• Journal of the korean academy of Pediatric Dentistry
• /
• v.26 no.2
• /
• pp.446-452
• /
• 1999

Ectopic eruption of the first permanent molar means the first permanent molar assumes an atypical path of eruption resulting in premature atypical resorption of the second primary molar. If the reversible eruption does not occur, early loss of the second primary molars results in space loss, mesial tipping of the first permanent molar, impaction of the second premolar, buccal segment crowding and overeruption of opposing tooth. The main objectives of treatment are (1) to prevent loss of the second deciduous molars so it can continue to serve as a space maintainer and (2) to regain lost arch length, allowing the second premolar to erupt into normal position. The optimal treatment approach depends on a number of factors including the clinical eruption status of /6/, the change in position of /6/, the amount of enamel ledge of /E/ entrapping /6/, the mobility of /E/, and the presence of pain or infection. Unilateral appliance to correct the mesial angulation of ectopic permanent first molars, as in the majority of the appliance designs, would produce a resultant force that would further enhance the space loss. A bilateral support similar to the holding arch design is recommended to maximize the anchorage. These case reports present the successful result of preserving space for the second premolar in treatment of ectopic eruption of the first permanent molar using Halterman appliance with bilateral anchorage on patients visiting department of pediatric dentistry in Samsung Medical Center.

• Journal of the Korea Society of Computer and Information
• /
• v.16 no.1
• /
• pp.133-142
• /
• 2011

When TCP operates in multi-hop wireless networks, it suffers from severe performance degradation due to the different characteristics of wireless networks and wired networks. This is because TCP reacts to wireless packet losses by unnecessarily decreasing its sending rate assuming the losses as congestion losses. Although several loss differentiation algorithms (LDAs) have been proposed to avoid such performance degradation, their detection accuracies are not high as much as we expect. In addition the schemes have a tendency to sacrifice the detection accuracy of congestion losses while they improve the detection accuracy of wireless losses. In this paper, we suggest a new sender-based loss differentiation scheme which enhances the detection accuracy of wireless losses while minimizing the sacrifice of the detection accuracy of congestion losses. Our scheme estimates the rate of queue usage which is highly correlated with the congestion in the network path between a TCP sender and a receiver, and it distinguishes congestion losses from wireless losses by comparing the estimated queue usage with a certain threshold. In the extensive experiments based on a network simulator, QualNet, we measure and compare each detection accuracy of wireless losses and congestion losses, and evaluate the performance enhancement in each scheme. The results show that our scheme has the highest accuracy among the LDAs and it improves the most highly TCP performance in multi-hop wireless networks.

• Journal of the Korea Academia-Industrial cooperation Society
• /
• v.17 no.9
• /
• pp.401-406
• /
• 2016

When measuring live cable faults and their location based on reflectometry, a coupler is placed between the cable and the test system. This coupler prevents damage to the test circuits by indirectly measuring the live voltage of the cable using reflectometry. It also provides a coupling path that allows the transmission and receive signal to pass into the cable. In this study, we design and construct a contact coupler to locate faults in both dead and live cables using reflectometry. The proposed coupler is of the inductive coupling type and is constructed after the calculation of the signal transmission loss by simulation. The performance of the developed coupler is tested by measuring the transmission loss and frequency flatness. The results showed that the transmission signal loss is less than -1.98dB in the frequency bandwidth above 1 Mhz. The reflectometry system was designed based on sequence time domain reflectometry (STDR) and spread spectrum time domain reflectometry (SSTDR) in order to apply it to the detection of faults and their location in live cables and tests on live cables were performed. The test results showed that the proposed coupler can be used in a reflectometry system for live cable fault detection.

• Journal of Advanced Navigation Technology
• /
• v.5 no.1
• /
• pp.62-73
• /
• 2001

In this paper, performance of DSRC systems is analyzed with considering the real roads and height of vehicles. The channels are modeled as 2-Ray and 4-Ray with a direct wave and reflected waves by a road and buildings in a physical layer because DSRC keeps LOS propagation characteristics, and the pass loss for each model is calculated respectively. Rician factor is obtained through the calculated path loss on two models for DSRC, and the performance of the systems is analyzed in AWGN and Rician fading channels, Impulsive noise and Rician fading channels respectively. As a result, in Rician fading channels with impulsive noise(A=0.2, ${\Gamma}^{\prime}=0.22$), BER is below $10^{-6}$ when the distance is farther than 80[m] and 40[m] in 2-Ray model and 4-Ray model respectively. For performance improvement, BCH coding scheme and MRC diversity scheme are adopted.

• The Journal of the Acoustical Society of Korea
• /
• v.32 no.1
• /
• pp.43-55
• /
• 2013

The window ventilation system based on the heat recovery device was developed which make air ventilation possible without opening the windows. However, mechanical and aerodynamic noises were come to pass which annoyed people in rooms. In the present study, noise of new window ventilation system was measured in both general room and anechoic chamber. Also, the noise path was detected to find cause of noise generation and vulnerable area of the device. Sound absorptive and insulation materials were applied to mitigate the noise. Finally, an alternative noise control method was suggested which can satisfy with the indoor noise standards. As a result, it was shown that the cause of noise was the low transmission loss in the ventilation system. As a result, it was shown that the main noise source of the ventilation system was the blower and the major cause of noise was the low transmission loss of the ventilation system. It is also concluded that the noise levels complies with the noise standards of 40 dBA when 2 mm rubber sheet is applied inside the ventilation system.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
• /
• v.19 no.4
• /
• pp.470-476
• /
• 2008

In this paper, we describe the cause of TDMA noise and distinction method of TDMA noise source in a GSM mobile phone. The causes of TDMA noise are composed of RF(Radio Frequency) energy coupling and low frequency energy coupling by burst ripple. We propose the distinction method of TDMA noise source from output(TDMA noise measurement) and frequency response of a system(audio path). Especially we propose a method of insertion loss($S_{21}$) analysis and the improvement method for RF energy coupling. Capacitor(40 pF) is a solution to reduce RF energy coupling and therefore TDMA noise was reduced by 10 dB.

• Journal of Broadcast Engineering
• /
• v.18 no.3
• /
• pp.445-454
• /
• 2013

By using the radio propagation prediction of Rec. ITU-R P.1546, geographic information system, and S-I plane, we presented the methodology of interference analysis based on the minimum coupling loss, and also suggested the local coordinate system for calculating azimuth and elevation angles between the victim receiver and the interferer for an arbitrary antenna pattern. To check the presented algorithm, the map with the land-sea mixed area was taken for the given area of $80{\times}60[km^2]$ as real geography information. Field strength, path profile, and protection ratio with maximum allowable interference level have been illustrated for radar and fixed wireless system for the assumed frequency. In addition interference power of the victim receiver was calculated asa function of azimuth and elevation angles of the interferer. The developed methodology of interference analysis in the VHF and UHF bands can be actually applied to assess interoperability as well as compatibility in the civil or military applications.

• Journal of Internet Computing and Services
• /
• v.6 no.3
• /
• pp.55-70
• /
• 2005

In order to provide mobile hosts with multicast service in mobile communication environment, we proposed a multicast mechanism named HXcast++ which is an extended version of the existing Xcast++ with hierarchical architecture, We assured that mobile hosts could get multicast service through an optimal path regardless of their location by making DR(Designated Router) join a group on behalf of the mobile hosts, In this present research we introduced hierarchical architecture in order to reduce the maintenance cost resulting from frequent handoff. We also proposed a GMA (Group Management Agent) based group management mechanism which enables the mobile hosts to join the group without waiting for a new IGMP Membership Query. A fast handoff method with L2 Mobile Trigger was, in this work, employed in order to reduce the amount of the packet loss which occurs as a result of the handoff, We also managed to curtail the packet loss caused by the latency of the group join by using a buffering and forward mechanism.

• Journal of KIISE:Information Networking
• /
• v.31 no.6
• /
• pp.620-632
• /
• 2004

IP is not suitable for mobile nodes by network-based routing because mobile nodes are dynamically change their network attachment point. Mobile-IP is an IETF standard providing continuous access to the Internet for mobile nodes, but it has the triangle routing problem. Also it has a performance degradation problem by massive packet loss during layer 3 handoff of mobile nodes. Especially, two IETF multicast support schemes for Mobile-IP do not guarantee the quality of realtime multimedia services because they have several problems such as long routing path, packet duplication(hi-directional tunneling) and massive packet loss(remote subscription). In this paper, we propose a lossless remote subscription scheme that guarantees the quality of realtime multimedia services over Mobile-IP. From the result of simulation, we verified that the proposed scheme in this paper can reduce the delay time of remote subscription by the effect of the low latency handoff scheme that is extended to apply to the multicast group management and it requires only 0.58% buffer spaces of the previously proposed lossless remote subscription scheme.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.37B no.9
• /
• pp.723-733
• /
• 2012

In wireless ad hoc networks, packet loss and latency are frequently occurred by movement of node. As the number of transmission hops increases in these networks, the throughput is increasingly deteriorated. In this paper, we design the wireless ad hoc network system based on IEEE 802.16j MMR (Mobile Multi-hop Relay), which improves the network performance. Our wireless ad hoc network system used adaptively the nearest BS and Farthest BS scheduling algorithms for efficient data transmission and chose the optimal path that minimize data loss and latency. In order to evaluate performance of the wireless ad hoc network system based on IEEE 802.16j MMR, we used the LWX (Light Wimax) model of NS-2 simulator. Through the simulation, we analyzed the network performance for various scenarios.