Eui-Jik Kim

Asynchronous inter-network interference avoidance for wireless body area networks

This paper considers the internetwork interference problem in environments with multiple wireless body area networks (WBANs). We propose an asynchronous internetwork interference avoidance scheme (abbreviated as AIIA), which is based on the hybrid multiple access of carrier sense multiple access with collision avoidance (CSMA/CA) and time division multiple access (TDMA). In AIIA, the gateway device of each WBAN maintains a table, called an AIIA table, which includes the timing offset and TDMA transmission schedule information corresponding to the interfering WBANs. By referring to the table, the conflicting TDMA schedule can be checked and updated by itself, in asynchronous and distributed manners. Extensive simulations are conducted to demonstrate the feasibility and effectiveness of AIIA.

Neighbor stability-based VANET clustering for urban vehicular environments

In this paper, we propose a neighbor stability-based VANET clustering (NSVC) that can efficiently deliver data in urban vehicular environments. The salient features of urban vehicles are their high mobility and unpredictable direction of movement, so vehicle-to-vehicle and vehicle-to-infrastructure (V2X) communication should take into consideration the frequent changes in the topology of vehicular ad hoc networks (VANETs). These technical challenges are addressed with NSVC by including a neighbor stability-based VANET clustering scheme and the corresponding supplementary transmission scheduling method. Thereby, NSVC supports fast cluster formation, minimizes the number of cluster head elections, and moreover guarantees the reliable delivery of data for emergency messages. The results of the simulation indicate that NSVC achieves better network performance when compared to existing approaches.

Adaptive multi-channel allocation for vehicular infrastructure mesh systems

This paper focuses on a wireless solution for vehicular infrastructure systems. In order to achieve both low cost and high efficiency, infrastructures can be connected to each other in vehicular networks by a wireless link similar to a mesh router in wireless mesh networks (WMNs). However, the existing WMN solutions cannot appropriately support various vehicular applications that require high rate and low latency communications. Therefore, in this paper, we present the design and performance evaluation of an adaptive multi-channel allocation for vehicular infrastructure mesh systems (abbreviated AMCA). In order to meet both high rate and low latency communications, AMCA is designed to provide optimal channel assignment duration for each flow to efficiently utilize multiple non-overlapping channels. The performance evaluation of AMCA is conducted by the QualNet 5.0 simulator under various network scenarios to consider diverse network conditions. Simulation results show that AMCA can achieve higher network throughput and lower average packet delay than other well known wireless solutions.

EBBS: Energy-efficient BOP-based Beacon transmission Scheduling for WSNs

This paper describes an energy-efficient BOP-based beacon transmission scheduling (EBBS) algorithm. EBBS is the centralized approach, in which a resource-sufficient node called as Topology Management Center (TMC) allocates the time slots to transmit a beacon frame to the nodes and manages the active/sleep schedules of them. We also propose EBBS with Adaptive BOPL (EBBS-AB), to adjust the duration to transmit beacon frames in every beacon interval, adaptively. Simulation results show that by using the proposed algorithm, the energy efficiency and the throughput of whole network can be significantly improved. EBBS-AB is also more effective for the network performance when the nodes are uniformly deployed on the sensor field rather than the case of random topologies.

Multi-level service differentiation scheme for the IEEE 802.15.4 sensor networks

In the sensor networks, the data packets transmitted by the different devices in the home networking and the industrial application maintain the different levels of importance. In this paper, we propose two mechanisms for the IEEE 802.15.4 sensor networks to provide the multi-level differentiated services which are required by each and every device. The mathematical model based on the discrete-time Markov chain is presented and is analyzed to measure the performances of the proposed mechanisms. The numerical results show the effect of the variation of the contention window size and the backoff exponent for the service differentiation on the 802.15.4 sensor networks. From the results, we derive that the contention window size is more affective than the backoff exponent on the service differentiation while the backoff exponent is more affective than the contention window size on the average delay of every device. Simulation results are also given to verify the accuracy of the numerical model.

Performance analysis for contention adaptation of M2M devices with directional antennas

In the directional carrier sense multiple access-collision avoidance (directional-CSMA-CA) mechanism, the efficient channel access mechanism with a collision-controlled concept is one of the most challenging topics. Especially, in capillary machine-to-machine (M2M) where wireless personal area network (WPAN) is a key technology for its wireless connectivity, a contention-based medium access control (MAC) protocol is crucial to enable numerous M2M devices to concurrently access the channel. This paper presents an analytical model for the slotted directional-CSMA-CA mechanism of 802.15.3c networks under saturated environment. By presenting a multi-class approach of directional-CSMA-CA mechanism to reduce collision occurrences, we support an adaptive back-off exponent of M2M device on the required frame payload size. For the mathematical analysis, the discrete-time Markov chain model is applied with the features of PHY/MAC of millimeter Wave (mm-Wave) network and the effects of directional antennas. In addition, extensive simulations are performed to verify the analytical model. The obtained results of the analytic and simulation show that the collision probability of M2M device is reduced by differentially adapting the contention rate to each M2M device, and the fairness property can be improved because there are remarkable increases of opportunities to transmit short-frame payloads.

Hybrid storage-based caching strategy for content delivery network services

This study proposes a novel caching scheme for content delivery network services. In general, video content users often watch the first part of video clips and then switch to other content. Therefore, a caching scheme is proposed, in which the first part of the frequently referenced content is stored on a solid state drive (SSD) while the remaining video content is stored on a hard disk drive (HDD),. The proposed hybrid (SSD/HDD) caching scheme offers several benefits, such as an improved average data output capacity due to the high average data rate and average hit capacity of the SSD. That is, performance can be significantly improved at a low extra cost with the cache server of a content delivery network (CDN).

Multi-hop-based opportunistic concurrent directional transmission in 60 GHz WPANs

In millimeter Wave wireless personal area networks (mmWave WPANs), the design of efficient concurrent transmission considered high modulations up to a few Gbps is one of the most challenging issues. Even for the concurrent transmission over mmWave networks, the use of directional antenna is highly recommended to guarantee high modulations and to overcome short propagation range caused to high path loss in mmWave frequency. Nevertheless the directional antenna has many advantages, users may suffer from performance degradation due to coverage limitation of wide beamwidth, when the concurrent transmission supports the multicast communication for the target applications such as conference room, wireless displays and room gaming. In this paper, we propose a multi-hop-based opportunistic concurrent directional transmission (M-OCDT) scheme for the directional multicast communication where the relay mechanism is generated depending on the locations of multicast users to maximize the sum rate. The proposed M-OCDT scheme is designed based on the IEEE 802.15.3c standard and supports the optimized searching algorithm for the relay users. Extensive simulation results demonstrate that the M-OCDT scheme can improve the average overall throughput by 81 to 89 % compared with the conventional non-relay directional multicast procedure.

Low-Concentration Indium Doping in Solution-Processed Zinc Oxide Films for Thin-Film Transistors

We investigated the influence of low-concentration indium (In) doping on the chemical and structural properties of solution-processed zinc oxide (ZnO) films and the electrical characteristics of bottom-gate/top-contact In-doped ZnO thin-film transistors (TFTs). The thermogravimetry and differential scanning calorimetry analysis results showed that thermal annealing at 400 °C for 40 min produces In-doped ZnO films. As the In content of ZnO films was increased from 1% to 9%, the metal-oxygen bonding increased from 5.56% to 71.33%, while the metal-hydroxyl bonding decreased from 72.03% to 9.63%. The X-ray diffraction peaks and field-emission scanning microscope images of the ZnO films with different In concentrations revealed a better crystalline quality and reduced grain size of the solution-processed ZnO thin films. The thickness of the In-doped ZnO films also increased when the In content was increased up to 5%; however, the thickness decreased on further increasing the In content. The field-effect mobility and on/off current ratio of In-doped ZnO TFTs were notably affected by any change in the In concentration. Considering the overall TFT performance, the optimal In doping concentration in the solution-processed ZnO semiconductor was determined to be 5% in this study. These results suggest that low-concentration In incorporation is crucial for modulating the morphological characteristics of solution-processed ZnO thin films and the TFT performance.

Monitoring Agent for Detecting Malicious Packet Drops for Wireless Sensor Networks in the Microgrid and Grid-Enabled Vehicles

Of the range of wireless communication technologies, wireless sensor networks (WSN) will be one of the most appropriate technologies for the Microgrid and Grid-enabled Vehicles in the Smartgrid. To ensure the security of WSN, the detection of attacks is more efficient than their prevention because of the lack of computing power. Malicious packet drops are the easiest means of attacking WSNs. Thus, the sensors used for constructing a WSN require a packet drop monitoring agent, such as Watchdog. However, Watchdog has a partial drop problem such that an attacker can manipulate the packet dropping rate below the minimum misbehaviour monitoring threshold. Furthermore, Watchdog does not consider real traffic situations, such as congestion and collision, and so it has no way of recognizing whether a packet drop is due to a real attack or network congestion. In this paper, we propose a malicious packet drop monitoring agent, which considers traffic conditions. We used the actual traffic volume on neighbouring nodes and the drop rate while monitoring a sending node for specific period. It is more effective in real network scenarios because unlike Watchdog it considers the actual traffic, which only uses the Pathrater. Moreover, our proposed method does not require authentication, packet encryption or detection packets. Thus, there is a lower likelihood of detection failure due to packet spoofing, Man- In-the Middle attacks or Wormhole attacks. To test the suitability of our proposed concept for a series of network scenarios, we divided the simulations into three types: one attack node, more than one attack nodes and no attack nodes. The results of the simulations meet our expectations.