Sungwon Jung

An efficient path computation model for hierarchically structured topographical road maps

In this paper, we have developed a HiTi (Hierarchical MulTi) graph model for structuring large topographical road maps to speed up the minimum cost route computation. The HiTi graph model provides a novel approach to abstracting and structuring a topographical road map in a hierarchical fashion. We propose a new shortest path algorithm named SPAH, which utilizes HiTi graph model of a topographical road map for its computation. We give the proof for the optimality of SPAH. Our performance analysis of SPAH on grid graphs showed that it significantly reduces the search space over existing methods. We also present an in-depth experimental analysis of HiTi graph method by comparing it with other similar works on grid graphs. Within the HiTi graph framework, we also propose a parallel shortest path algorithm named ISPAH. Experimental results show that inter query shortest path problem provides more opportunity for scalable parallelism than the intra query shortest path problem.

A tree-structured index allocation method with replication over multiple broadcast channels in wireless environments

Broadcast has often been used to disseminate frequently requested data efficiently to a large volume of mobile units over single or multiple channels. Since mobile units have limited battery power, the minimization of the access and tuning times for the broadcast data is an important problem. There have been many research efforts that focus on minimizing access and tuning times by providing indexes on the broadcast data. We have studied an efficient index allocation method for broadcast data with skewed access frequencies over multiple physical channels which cannot be coalesced into a single high bandwidth channel. Previously proposed index allocation techniques have one of two problems. The first problem is that they require equal size for both index and data. The second problem is that their performance degrades when the number of given physical channels is not enough. These two problems result in an increased average access time for the broadcast data. To cope with these problems, we propose a tree-structured index allocation method. Our method minimizes the average access time by broadcasting the hot data and their indices more frequently than the less hot data and their indexes over the dedicated index and data channels. We present an in-depth experimental and theoretical analysis of our method by comparing it with other similar techniques. Our performance analysis shows that it significantly decreases the average access and tuning times for the broadcast data over existing methods.

Data Model and Comparison and Query Methods for Interacting Classifications in a Taxonomic Database

An information-theoretic view has been applied to biological classification to capture taxonomic concepts as taxonomic data entities and to develop a system for managing these concepts and the lineage relationships among them. In order to develop the data model, it has been necessary to apply explicit definitions to several taxonomic terms that generally have not been precisely defined and to coin and define several new terms and concepts. Methods are outlined for comparing interacting classifications and querying hierarchical taxonomic databases. A program/database system called HICLAS, which provides an X-Window interface to query classification data, is available on the Internet.

A cost effective cache consistency method for mobile clients in wireless environment

When a mobile client disconnects for a prolonged time which exceeds the window size of the server’s invalidation reports , the mobile client’s cache is discarded even if most of data are still valid. In this paper, we propose an efficient cache consistency method called CCI(Cost-based Cache Invalidation) for mobile clients, which takes into account not only the disconnection time but also the update frequencies at a server.

A concurrency control scheme for mobile transactions in broadcast disk environments

Broadcast disk technique has been often used to disseminate frequently requested data efficiently to a large volume of mobile clients over wireless channels. In broadcast disk environments, a server often broadcasts different data items with differing frequencies to reflect the skewed data access patterns of mobile clients. Previously proposed concurrency control methods for mobile transactions in wireless broadcast environments are focused on the mobile transactions with uniform data access patterns. These protocols perform poorly in broadcast disk environments where the data access patterns of mobile transactions are skewed. In broadcast disk environments, the time length of a broadcast cycle usually becomes large to reflect the skewed data access patterns. This will often cause read-only transactions to access old data items rather than the latest data items. Furthermore, updating mobile transactions will be frequently aborted and restarted in the final validation stage due to the update conflict of the same data items with high access frequencies. This problem will increase the average response time of the update mobile transactions and waste the uplink communication bandwidth. In this paper, we extend the existing FBOCC concurrency control method to efficiently handle mobile transactions with skewed data access patterns in broadcast disk environments. Our method allows read-only transactions to access the more updated data, and reduces the average response time of updating transactions through early aborts and restarts. Our method also reduces the amount of uplink communication bandwidth for the final validation of the update transactions. We present an in-depth experimental analysis of our method by comparing with existing concurrency control protocols. Our performance analysis shows that it significantly decreases the average response time and the amount of uplink bandwidths over existing methods.

A new data model for biological classification

In the domain of biological classification, classifications are performed hierarchically. There are no standard classifications which are unanimously accepted by the community of each domain; many different interacting views of classification exist about the same data, and the discovery of new data results in changes to the existing classification. Even a single individual may change his or her own classification of a particular group. Since multiple classification views interact, they are semantically related. It is difficult to model this kind of dynamically evolving and semantically interacting classification system using traditional data models, which lack the structural flexibility necessary to support dynamic views of hierarchic classifications, and cannot properly capture the history of these complex interactions. We have developed a new data model which is suitable for supporting semantically interacting dynamic views of hierarchic biological classifications. On the basis of our new data model we have developed a prototype database system called HICLAS (HIerarchical CLAssification System); its domain is plant taxonomy. HICLAS is available through the Internet and an X-window interface has been implemented to support queries to classification data.

An energy-efficient mobile transaction processing method using random back-off in wireless broadcast environments

Broadcast is widely accepted as an efficient technique for disseminating data to a large number of mobile clients over a single or multiple channels. Due to the limited uplink bandwidth from mobile clients to server, conventional concurrency control methods cannot be directly applied. There has been many researches on concurrency control methods for wireless broadcast environments. However, they are mostly for read-only transactions or do not consider exploiting cache. They also suffer from the repetitive aborts and restarts of mobile transactions when the access patterns of mobile transactions are skewed. In this paper, we propose a new optimistic concurrency control method suitable for mobile broadcast environments. To prevent the repetitive aborts and restarts of mobile transactions, we propose a random back-off technique. To exploit the cache on mobile clients, our method keeps the read data set of mobile transactions and prefetches those data items when the mobile transactions are restarted. As other existing optimistic concurrency control methods for mobile broadcast environments does, it works for both read-only and update transactions. Read-only transactions are validated and locally committed without using any uplink bandwidth. Update transactions are validated with forward and backward validation, and committed after final validation consuming a small amount of uplink bandwidth. Our performance analysis shows that it significantly decreases uplink and downlink bandwidth usage compared to other existing methods.

An Efficient Tree-Structure Index Allocation Method over Multiple Broadcast Channels in Mobile Environments

Broadcast has been often used to disseminate the frequently requested data efficiently to a large volume of mobile units over a single or multiple channels. Since the mobile units have limited battery power, the minimization of the access time for the broadcast data is an important problem. In this paper, we studied an efficient index allocation method for the broadcast data over multiple physical channels to minimize the access time. Previously proposed index allocation techniques either require the equal size of index and data or have a performance degradation problem when the number of given physical channels is not enough. To cope with these problems, we propose an efficient tree-structured index allocation method for the broadcast data with different access frequencies over multiple physical channels.

A Dynamic Window-Based Approximate Shortest Path Re-computation Method for Digital Road Map Databases in Mobile Environments

In this paper, we have studied the shortest path re-computation problem that arises in the dynamic route guidance system (DRGS) in ATIS where the cost of topological digital road map is frequently updated as traffic condition changes dynamically. Previously suggested methods are trivial in that they do not intelligently utilize the previously computed shortest path information. We have developed an efficient approximate shortest path recomputation method based on the dynamic window scheme. The proposed method recomputes an approximate shortest path very quickly by utilizing the previously computed shortest path information. We first show the theoretical analysis of our methods and then present an in-depth experimental performance analysis by implementing it on grid graphs.

Enhanced Cost Effective Cache Invalidation for Mobile Clients in Stateless Server Environments

In [1], we proposed a cost-based cache invalidation method for mobile clients. However, our previous work did not concern a remarkable difference of data transfer rate between download and upload links. In this paper, we present an enhanced cost-effective cache validation scheme with a new cost function. We prove that our proposed scheme greatly increases the cache reusability and the uplink message overhead does not lead to serious performance degradation through experiments.