Beakcheol Jang

An asynchronous scheduled MAC protocol for wireless sensor networks

Wireless sensor networks (WSNs) comprise a number of autonomous sensors and one or more sinks to cooperatively monitor physical or environmental conditions. Energy efficiency is a key design factor of a MAC protocol for WSNs. Due to the importance of the problem, a number of energy efficient MAC protocols have been developed for WSNs. Preamble-sampling based MAC protocols (e.g., B-MAC and X-MAC) have overheads due to their preambles, and are inefficient at large wakeup intervals. SCP-MAC, a synchronous scheduled energy-efficient scheduling MAC protocol, minimizes the preamble by combining preamble sampling and scheduling techniques; however, it does not prevent energy loss due to overhearing; in addition, due to its synchronization procedure, it results in increased contention and delay. In this paper, we present an energy efficient MAC protocol for WSNs that avoids overhearing and reduces contention and delay by asynchronously scheduling the wakeup time of neighboring nodes. We provide an energy consumption analysis for multi-hop networks. To validate our design and analysis, we implement the proposed scheme in TinyOS. Experimental results show that AS-MAC considerably reduces energy consumption, packet loss and delay when compared with existing energy efficient MAC protocols.

IEEE 802.11 saturation throughput analysis in the presence of hidden terminals

Due to its usefulness and wide deployment, IEEE 802.11 has been the subject of numerous studies, but still lacks a complete analytical model. Hidden terminals are common in IEEE 802.11 and cause the degradation of throughput. Despite the importance of the hidden terminal problem, there have been a relatively small number of studies that consider the effect of hidden terminals on IEEE 802.11 throughput, and many are not accurate for a wide range of conditions. In this paper, we present an accurate new analytical saturation throughput model for the infrastructure case of IEEE 802.11 in the presence of hidden terminals. Simulation results show that our model is accurate in a wide variety of cases.

AS-MAC: An asynchronous scheduled MAC protocol for wireless sensor networks

Energy efficiency of the MAC protocol is a key design factor for wireless sensor networks (WSNs). Due to the importance of the problem, a number of energy efficient MAC protocols have been developed for WSNs. Preamble-sampling based MAC protocols (e.g., B-MAC and X-MAC) have overheads due to their preambles, and are inefficient at large wakeup intervals. SCP-MAC, a very energy efficient scheduling MAC protocol, minimizes the preamble by combining preamble sampling and scheduling techniques; however, it does not prevent energy loss due to overhearing; in addition, due to its synchronization procedure, it results in increased contention and delay. In this paper, we present an energy efficient MAC protocol for WSNs that avoids overhearing and reduces contention and delay by asynchronously scheduling the wakeup time of neighboring nodes. To validate our design and analysis, we implement the proposed scheme on the MicaZ platform. Experimental results show that AS-MAC considerably reduces energy consumption, packet loss and delay when compared with SCP-MAC.

Enhanced Non-Seamless Offload for LTE and WLAN Networks

Non-seamless WLAN offload is a currently available approach to the data explosion issue in cellular networks without requiring new hardware and functional deployment on network sides. In this article, we propose two enhanced offloading methods that guarantee session continuity for non-seamless WLAN offload. We describe how the proposed methods manipulate multi-radio (LTE and WiFi) interfaces to maintain IP sessions in the dynamics of available radios. We present a performance analysis that studies the throughput and energy consumption of smart devices.

RaPTEX: Rapid prototyping tool for embedded communication systems

Advances in microprocessors, memory, and radio technology have enabled the emergence of embedded systems that rely on communication systems to exchange information and coordinate their activities in spatially distributed applications. However, developing embedded communication systems that satisfy specific application requirements is a challenge due to the many tradeoffs imposed by different choices of underlying protocols and their parameters. Furthermore, evaluating the correctness and performance of the design and implementation before deploying it is a nontrivial task due to the complexity of the resulting system. This article presents the design and implementation of RaPTEX, a rapid prototyping tool for embedded communication systems, especially well suited for wireless sensor networks (WSNs), consisting of three major subsystems: a toolbox, an analytical performance estimation framework, and an emulation environment. We use a hierarchical approach in the design of the toolbox to facilitate the composition of the network stack. For fast exploration of the tradeoff space at design time, we build an analytical performance estimation model for energy consumption, delay, and throughput. For realistic performance evaluation, we design and implement a hybrid, accurate, yet scalable, emulation environment. Through three use cases, we study the tradeoff space for different protocols and topologies, and highlight the benefits of using RaPTEX for designing and evaluating embedded communication systems for WSNs.

MCAS-MAC: A multichannel asynchronous scheduled MAC protocol for wireless sensor networks

Abstract Due to their energy efficiency, many duty cycling-based MAC protocols have been proposed in WSNs. Although these protocols considerably reduce energy consumption by minimizing idle listening and overhearing, the energy efficiency comes at the cost of decreased packet delivery ratio and increased delay. In this paper, we present a multichannel asynchronous scheduled MAC protocol, called MCAS-MAC, which inherits the basic asynchronous scheduling operation from AS-MAC and adds back-to-back packet transmissions and multichannel support for high traffic dense WSN. Using RaPTEX, we evaluate the performance of MCAS-MAC by comparing it with existing duty cycling MAC protocols including BMAC, SMAC and AS-MAC.

Performance impact of large file transfer on web proxy caching: A case study in a high bandwidth campus network environment

Since large objects consume substantial resources, web proxy caching incurs a fundamental trade-off between performance (i.e., hit-ratio and latency) and overhead (i.e., resource usage), in terms of caching and relaying large objects to users. This paper investigates how and to what extent the current dedicated-server based web proxy caching scheme is affected by large file transfers in a high bandwidth campus network environment. We use a series of trace-based performance analyses and profiling of various resource components in our experimental squid proxy cache server. Large file transfers often overwhelm our cache server. This causes a bottleneck in a web network, by saturating the network bandwidth of the cache server. Due to the requests for large objects, response times required for delivery of concurrently requested small objects increase, by a factor as high as a few million, in the worst cases. We argue that this cache bandwidth bottleneck problem is due to the fundamental limitations of the current centralized web proxy caching model that scales poorly when there are a limited amount of dedicated resources. This is a serious threat to the viability of the current web proxy caching model, particularly in a high bandwidth access network, since it leads to sporadic disconnections of the downstream access network from the global web network. We propose a peer-to-peer cooperative web caching scheme to address the cache bandwidth bottleneck problem. We show that it performs the task of caching and delivery of large objects in an efficient and cost-effective manner, without generating significant overheads for participating peers.

DNS resolution with renewal using piggyback

Domain name system (DNS) is a primary identification mechanism for Internet applications. However, DNS resolutions often take an unbearably long time, and this could seriously impair the consistency of the service quality of Internet applications based on DNS such as World Wide Web. Several approaches reduce DNS resolution time by proactively refreshing expired cached records or prefetching available records beforehand, but these approaches have an inherent problem in that they cause additional DNS traffic. In this paper, we propose a DNS resolution time reduction scheme, named renewal using piggyback (RUP), which refreshes expired cached records by piggybacking them onto solicited DNS queries instead of by issuing additional DNS queries. This method decreases both DNS resolution time and DNS traffic since it reduces the number of queries generated to handle a given DNS resolution without generating additional DNS messages. Simulation results based on two large independent DNS traces show that our proposed approach much reduces not only the DNS resolution time but also the DNS traffic.

DiTeX: Disease-related topic extraction system through internet-based sources

This paper describes the web-based automated disease-related topic extraction system, called to DiTeX, which monitors important disease-related topics and provides associated information. National disease surveillance systems require a considerable amount of time to inform people of recent outbreaks of diseases. To solve this problem, many studies have used Internet-based sources such as news and Social Network Service (SNS). However, these sources contain many intentional elements that disturb extracting important topics. To address this challenge, we employ Natural Language Processing and an effective ranking algorithm, and develop DiTeX that provides important disease-related topics. This report describes the web front-end and back-end architecture, implementation, performance of the ranking algorithm, and captured topics of DiTeX. We describe processes for collecting Internet-based data and extracting disease-related topics based on search keywords. Our system then applies a ranking algorithm to evaluate the importance of disease-related topics extracted from these data. Finally, we conduct analysis based on real-world incidents to evaluate the performance and the effectiveness of DiTeX. To evaluate DiTeX, we analyze the ranking of well-known disease-related incidents for various ranking algorithms. The topic extraction rate of our ranking algorithm is superior to those of others. We demonstrate the validity of DiTeX by summarizing the disease-related topics of each day extracted by our system. To our knowledge, DiTeX is the world’s first automated web-based real-time service system that extracts and presents disease-related topics, trends and related data through web-based sources. DiTeX is now available on the web through http://epidemic.co.kr/media/topics.

Worldwide emerging disease-related information extraction system from news data

Although there have been many researches on the disease information system with the increased interest in disease, the existing systems have limitations in terms of emerging disease monitoring and internationalization. The purpose of this study is to develop a worldwide emerging disease-related information extraction system from news data, which provides nation-specific disease related information, disease-related topic ranking, map-based number of news articles per region, and various charts showing top disease regions and diseases. Our system is available on the web through http://www.epidemic.co.kr/worldwide.