Byoung-Hoon Lee

Lease-based consistency schemes in the web environment

There are many methods to maintain consistency in the distributed computing environment. Ideally, efficient schemes for maintaining consistency should take into account the following factors: lease duration of replicated data, data access pattern and system parameters. One method used to supply strong consistency in the web environment is the lease method. During the proxys lease time from a web server, the web server can notify the modification to the proxy by invalidation or update. In this paper, we analyze lease protocol performance by the varying update/invalidation scheme, lease duration and read rates. By using these analyses, we can choose the adaptive lease time and proper protocol (invalidation or update scheme of the modification for each proxy in the web environment). As the number of proxies for web caching increases exponentially, a more efficient method for maintaining consistency needs to be designed. We also present three-tier hierarchies on which each group and node independently and adaptively choose the proper lease time and protocol for each proxy cache. These classifications of the scheme make proxy caching adaptive to client access pattern and system parameters.

Scalable real-time monitoring system for ubiquitous smart space

Scalable real-time monitoring is one of important requirements to supply smart services for customers, on time, in ubiquitous smart space. We propose a scalable real-time monitoring method to aggregate large amounts of data from various sensor devices distributed over different domain areas. The real-time monitoring scheme can process data from sensor devices within deadline and is scalable based on the number of sensor devices. The scalability of monitoring can be improved by employing a hierarchical monitoring agent for sensor devices while still satisfying the data deadlines. Simulation results show that our real-time monitoring scheme can improve the met deadline ratio up to 27% compared to the previous schemes.

Diversity and fault avoidance for dependable replication systems

In the hot-standby replication system, the system cannot process its tasks anymore when all replicated nodes have failed. Thus, the remaining living nodes should be well-protected against failure when parts of replicated nodes have failed. Design faults and system-specific weaknesses may cause chain reactions of common faults on identical replicated nodes in replication systems. These can be alleviated by replicating diverse hardware and software. Going one-step forward, failures on the remaining nodes can be suppressed by predicting and preventing the same fault when it has occurred on a replicated node. In this paper, we propose a fault avoidance scheme which increases system dependability by avoiding common faults on remaining nodes when parts of nodes fail, and analyze the system dependability.

Minimum energy and latency MAC protocol for wireless sensor networks

Wireless sensor network consists of sensor nodes having limited resources. Energy saving is very important to increase the life time of these networks. In this paper, we propose Minimum Energy and Latency MAC (MELM) protocol for wireless sensor networks. MELM uses dual frequency radio setup inspired by STEM and LEEM to minimize the energy consumption and the latency in a multi-hop environment. Both LEEM and STEM protocols squander energy and increase latency because of overhearing, over-transmission and packet collisions. MELM eliminates these problems by maintaining a scheduling table in all nodes having logical 2-hop distance from the receiver node and by using advance channel reservation mechanism. MELM eradicates the hidden terminal and the exposed terminal problems, which is a key advantage, by maintaining a 2-hop receiver centric scheduling table.

Dependency-Based dynamic component reconfiguration for wireless computing systems

Wireless computing system has to satisfy the changeable requirements of various users and environments. Therefore the coming wireless computing requires systems to be more available, flexible and adaptable. Dynamic reconfiguration certainly became the necessary technique because it provides adaptability and flexibility by reconstructing system components. Dynamic reconfiguration of component based wireless computing system consists of modifying the configuration of components of a system during runtime. As system can execute dynamic reconfiguration, system can have ability to easily adapt to new environment and to be more easily extensible. Therefore, the development of dynamic reconfiguration is essential for wireless computing systems. But dynamic reconfiguration has crucial problems, performance, QoS and fault tolerance. We need efficient management for dynamic reconfiguration to solve these problems. We propose an efficient method to manage dynamic component reconfiguration using the dependency relations between components for wireless computing systems. Performance analysis shows that our scheme can reduce execution time and down-time by considering dependency between components.

Process scheduling policy based on rechargeable power resource in wireless sensor networks

Many multi-process scheduling schemes have been developed to utilize system resources efficiently. In the previous researches, most of the scheduling schemes were focused on how to allocate computing resources efficiently. However, in the environment of wireless sensor networks, scheduling policy for power resources of mobile device is also essential as high power drain of mobile sensor device will lead to immediate non-functioning of device due to limited capacity of its battery. Sensor nodes can also be powered by rechargeable power resources such as solar cell or gravitation. In these cases, residual energy of rechargeable power resource is dynamic over time. This motivated us to design a scheduling policy based on rechargeable power resource for increasing the network life time and reducing the response time of service. In this paper, we present and evaluate six process scheduling schemes based on execution time and energy consumption of process.