Performance impact of interlayer dependence in infrastructure WLANs

Choi, Sunwoong,Park, Kihong,Kim, Chong-kwon IEEE 2006 IEEE TRANSACTIONS ON MOBILE COMPUTING Vol.5 No.7

Widespread deployment of infrastructure WLANs has made Wi-Fi an integral part of today's Internet access technology. Despite its crucial role in affecting end-to-end performance, past research has focused on MAC protocol enhancement, analysis, and simulation-based performance evaluation without sufficient consideration for modeling inaccuracies stemming from interlayer dependencies, including physical layer diversity, that significantly impact performance. We take a fresh look at IEEE 802.11 WLANs and using experiment, simulation, and analysis demonstrate its surprisingly agile performance traits. Our findings are two-fold. First, contention-based MAC throughput degrades gracefully under congested conditions, enabled by physical layer channel diversity that reduces the effective level of MAC contention. In contrast, fairness degrades and jitter increases significantly at a critical offered load. This duality obviates the need for link layer flow control for throughput improvement. Second, TCP-over-WLAN achieves high throughput commensurate with that of wireline TCP under saturated conditions, challenging the widely held perception that TCP throughput fares poorly over WLANs when subject to heavy contention. We show that TCP-over-WLAN prowess is facilitated by the self-regulating actions of DCF and TCP feedback control that jointly drive the shared channel at an effective load of two to three wireless stations, even when the number of active stations is large. We show that the mitigating influence of TCP extends to unfairness and adverse impact of dynamic rate shifting under multiple access contention. We use experimentation and simulation in a complementary fashion, pointing out performance characteristics where they agree and differ.

Minimizing Device-to-Device Variation in the Spectral Response of Portable Spectrometers

Choi, Sunwoong,Kim, Youngsik,Lee, Jae-Hyung,You, Hanjong,Jang, Byung-Jun,Jung, Kyeong-Hoon Hindawi Limited 2019 Journal of sensors Vol.2019 No.-

<P>As portable spectrometers have been developed, the research of spectral analysis has evolved from a traditional laboratory-based closed environment to a network-connected open environment. Consequently, its application areas are expanding in combination with machine learning techniques. The device-to-device variation in the spectral response of portable spectrometers is a critical issue in a machine learning-based service scenario since the classification performance is highly dependent on the consistency of spectral responses from each spectrometer. To minimize device-to-device variation, a cuboid prism is employed instead of a combination of mirrors and prism to construct an optical system for the spectrometer. The spectral responses are calibrated to correct pixel shift on the image sensor. Experimental results show that the proposed method can minimize the device-to-device variation in spectral response of portable spectrometers.</P>

MRCT: An Efficient Tag Identification Protocol in RFID Systems with Capture Effect

( Sunwoong Choi ),( Jaehyuk Choi ),( Joon Yoo ) 한국인터넷정보학회 2013 KSII Transactions on Internet and Information Syst Vol.7 No.7

In RFID systems, one important issue is how to effectively address tag collision, which occurs when multiple tags reply simultaneously to a reader, so that all the tags are correctly identified. However, most existing anti-collision protocols assume isotropic collisions where a reader cannot detect any of the tags from the collided signals. In practice, this assumption turns out to be too pessimistic since the capture effect may take place, in which the reader considers the strongest signal as a successful transmission and the others as interference. In this case, the reader disregards the other collided tags, and in turn, fails to read the tag(s) with weaker signal(s). In this paper, we propose a capture effect-aware anti-collision protocol, called Multi-Round Collision Tree (MRCT) protocol, which efficiently identifies the tags in real RFID environments. MRCT deals with the capture effect as well as channel error by employing a multi-round based identification algorithm. We also analyze the performance of MRCT in terms of the number of slots required for identifying all tags. The simulation results show that MRCT significantly outperforms the existing protocol especially in a practical environment where the capture effect occurs.

Fast transition for couple-resolution blocking protocol in RFID systems

Choi, Hae-il,Choi, Sunwoong Hikari, Ltd. 2016 Contemporary Engineering Sciences Vol.9 No.-

Standards and codes activities in fusion jointing of polyethylene pressure pipes

Sunwoong Choi(최선웅) 대한용접·접합학회 2018 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2018 No.11

Development of Smart Mobile Manipulator Controlled by a Single Windows PC Equipped with Real-Time Control Software

Sunwoong Choi,Shinsuk Park 한국정밀공학회 2021 International Journal of Precision Engineering and Vol.22 No.10

Despite significant advances in robotic technologies, few affordable robots for domestic applications are commercially available. This study aimed to develop an affordable yet practical mobile manipulator for use in household environments. The proposed mobile manipulator features a software-based real-time controller to control the motion of the manipulator and mobile base. The real-time controller was implemented on a single on-board Windows PC and can control the actuator and IO devices via EtherCAT communication. The major functionalities of the mobile manipulator include the pick-and-place of household objects and autonomous navigation within a domestic environment. For pick-and-place tasks, a deep neural network (DNN) was employed to recognize the object to pick up. For autonomous navigation, open-source ROS packages for SLAM and navigation were used along with the measurements from a LiDAR sensor and odometry. The feasibility of using the developed robot for domestic applications was experimentally evaluated.

Adaptive couple-resolution blocking protocol for repeated tag identification in RFID systems

Choi, Sunwoong,Choi, Hae-il SpringerOpen 2016 Eurasip Journal on Wireless Communications and Net Vol.2016 No.-

<P>RFID applications such as monitoring an object for a long time need to identify tags repeatedly within the scope of the reader. Re-identification process can be improved using the information obtained from the previous tag identification process. Couple-resolution blocking (CRB) protocol utilizes the blocking technique that prevents staying tags from being collided by newly arriving tags. Staying tags can be efficiently re-identified by utilizing the retained information. After staying tags are separately all identified, arriving tags are identified. In this paper, we argue that CRB may work more poorly than other protocols which do not consider the repeated tag identification, such as query tree (QT) and collision tree (CT) protocol, when only few tags stay. To tackle the problem, we propose an adaptive CRB (ACRB) protocol. In ACRB, the reader estimates the tag staying ratio during the re-identification process for staying tags. If the estimated ratio is lower than a certain threshold, the blocking technique is immediately abandoned. Instead, staying tags and arriving tags are identified together without considering the retained information. In addition, we propose to improve CRB further using CT protocol, instead of QT protocol. Through computer simulation, we show that ACRB improves the identification efficiency of CRB, especially when the tag staying ratio is low.</P>

RFID 태그 식별 시의 캡쳐 효과에 대처하기 위한 효율적인 충돌 방지 프로토콜

최선웅(Sunwoong Choi),최재혁(Jaehyuk Choi),유준(Joon Yoo) 한국정보과학회 2011 정보과학회논문지 : 정보통신 Vol.38 No.6

충돌 트리(Collision Tree) 프로토콜은 맨체스터 코드(Manchester code)를 활용하여 질의 트리(Query Tree) 프로토콜의 성능을 향상시킨 RFID 충돌 방지 프로토콜이다. 하지만 충돌 트리 프로토콜은 캡쳐 효과(Capture effect)에 대해 고려하지 않고 있다. 본 논문에서는 캡쳐 효과가 발생하는 상황에서, 충돌 트리 프로토콜이 일부 태그를 식별되지 못하는 문제가 발생할 수 있다는 것을 지적한다. 그리고 이를 해결할 수 있는 다단계 충돌 트리(Multi-Round Collision Tree) 프로토콜을 제안한다. 다단계 충돌 트리 프로토콜에서 전체 태그 식별을 위해 필요한 질의 메시지의 수를 분석하고 컴퓨터 시뮬레이션 결과를 보인다. Collision Tree (CT) protocol improves Query Tree (QT) protocol using Manchester code. However, CT does not consider the capture effect. In this paper, we note that it is possible that some tags cannot be identified by the CT under the capture effect. We then propose Multi-Round Collision Tree (MRCT) protocol which guarantees to identify all the tags within the reader’s transmission range. We analyze the number of Query messages for identifying all tags and show the computer simulation results.

물리 계층의 다양성과 무선 랜의 성능에 미치는 영향

최선웅(Sunwoong Choi),박기홍(Kihong Park),김종권(Chong kwon Kim) 한국정보과학회 2005 정보과학회논문지 : 정보통신 Vol.32 No.6

오늘날 인프라 무선 랜은 많은 사용자들이 사용하는 중요한 인터넷 접속 기술이다. 지금까지 무선 랜에 관한 연구들은 물리 계층의 채널 다양성으로 인하여 발생하는 모델링 부정확성에 대해 충분히 고려하지 않고, MAC 프로토콜의 향상 및 분석 그리고 시뮬레이션을 통한 성능 평가에만 초점을 맞추어 왔다. 본 논문에서는 계층 상호 간의 의존성의 중요성에 주목하는 새로운 시각으로 IEEE 802.11 무선 랜의 특성을 고찰한다. 실제 무선 랜 시스템 상에서 실험을 수행하여 물리 계층에서 발생하는 불평등성을 관찰하였다. 그리고, 그것이 노드들 간의 혼잡 수준을 떨어뜨리는 역할을 하여 경쟁 기반의 MAC 처리율이 혼잡 상태에서도 서서히 감소하는 것을 보인다. 반면에 노드들 간의 공평성과 노드 처리율의 안정성은 시스템 입력 로드가 특정 수준을 넘으면 크게 저하되는 것을 보인다. 이와 같은 결과를 통하여, 처리율을 높이기 위해 링크 계층에서 제어를 할 필요성은 적은 반면에, 공평한 자원 분배나 서비스의 품질을 보장하기 위해서는 트래픽 제어가 필요하다는 것을 알 수 있다. 시스템의 성능 평가를 위하여 시뮬레이션과 실험을 병행하였다. Wide spread deployment of infrastructure WLANs has made Wi Fi an integral part of today’s Internet access technology. Despite its crucial role in affecting end to end performance, past research has focused on MAC protocol enhancement, analysis and simulation based performance evaluation without sufficient consideration for modeling inaccuracies stemming from inter layer dependencies, including physical layer diversity, that significantly impact performance. We take a fresh look at IEEE 802.11 WLANs, and using experiment, simulation, and analysis demonstrate its surprisingly agile performance traits. Contention based MAC throughput degrades gracefully under congested conditions, enabled by physical layer channel diversity that reduces the effective level of MAC contention. In contrast, fairness and jitter significantly degrade at a critical offered load. This duality obviates the need for link layer flow control for throughput improvement but necessitates traffic control for fairness and QoS. We use experimentation and simulation in a complementary fashion, pointing out performance characteristics where they agree and differ.

패킷 버퍼링을 이용한 TCP 처리율 보장 방법

최선웅(Sunwoong Choi),김종권(Chongkwon Kim) 한국정보과학회 2001 정보과학회논문지 : 정보통신 Vol.28 No.2

본 논문에서는 차별화 서비스(Differentiated Services) 망에서 TCP 플로우(flow)의 처리율을 보장하는 문제를 연구하였다. 확신 서비스는 망의 혼잡 시에도 약속한 대역폭을 보장하기 위하여 패킷 폐기율을 차별화하는 방법을 사용한다. 그러나 토큰 버퍼 표시자는 TCP의 혼잡제어과 잘 동작하지 않아 적절한 성능을 보이지 못한다. 본 논문에서는 토큰 버퍼와 함께 데이타 버퍼를 사용하는 표시자를 제안하였다. 데이타 버퍼를 사용하는 표시자는 TCP 트래픽을 평활화하는 효과가 있어 확신 서비스 메커니즘과 잘 동작한다. 컴퓨터 시뮬레이션 결과 데이타 버퍼를 사용하는 표시자가 토큰 버퍼만을 사용하는 표시자에 비해 목표 대역폭에 보다 근접한 성능을 보이는 것을 확인하였다. 또한 최적 데이타 버퍼 크기는 예약 대역폭과 RTT에 비례하는 것을 관찰하였다. This paper deals with the TCP bandwidth guarantee problem in a differentiated services(Diffserv) network. The Diffserv assured service differentiates packet drop probabilities to guarantee the promised bandwidth even under network congestion. However a token buffer marker fails to show adequate performance because TCP generates packets according to the unique TCP congestion control mechanism. We propose a marker that uses a data buffer as well as a token buffer. The marker with a data buffer works well with the assured service mechanism because it smooths TCP traffic. We showed that the marker with a data buffer achieves the target throughput better than a marker with a token buffer only. We also showed that the optimal buffer size is proportional to reserved throughput and RTT.