JoAnne Holliday

Distributed energy-efficient hierarchical clustering for wireless sensor networks

Since nodes in a sensor network have limited energy, prolonging the network lifetime and improving scalability become important. In this paper, we propose a distributed weight-based energy-efficient hierarchical clustering protocol (DWEHC). Each node first locates its neighbors (in its enclosure region), then calculates its weight which is based on its residual energy and distance to its neighbors. The largest weight node in a neighborhood may become a clusterhead. Neighboring nodes will then join the clusterhead hierarchy. The clustering process terminates in O(1) iterations, and does not depend on network topology or size. Simulations show that DWEHC clusters have good performance characteristics.

Epidemic algorithms for replicated databases

We present a family of epidemic algorithms for maintaining replicated database systems. The algorithms are based on the causal delivery of log records where each record corresponds to one transaction instead of one operation. The first algorithm in this family is a pessimistic protocol that ensures serializability and guarantees strict executions. Since we expect the epidemic algorithms to be used in environments with low probability of conflicts among transactions, we develop a variant of the pessimistic algorithm which is optimistic in that transactions commit as soon as they terminate locally and inconsistencies are detected asynchronously as the effects of committed transactions propagate through the system. The last member of the family of epidemic algorithms is pessimistic and uses voting with quorums to resolve conflicts and improve transaction response time. A simulation study evaluates the performance of the protocols.

Disconnection modes for mobile databases

As mobility permeates into todays computing and communication arena, we envision application infrastructures that will increasingly rely on mobile technologies. Traditional database applications and information service applications will need to integrate mobile entities: people and computers. In this paper, we develop a distributed database framework for mobile environments. A key requirement in such an environment is to support frequent connection and disconnection of database sites. We present algorithms that implement this framework in an asynchronous system.

Data Management and Layout for Shingled Magnetic Recording

Ultimately the performance and success of a shingled write disk (SWD) will be determined by more than the physical hardware realized, but will depend on the data layouts employed, the workloads experienced, and the architecture of the overall system, including the level of interfaces provided by the devices to higher levels of system software. While we discuss several alternative layouts for use with SWD, we also discuss the dramatic implications of observed workloads. Example data access traces demonstrate the surprising stability of written device blocks, with a small fraction requiring multiple updates (the problematic operation for a shingled-write device). Specifically, we discuss how general purpose workloads can show that more than 93% of device blocks can remain unchanged over a day, and that for more specialized workloads less than 0.5% of a shingled-write disks capacity would be needed to hold randomly updated blocks. We further demonstrate how different approaches to data layout can alternatively improve or reduce the performance of a shingled-write device in comparison to the performance of a traditional non-shingled device.

Partial database replication using epidemic communication

Data replication in distributed databases has been investigated extensively with the hope that it will improve performance, reliability, and availability. However the growth of the Internet has shown us that current replica management do not work well when the replicas are connected by an unreliable network, subject to congestion and dynamic topology changes. In this paper we present a replica update protocol that handles an adaptive partial replication scheme on such a network.

Replicated database recovery using multicast communication

Database replication with update-anywhere capability and one-copy serializability has long been thought impractical. Protocols have been proposed for replicated databases that take advantage of atomic broadcast systems to simplify message passing and conflict resolution. The paper presents global recovery algorithms to handle site failures when such protocols are used.

The performance of database replication with group multicast

Replication with update-anywhere capability while maintaining global synchronization and isolation has long been thought impractical. Protocols have been proposed for distributed replicated databases that take advantage of atomic broadcast systems to simplify message passing and conflict resolution in hopes of making replication efficient. This paper presents performance measurements on a simulation of a replicated database using those protocols. The results show that with the proper group broadcast mechanism, replication with update-anywhere capability is indeed practical.

Optimal Super-peer Selection for Large-scale P2P System

The peer-to-peer (P2P) systems have grown significantly over the last few years due to their potential for sharing various resources. Unstructured hybrid P2P system can improve the performance of the entire network and system using SP (Super-peer), which has the responsibility for query processing instead of OPs (Ordinary-Peer). In these systems, selecting the best SP to join is an important problem, but it is difficult to choose the optimal SP by the various reasons such as heterogeneous capacity, content similarity and dynamic capacity change. In this paper, we present the SP selection¿s problem and SP selection strategy based on dynamic capacity of the SP and content similarity. Also we measure the SP¿s score with weight to the factors distance cost, processing power and content similarity. Through the simulation, we show query processing performance is improved when OPs use our strategy to choose the best SP

Improving the security of wireless LANs by managing 802.1x disassociation

802.1x is a security protocol based on the frame structure of 802.11. It attempts to provide strong authentication, access control, and WEP (wired equivalent privacy) key management for wireless LANs. Unfortunately, 802.1x misses its goals in access control denial-of-service attacks. Currently, there are no IEEE approved ways to solve this security hole. We propose a central manager, not only to take the responsibility of an authentication server, but also to add functionality to prevent denial of service attacks. We also analyze how the 802.11 MAC layer protocol works with our solution.

Epidemic quorums for managing replicated data

In the epidemic model an update is initiated on a single site and is propagated to other sites in a lazy manner. When combined with version vectors and event logs, this propagation mechanism delivers updates in causal order despite communication failures. We integrate quorums into the epidemic model to process transactions on replicated data while ensuring global serializability. We present a detailed simulation of a distributed replicated database and demonstrate the performance improvements.