Il-Kyu Park

Improving TCP performance in multipath packet forwarding networks

This paper investigates schemes to improve TCP performance in multipath forwarding networks. In multipath routing, packets to the same destination are sent to multiple next-hops in either packet-level or flow-level forwarding mode. Effective bandwidth is increased since we can utilize unused capacity of multiple paths to the destination. In packet-level multipath forwarding networks, TCP performance may not be enhanced due to frequent out-of-order segment arrivals at the receiver because of different delays among paths. To overcome this problem, we propose simple TCP modifications. At the sender, the fast retransmission threshold is adjusted taking the number of paths into consideration. At the receiver, the delayed acknowledgment scheme is modified such that an acknowledgment for an out-of-order segment arrival is delayed in the same way for the in-order one. The number of unnecessary retransmissions and congestion window reductions is diminished, which is verified by extensive simulations. In flow-level multipath forwarding networks, hashing is used at routers to select outgoing link of a packet. Here, we show by simulations that TCP performance is increased in proportion to the number of paths regardless of delay differences.

Hierarchical ZnO Nanorods on Si Micropillar Arrays for Performance Enhancement of Piezoelectric Nanogenerators

Enhanced output power from a ZnO nanorod (NR)-based piezoelectric nanogenerator (PNG) is demonstrated by forming a heterojunction with Si micropillar (MP) array. The length of the SiMP array, which was fabricated by electrochemical etching, was increased systematically from 5 to 20 μm by controlling the etching time. Our structural and optical investigations showed that the ZnO NRs were grown hierarchically on the SiMPs, and their crystalline quality was similar regardless of the length of the underlying SiMPs. The peak output voltage from the ZnO NR-based PNG was greatly increased by ∼5.7 times, from 0.7 to 4.0 V, as the length of the SiMP arrays increased from 0 (flat substrate) to 20 μm. The enhancement mechanism was explained based on the series connection of the ZnO NRs regarded as a single source of piezoelectric potential by creating a heterojunction onto the SiMP arrays.

Emission Pattern Control of GaN-Based Light-Emitting Diodes with ZnO Nanostructures

We report a controllable way of changing the emission patterns of GaN-based blue light-emitting diodes (LEDs) using ZnO nanorods (NRs) grown hydrothermally. The shape of the ZnO NRs was controlled using seed layers for flower, askance, and vertical structures. The electrical properties of the LEDs with the ZnO NRs did not degrade, while the integrated electroluminescence intensity increased compared with that of the bare LEDs. The emission patterns of the LEDs were broadened as the inclination angle of the ZnO NRs increased. These are attributed to the ZnO NRs acting a role in scattering and guiding the light wave efficiently.

VENUS: the online game simulator using massively virtual clients

In this paper, we present an efficient method for simulating massively virtual clients in an online game environment. Massively multiplayer online games and other multi-user based networked applications are becoming more attractive to the gamer players. Such kind of technology has long been researched in the area called Networked Virtual Environments. In the game development process, a set of beta tests is used to ensure the stability of online game servers. A set of testing processes consumes a lot of development resources such as cost, time, and etc. The purpose of the VENUS (Virtual Environment Network User Simulator) system is to provide an automated beta test environment to the game developers to efficiently test the online games to reduce development resources.

Fabrication of ZnO Nanorod/polystyrene Nanosphere Hybrid Nanostructures by Hydrothermal Method for Energy Generation Applications

We report on the successful fabrication of ZnO nanorod (NR)/polystyrene (PS) nanosphere hybrid nanostructure by combining drop coating and hydrothermal methods. Especially, by adopting an atomic layer deposition method for seed layer formation, very uniform ZnO NR structure is grown on the complicated PS surfaces. By using zinc nitrate hexahydrate and hexamine as sources for Zn and O in hydrothermal process, hexagonal shaped single crystal ZnO NRs are synthesized without dissolution of PS in hydrothermal solution. X-ray diffraction results show that the ZnO NRs are grown along c-axis with single crystalline structure and there is no trace of impurities or unintentionally formed intermetallic compounds. Photoluminescence spectrum measured at room temperature for the ZnO NRs on flat Si and PS show typical two emission bands, which are corresponding to the band-edge and deep level emissions in ZnO crystal. Based on these structural and optical investigations, we confirm that the ZnO NRs can be grown well even on the complicated PS surface morphology to form the chestnut-shaped hybrid nanostructures for the energy generation and storage applications.

Fabrication of ZnO Nanorod based Robust Nanogenerator Metal Substrate

We report on the succesful fabrication of ZnO nanorod (NR)-based robust piezoelectric nanogenerators (PNGs) by using Cu foil substrate. The ZnO NRs are successfully grown on the Cu foil substrate by using all solution based method, a two step hydrothermal synthesis. The ZnO NRs are grown along c-axis well with an average diameter of 75~80 nm and length of . The ZnO NRs showed abnormal photoluminescence specrta which is attributed from surface plasmon resonance assistant enhancement at specific wavelength. The PNGs on the SUS substrates show typical piezoelectric output performance which showing a frequency dependent voltage enhancement and polarity dependent charging and discharging characteristics. The output voltage range is 0.79~2.28 V with variation of input strain frequency of 1.8~3.9 Hz. The PNG on Cu foil shows reliable output performance even at the operation over 200 times without showing degradation of output voltage. The current output from the PNG is which is a typical out-put range from the ZnO NR-based PNGs. These performance enhancement is attributed from the high flexibility, high electrical conductivity and excellent heat dissipation properties of the Cu foil as a substrate.

Mobile authoring platform and mobile app for high quality stereoscopic contents

After introducing mobile digital devices with camera broadly, users have changed their stance from the consumers to the prosumers that produce and share contents themselves. Lately, as the 3D image output devices are provided, the image processing technologies are extended from 2D to 3D image. In this paper, we propose the design method for 3D image authoring platform and mobile apps using the platform. The 3D image authoring platform provides three functions: object recognition and segmentation, hole filling in occlusion area, and object removal and hole filling in the removed area. We tested the 3D image authoring platform through developing mobile apps for general users and professional users. As 3D image output devices like stereo cameras and light field cameras are introduced in the future, we will support newly introduced devices using our 3D image authoring platform.