Haengrae Cho

Study on a cluster-chain routing protocol in wireless sensor networks

The energy constraint and the low power communication requirements amplify the significance of the energy efficient routing protocols in wireless sensor networks (WSNs). The primary aim to design a routing protocol for WSN is to improve the energy efficiency and prolong the network lifetime. In this paper, we propose a cluster-chain routing protocol (CCRP), which adopts a more balanced cluster-head selection algorithm and an improved data transmission mechanism from the cluster-heads to the base station. Simulation results indicate that, for a 100 mtimes100 m network with the initial energy 0.5 J in each node, the proposed CCRP outperforms LEACH by about 78%, 150% and 130% in terms of rounds when 1%, 50%, and 100% of nodes die, respectively.

Checkpointing schemes for fast restart in main memory database systems

The potential for substantial performance improvement in a main memory database system (MMDB) is promising, since I/O activity is kept at minimum. On the other hand, due to the volatility of main memory, the issue of failure recovery becomes more complex than in traditional disk resident database systems. We present four checkpointing schemes for the MMDB. The proposed schemes are novel in the sense that they can reduce the amount of disk I/O by exploiting recovery memory as a checkpointing buffer. Furthermore, logging overhead is reduced by maintaining only REDO logs in the stable memory. We evaluate the performance of the proposed schemes under a variety of database workloads using a centralized database simulation model.

Maintaining cache coherency in a multisystem data sharing environment

Abstract A multisystem data sharing environment (MDSE) refers to a distributed architecture for high performance transaction processing. In the MDSE, the computing nodes are locally coupled via a high-speed network and share a common database at the disk level. To reduce the amount of expensive and slow disk I/O, each node caches database pages in its main memory buffer. However, since a particular page may be simultaneously cached in different nodes, cache coherency should be maintained to ensure that the nodes always see the most recent version of database pages. In this paper, we propose two coherency control schemes for the MDSE. The proposed schemes improve the performance by alleviating the amount of message traffic or by reducing the number of cache misses. Using a distributed database simulation model, we evaluate the performance of the proposed schemes under a wide variety of database workloads. Our results indicate that the proposed schemes exhibit substantial performance improvement over the previous schemes when data contention is high or when transactions referencing similar data are clustered together to be executed on the same node.

A prefetching Web caching method using adaptive search patterns

As the Internet continues to grow exponentially, the Web caching technology becomes one of very important technologies in computer networks. The mechanism of a web caching affects the performance of Internet servers or proxy servers. This paper presents a new approach to predict Web documents by tracking search patterns of users and by managing documents depending upon the number of hits. Several hot spot documents and their linked documents are stored in cache servers and transmitted to clients for fast response. Generally cache servers store not only the first document of a site but also all documents navigated through linked documents. The adaptive prefetching method, using search patterns of users, analyzes documents along with the navigation patterns and marks several popular documents. If one of these marked documents is hit, all marked documents are loaded into cache at a time. Cache servers can save their cache memory space as well as provide fast response to clients. The results show that the response time of the proposed method decreases to 20% compared with other methods.

An energy-efficient single-hop wireless sensor network using Repeat-Accumulate codes

Error-control coding (ECC) is a classic approach to save transmission energy in wireless sensor networks (WSN). In this paper, we consider a single-hop wireless sensor network, where Repeat-Accumulate (RA) codes are utilized. RA codes, which are asymmetric codes with low encoding complexity and high decoding complexity, are applied to WSN for the transmission from sensors to the base station. Experiments demonstrate that our proposed scheme can bring 24.1% energy cut compared to that with convolutional codes, and can extend the feasible single-hop range to 185 m. Therefore, the performance of the proposed scheme is superior to that with convolutional codes.

Path conscious caching of B+ tree indexes in a shared disks cluster

A shared disks (SD) cluster is a representative architecture to couple multiple computing nodes for high performance transaction processing. Every node in the SD cluster shares a common database at the disk level. To avoid frequent disk accesses, each node caches both data pages and index pages in its local memory buffer. In general, index pages are accessed more often and thus cached at more nodes than their corresponding data pages. Furthermore, tree-based indexes such as B^+ trees require complicated operations, e.g., root-to-leaf traversal and structure modification operation (SMO) due to a page split or a page concatenation. This means that it is strongly required to devise a dedicated cache coherency scheme (CCS) for index pages that takes advantage of the semantics of their access. In this paper, we propose a new CCS for B^+ tree indexes in the SD cluster. The proposed scheme can reduce the message traffic between nodes and the number of tree re-traversals. An extensive performance study shows that the proposed scheme exhibits substantial performance improvement over the previous schemes, especially when there are large number of nodes or the update probability is high.

Prototyping DIME, a tightly coupled heterogeneous distributed database system

In this paper, we design and implement a prototypical heterogeneous distributed database system, named DIME (Distributed Information Management). DIME has the following salient properties. First, DIME allows both global retrieval operations and global update operations where different concurrency control schemes are used in different local database systems (LDBSs). Second, DIME implements international standard protocols on the distributed transaction processing and the remote database access. Last, DIME provides distribution transparency, and thus users can generate not only single site queries (including remote site queries) but also inter-site queries without considering data distribution in LDBSs.<<ETX>>

An Update Propagation Algorithm for P2P File Sharing over Wireless Mobile Networks

Peer-to-peer (P2P) file sharing systems often replicate files to multiple nodes. File replication is beneficial in the sense that it can achieve good query latency, load balance, and reliability. However, it introduces another problem of maintaining mutual consistency among replica when a file is updated. The new file has to be propagated to all of its replica. In this paper, we propose an update propagation algorithm for P2P file sharing over wireless mobile networks (MONET). Compared to the previous algorithms proposed in wired P2P file sharing systems, our algorithm has low communication overhead. It also guarantees safe delivery of updates even when the underlying network is unreliable. This means that our algorithm is well matched to the characteristics of MONET such as limited battery power of mobile nodes, lower network reliability, and frequent disconnection of mobile nodes.

Dynamic Affinity Cluster Allocation in a Shared Disks Cluster

A shared disks (SD) cluster couples multiple computing nodes for high performance transaction processing, and all nodes share a common database at the disk level. In the SD cluster, a front-end router selects a node for an incoming transaction to be executed. An affinity-based routing can increase the buffer hit ratio of each node by clustering transactions referencing similar data to be executed on the same node. However, the affinity-based routing is non-adaptive to the changes of the system load. This means that a specific node would be overloaded if corresponding transactions rush into the system. In this paper, we propose a new transaction routing algorithm, named Dynamic Affinity Cluster Allocation (DACA). DACA can make an optimal balance between the affinity-based routing and indiscriminate sharing of load in the SD cluster. As a result, DACA can increase the buffer hit ratio and reduce the frequency of inter-node buffer invalidations while achieving the dynamic load balancing.

Semantic load shedding for prioritized continuous queries over data streams

A data stream management system (DSMS) has to handle high-volume and bursty data streams with large number of continuous queries. When an input rate of any data stream exceeds the system capacity, the DSMS has to shed load by dropping some fraction of unprocessed data items. In this paper, we propose a new load shedding algorithm for continuous queries over data streams. Unlike previous algorithms assuming that all queries are equally important, we consider the priority of each query so that more important queries make more convincing outputs. As a result, the proposed algorithm can support differentiated quality of services by exploiting semantics inherent to applications. We also report the experiment results confirming the benefits of the proposed algorithm.