Ha-Joo Song

A log buffer-based flash translation layer using fully-associative sector translation

Flash memory is being rapidly deployed as data storage for mobile devices such as PDAs, MP3 players, mobile phones, and digital cameras, mainly because of its low electronic power, nonvolatile storage, high performance, physical stability, and portability. One disadvantage of flash memory is that prewritten data cannot be dynamically overwritten. Before overwriting prewritten data, a time-consuming erase operation on the used blocks must precede, which significantly degrades the overall write performance of flash memory. In order to solve this “erase-before-write” problem, the flash memory controller can be integrated with a software module, called “flash translation layer (FTL).” Among many FTL schemes available, the log block buffer scheme is considered to be optimum. With this scheme, a small number of log blocks, a kind of write buffer, can improve the performance of write operations by reducing the number of erase operations. However, this scheme can suffer from low space utilization of log blocks. In this paper, we show that there is much room for performance improvement in the log buffer block scheme, and propose an enhanced log block buffer scheme, called FAST (full associative sector translation). Our FAST scheme improves the space utilization of log blocks using fully-associative sector translations for the log block sectors. We also show empirically that our FAST scheme outperforms the pure log block buffer scheme.

System software for flash memory: a survey

Recently, flash memory is widely adopted in embedded applications since it has several strong points: non-volatility, fast access speed, shock resistance, and low power consumption. However, due to its hardware characteristic, namely “erase before write”, it requires a software layer called FTL (Flash Translation Layer). This paper surveys the state-of-the-art FTL software for flash memory. This paper also describes problem definitions, several algorithms proposed to solve them, and related research issues. In addition, this paper provides performance results based on our implementation of each of FTL algorithms

Localization through Map Stitching in Wireless Sensor Networks

Node localization is an essential problem in ad hoc wireless sensor networks. Map stitching is a type of localization algorithm that has received a great deal of attention recently. In this algorithm, the network is divided into small overlapping sub-regions, each of which creates a local map. Then, the local maps are stitched together to form a single global map. In this paper, we first propose a new technique for map-to-map stitching that exploits every available distances between two maps. Next, we propose a few anchor-free localization algorithms based on our stitching technique. We experimentally studied the performance of our algorithms under various settings. The results show that our method achieves a significant performance improvement upon the existing method.

Anchor-Free Localization through Flip-Error-Resistant Map Stitching in Wireless Sensor Network

In patch-and-stitch localization algorithms, a flip error refers to the kind of error in which a patch is stitched to the map as being wrongly reflected. In this paper, we present an anchor-free localization algorithm which tries to detect and prevent flip errors. The flip error prevention is achieved by two filtering mechanisms: the flip ambiguity test and the flip conflict detection. Based on two techniques, we devised an anchor-free localization algorithm and evaluated the performance of the proposed algorithm though simulations. The results show that our algorithm achieves significant performance improvement over the existing algorithms.

An Efficient Buffer Management Scheme for Implementing a B-Tree on NAND Flash Memory

Recently, NAND flash memory has been used for a storage device in various mobile computing devices such as MP3 players, mobile phones and laptops because of its shock-resistant, low-power consumption, and none-volatile properties, However, due to the very distinct characteristics of flash memory, disk based systems and applications may result in severe performance degradation when directly adopting them on flash memory storage systems. Especially, when a B-tree is constructed, intensive overwrite operations may be caused by record inserting, deleting, and its reorganizing, This could result in severe performance degradation on NAND flash memory because of its distinct characteristics. In this paper, we propose an efficient buffer management scheme, called IBSF, which eliminates redundant index units in the index buffer and then delays the time that the index buffer is filled up. Consequently, IBSF significantly reduces the number of write operations to a flash memory when constructing a B-tree. We also show that IBSF yields a better performance on a flash memory by comparing it to the related technique called BFTL through various experiments.

A New Map Stitching Method for Anchor-free Localization in Wireless Sensor Networks

The map stitching is a type of localization algorithm in which the network is divided into small overlapping subregions, each of which creates a local map, and then, the local maps are stitched together to form a single global map. Although there are several methods of constructing local maps, every existing algorithms uses the same method for stitching two maps, called the absolute orientation method. In this paper, we extend the absolute orientation method so that it can handle every available distance constraints between two maps. We experimentally studied the performance of our proposing method under various settings. The results show that our method achieves a significant performance improvement upon the existing method.

Counting-based distance estimations and localizations in wireless sensor networks

We present two distributed algorithms for localizing sensor nodes of a wireless sensor network. Our algorithms determine locations of nodes based on the connectivity between nodes. The basic idea behind our algorithms is to estimate distances between nearby nodes by counting their common neighbors. We analyze the performance of our algorithms experimentally. The results of experiments show that our algorithms achieve performance improvements upon the existing algorithms.

Tag localization in a two-dimensional RFID tag matrix

Localization schemes based on RFID technology are mostly estimating the location of tags either in a two dimensional or in a three dimensional space. In some applications, however, only the relative positions of the tags are needed instead of the exact locations. In this paper, we propose a RFID based localization scheme that estimates the row and column indexes of tags that are spread in a two-dimensional matrix using the movement of an RFID reader. The movement of a reader brings changes in the set of tags that are scanned in consecutive read operations. Based on the changes, the proposed scheme estimates relative positions of tags in a tag matrix. Proposed scheme does not need additional equipment such as reference tags and readers. Therefore it can be easily applied to various applications that need relative positions among tagged objects. Localization of passive RFID tags in a two dimensional space.Estimation of relative tag positions using a moving RFID reader.The proposed scheme can be useful for practical usages.

20 mA bidirectional laser triggering in planar devices based on vanadium dioxide thin films using CO 2 laser

By utilizing a CO2 laser centered at ~10.6 μm as an optical stimulus, we demonstrated bidirectional laser triggering in a two-terminal planar device based on a highly resistive vanadium dioxide (VO2) thin film. The break-over voltage of the VO2-based device was measured as large as ~294.8 V, which resulted from the high resistivity of insulating VO2 grains comprising the thin film and the large electrode separation of the device. The bidirectional current switching of up to 20 mA was achieved by harnessing the dramatic resistance variation of the device photo-thermally induced by the laser illumination. The transient responses of laser-triggered currents were also analyzed when laser pulses excited the device at a variety of pulse widths and repetition rates. In the transient responses, a maximum switching contrast between off- and on-state currents was measured as ~7067 with an off-state current of ~2.83 μA, and rising and falling times were measured as ~30 and ~16 ms, respectively, for 100 ms laser pulses.

Flip Error Resistant Stitching in Sensor Network Localizations

In patch-and-stitch localization algorithms, a flip-error refers to the kind of error in which a patch is stitched to the map as being wrongly reflected. In this paper, we present an anchor-free localization algorithm which tries to detect and prevent flip errors. The flip error prevention is achieved by two filtering mechanisms: the flip-ambiguity test and the flip-conflict detection. We evaluate the performances of proposed techniques though simulations and show that they achieve significant performance improvements.