mesh Networks," in Proc. of KCC 2008, Phoenix Park, Gangwon-Do, June 2008. [PDF]
• Dukhyun Chang, Yoonbo Shim, Gilsoo Kim, Nakjung Choi, Jiho Ryu, Taekyoung Kwon, and Yanghee Choi, "MIR: Mobility Information based Routing for Delay and Disruption Tolerant Network," in Proc. of KCC 2008, Phoenix Park, Gangwon-Do, June 2008. [PDF]
• Youngbin Im, Jiho Ryu, Kunwoo Park, Taekyoung Kwon, Yanghee Choi, Taewan You, and Seungyun Lee, "A study on the L3Shim Implementation for IPv6 Site Multihoming based on Linux Kernel," in Proc. of KICS Conference Fall 2007, Seoul, November 2007. [PDF]
  본 논문은 현재 IETF에서 표준화가 진행되고 있는 L3Shim 사이트 멀티호밍 프로토콜을 리눅스 커널에 구현하는 방법을 기술한 것이다. L3Shim은 기존 호스트와 shim-enabled 호스트 사이의 통신 및 shim-enabled 호스트들끼리의 통신에서 단말식별자와 위치식별자를 구분함으로써 현재 맺어져 있는 커넥션을 유지하면서 멀티호밍을 지원하는 프로토콜이다. 본 연구진은 단말들이 L3Shim을 사용할 경우 일어날 수 있는 성능저하요소를 제거하기 위하여 L3Shim의 핵심 부분은 모듈의 형태로 리눅스 커널에 직접 구현하였다. 그리고 L3Shim의 나머지 부분은 하나의 응용프로그램으로 구현함과 동시에 API를 제공함으로써 사용자 또는 타 개발자가 접근하기 쉽도록 설계했다. 본 연구를 통하여 L3Shim은 낮은 오버헤드를 가지고 동작할 수 있음을 보이고 추가적인 draft 형태로 존재하는 signaling 프로토콜을 통하여 오류 자동 감지 및 자동 복구에의 활용 가능성을 확인한다.
  ※ The original paper was written in Korean.
• Jiho Ryu, Nakjung Choi, Taekyoung Kwon, Yanghee Choi, and Eunkyoung Paik, "The Mechanism to Bypass Ingress Filtering for Multihomed Mobile Networks," in Proc. of KISS Conference Fall 2006, Seoul, October 2006. [PDF][PPT]
  본 논문에서는 멀티호밍 모바일 네트워크에서 발생하는 다양한 이슈 중 인그레스 필터링(ingress filtering) 문제에 대한 해결책을 제시하고자 한다. 본 저자들은 먼저 다수의 모바일 라우터가 존재하는 모바일 네트워크 환경에서 이웃 모바일 라우터 사이에 '프리픽스(prefix) 이웃' 관계를 제안한다. 그리고 이런 관계를 활용하여 모바일 라우터의 서비스를 받는 단말들이 자신의 주소를 변경하지 않고도 이웃 모바일 라우터를 통하여 릴레이 서비스를 받을 수 있도록 하는 인그레스 필터링 우회 기법도 제안한다. 또한 제안된 기법들을 ns-2 시뮬레이터 상에서 구현하고, 모의 실험을 수행하여 제안된 기법의 성능 향상을 검증한다.
  ※ The original paper was written in Korean.
• Yongsub Nam, Jiho Ryu, Nakjung Choi, Taekyoung Kwon, and Yanghee Choi, "Contention-based Multi-Polling Mechanism for Wireless LANs Using Bitmap," in Proc. of KICS Conference Summer 2006, Jeju Island, July 2006. [PDF] [PPT]
  IEEE 802.11 무선 랜의 DCF 기법은 backoff 로 인한 채널 유휴 시간과 충돌로 인해 실제 가능한 링크 대역폭 에 못 미치는 효율을 보인다. 이의 대안으로 폴링 기법을 고려할 수 있으나, 기존의 폴링 기법은 개개의 단말을 폴링해야 하고, 또한 보낼 프레임이 없는 단말에게도 폴링을 수행하게 되어 채널을 낭비하게 된다. 본 논문은 무선 랜의 성능을 높이기 위해 비트맵을 이용한 경쟁 기반 다중 폴링 기법을 제안한다. 액세스 포인트가 주기적으로 전송 순서를 기록한 비트맵을 브로드캐스트하며, 이를 송신한 서비스 지역 내의 모든 단말이 자신의 전송 순서를 유추한다. 전송 순서가 결정되어 있으므로 충돌이나 backoff 로 인한 채널 낭비가 발생하지 않아 채널을 효율적으로 사용한다. 또한, 하나의 프레임으로 BSS 내의 모든 단말을 폴링하며, 청크 기반으로 폴링 목록을 전송하므로 폴링 오버헤드 역시 감소한다. 모의 실험을 통한 성능 평가 결과는 제안 기법의 우수한 성능을 입증한다.
  ※ The original paper was written in Korean.
• Yongsub Nam, Nakjung Choi, Jiho Ryu, Taekyoung Kwon, and Yanghee Choi, "Dynamic Multi-Rate Routing Sub-Layer for Mobile Ad hoc Networks," in Proc. of KCC 2006, Yongpyong, June 2006. [PDF][PPT]
  IEEE 802.11 표준은 채널 상태에 따라 다양한 전송 속도를 지원하며, 무선랜 환경에서 이러한 특성을 활용하는 연구들이 진행되어 왔다. 그러나, 모바일 애드 혹 네트워크 환경에서 다중전송속도를 활용하는 연구는 거의 이루어지지 않았다. 본 논문은 모바일 애드 혹 네트워크에서 동적으로 다중전송속도를 활용하여 채널 효율을 높이는 알고리즘을 제안한다. 제안 기법은 MAC 계층과 네트워크 계층 중간에 위치하며 각 계층에 독립적으로 동작한다. 라우팅 프로토콜에서 결정한 다음 홉에 더불어, 제안 기법은 더 빠른 전송 속도를 갖는 중계 경로를 탐색한다. 또한, 패킷의 크기에 따라 경쟁 부하를 고려하여 선택적으로 전송 경로를 변경한다. 모의 실험을 통한 성능 평가는 제안 기법의 우수한 성능을 입증한다.
  ※ The original paper was written in Korean.
• Yongsub Nam, Jiho Ryu, Taekyoung Kwon, and Yanghee Choi, "Optimal topology in Wireless Mesh Networks Using Genetic Algorithm," in Proc. of KISS Conference Fall 2005, Seoul, November 2005. [PDF]
  무선 매쉬 네트워크에서의 액세스 포인트의 배치는 예상 가입자의 위치와 서비스 공급자의 위치, 액세스 포인트의 수를 모두 고려해야 하는 NP-complete 문제이나, 효율과 비용의 측면에서 간과할 수 없는 매우 중요한 문제이다. 본 연구에서는 대규모 매쉬 네트워크의 효율적인 배치를 위해, 유전 알고리즘을 사용하여 최소한의 액세스 포인트를 사용하여 최대한의 가입자를 수용할 수 있는 무선 메쉬 네트워크의 배치 방안을 모색한다. 구현을 통한 성능평가 결과와 수치적 분석은 본 문제 공간에서의 유전 알고리즘의 효용성을 입증한다.
  ※ The original paper was written in Korean.
• Jiho Ryu, Nakjung Choi, Yongho Seok, Taekyoung Kwon, and Yanghee Choi, "Improving TCP Uploading Throughput in IEEE 802.11 Wireless LANs," in Proc. of KICS Conference Summer 2005, Yongpyong, June/July 2005. [PDF][PPT]
  최근 유비퀴터스 네트워크를 지원하기 위한 다양한 무선 네트워킹 기술이 등장하였고, 그 중 무선랜은 쉬운 설치와 값싼 비용으로 인하여 가장 성공적인 기술로 알려져 있다. 그러나 점차 사용자가 늘어남에 따라 더 많은 사용자, 더 나은 서비스를 제공하기 위한 효율적인 AP 배치와 무선랜의 성능 향상 기법이 중요해지고있다. 특히나 기존의 응용 프로그램 대부분이 TCP 기반으로 동작한다는 점을 감안하면, 무선랜에서 TCP 연결의 성능 향상은 가장 중요한 이슈 중에 하나로 부각되고 있다. 본 논문에서는 IEEE 802.11 기반 무선랜에서 TCP 성능 향상에 대한 기존 연구의 문제점을 지적하고, 최적의 네트워크 성능을 달성하기 위한 알고리즘을 제시한다. 또한 실험을 통하여 제안된 알고리즘이 기존의 IEEE 802.11 DCF 보다 우수한 성능을 보임을 증명한다.
  ※ The original paper was written in Korean.

Internet Drafts

• Jiho Ryu, Nakjung Choi, Eunkyoung Paik, Taekyoung Kwon, and Chulhyun Park, "Failover for Multiple Mobile Routers in a Mobile Network," Internet Draft draft-ryu-nemo-mr-failover-02.txt, submitted to Internet Engineering Task Force (IETF) 67th Meeting, San Diego, USA, October 2006, Expired.
• Chulhyun Park , Nakjung Choi, Jiho Ryu, Taekyoung Kwon, and Eunkyoung Paik, "Interface selection for a multihomed mobile network," Internet Draft draft-park-mis-01.txt, submitted to Internet Engineering Task Force (IETF) 67th Meeting, San Diego, USA, October 2006, Expired.
• Jiho Ryu, Nakjung Choi, Eunkyoung Paik, Taekyoung Kwon, and Chulhyun Park, "Bypassing Ingress Filtering for Multihomed mobile network," Internet Draft draft-ryu-nemo-ingress-filtering-00.txt, submitted to Internet Engineering Task Force (IETF) 66th Meeting, Montreal, Canada, July 2006, Expired.
• Chulhyun Park , Nakjung Choi, Jiho Ryu, Taekyoung Kwon, and Eunkyoung Paik, "Interface selection for a multihomed mobile network," Internet Draft draft-park-mis-00.txt, submitted to Internet Engineering Task Force (IETF) 66th Meeting, Montreal, Canada, July 2006, Expired.
• Jiho Ryu, Nakjung Choi, Eunkyoung Paik, Taekyoung Kwon, and Chulhyun Park, "Failover for Multiple Mobile Routers in a Mobile Network," Internet Draft draft-ryu-nemo-mr-failover-01.txt, submitted to Internet Engineering Task Force (IETF) 65th Meeting, Dallas, USA, March 2006, Expired.
• Jiho Ryu, Nakjung Choi, Eunkyoung Paik, Taekyoung Kwon, and Minji Nam, "Failover for Multiple Mobile Routers in a Mobile Network," Internet Draft draft-ryu-nemo-mr-failover-00.txt, submitted to Internet Engineering Task Force (IETF) 63th Meeting, Paris, France, July 2005, Expired.

Patent

• Nakjung Choi, Chulhyun Park, Jiho Ryu, Taekyoung Kwon, Yanghee Choi, and Eunkyoung Paik "Multiple network interface apparatus and method, and traffic control option information transmission method and network interface selection method in its," pending Korea 2006-0127423, December, 13, 2006.
• Nakjung Choi, Jiho Ryu, Yanghee Choi, Eunkyoung Paik, Minji Nam, Sejun Park, and Byungsoo Chang, "Peer mobile router authentication method, and multiple peer care-of addresses registration method, and mobile router failover method for multi-homed mobile networks," pending Korea 2005-0061232, July, 7, 2005.
• Nakjung Choi, Jiho Ryu, Yanghee Choi, Eunkyoung Paik, Minji Nam, Sejun Park, and Byungsoo Chang, "Peer mobile router authentication method, and multiple peer care-of addresses registration method, and mobile router failover method for multi-homed mobile networks," pending PCT/KR2006/002485, June 27, 2006.

§ Project Experiences

• Future Internet Network Model Development sponsored by KRCF (2009.1 - current)
• Research on Interconnection between Heterogeneous Networks in Fundamental technologies for the Future Internet sponsored by MKE (2008.3 - 2009.6)
• Trusted Overlay Networks sponsored by IITA (2008.3 - 2009.2)
• Mesh-based Access Network in the Future Internet sponsored by NIA (2007.04 - 2007.12)
• Research on L3SHIM support API for IPv6 site multihoming sponsored by ETRI (2007.03 - 2007.09)
• Testbed implementation and evaluation of mobility support platform for IPv6 mobile nodes and networks sponsored by KT (2006.02 - 2006.12)
• Research on IPv6 network mobility and multi-homing for portable internet sponsored by KT (2005.03 - 2005.12)

§ Other Experiences

International Visiting Research

• The Ohio State University. (2009. 7 - 2009. 12) (OSU)
 (Advisor : Ness B. Shroff)
• The University of Connecticut. (2008. 6 - 2008. 8) (UConn)
 (Advisor : Yoo-Ah Kim)

Awards and Honors

• The prize for excellence.
  In the 2nd KARUS RFID/USN Research Paper Contest, awarded by the President of KARUS, November 2006.
  Title of paper: A Hybrid Query Tree Protocol for Tag Collision Arbitration in RFID systems.
  (Jiho Ryu, Yongho Seok, Hojin Lee, Taekyoung Kwon, and Yanghee Choi)
• The best student paper award(3rd place).
  IEEE Consumer Communications & Networking Conference (CCNC) 2006, January 2006.
  Title of paper: Unicast-Friendly Multicast in IEEE 802.11 Wireless LANs.
  (Nakjung Choi, Jiho Ryu, Yongho Seok, Yanghee Choi, and Taekyoung Kwon)

Reviewer

• Ad Hoc Networks, SECON 2010, JCCI 2010, IEEE WCNC 2010 - MAC
• MSWiM 2009, ISWCS'09, ICOIN 2009
• ICCCN 2008, ICC'08 WCS, JCCI 2008
• IEEE Globecom 2007 Ad-hoc and Sensor Networking Symposium, IEEE/Create-Net /ICST COMSWARE 2007.

Internship

• Internship at Electronics and Telecommunications Research Institute. (2004. 7 - 2004. 8) (ETRI)
• Internship at LG Electronics Institute of Technology. (2004. 1 - 2004. 2) (LG Elite)

Activities and Memberships

• Student member of Institute of Electrical and Electronics Engineers, Inc. [2007.03 -] (IEEE)
• Student member of Korean Institute of Information Scientists and Engineers. [2006.01 -] (KIISE)
• Student member of Korean Information and Communications Society. [2005.06 -] (KICS)
• Was a vice-chairman and chairman of KAIST Kumdo Club (검우회 in Korean) in 2002 and 2003 respectively.

§ Computer Related Skills

• Programming Language : C, C++, Perl, Java
• OS : MS Windows, Linux/FreeBSD, MAC OS X
• Tools : NS-2, QualNet

§ Link

• My cyworld home
• The networking conference statistics page maintained by Kevin Almeroth.
• Estimated impact of publication venues in Computer Science
• Call for Papers on wireless and mobile network control
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