Szu-Chi Wang

Communication strategies for heartbeat-style failure detectors in wireless ad hoc networks

Heartbeat-style failure detectors are a commonly used building block in practical fault-tolerant distributed systems over unreliable and asynchronous networks. A basic requirement to implement such failure detectors is to diffuse heartbeat information across the underlying network. In wireless ad hoc networks, however, the dynamics of mobility and lack of resource make information dissemination a formidable task. Moreover, as a middleware service, the total bandwidth used for failure detection should be constrained. This paper describes several communication strategies on which heartbeat-style failure detectors can be developed in wireless ad hoc networks. The design goal is to support an effective and robust means of gossiping under a fixed message transmission rate. We show through simulations that the proposed gossiping schemes are resilient to message losses and topology changes. The simulation results also show that the performance of gossiping can be improved by introducing the concept of transient hierarchy under varied network characteristics.

A topology control algorithm for constructing power efficient wireless ad hoc networks

In this paper, we present a localized algorithm for constructing power efficient topology for wireless ad hoc networks. Each mobile node determines its own transmission power based only on local information. The proposed algorithm first constructs the constrained Gabriel graph from the given unit disk graph and then reduces the total transmission power by allowing each node individually excises some replaceable links. The constructed topology is sparse, has a constant bounded power stretch factor, and the total transmission power is lower than those obtained from other proposed algorithms. In addition, compared with others, our algorithm requires lower time complexity to generate a solution, and can thus further save the energy for each mobile node. We demonstrate the performance improvements of our algorithm through simulations.

An SPT-based topology control algorithm for wireless ad hoc networks

In this paper, we present a localized Shortest-Path-Tree (SPT) based algorithm that copes with the topology control problem in wireless ad hoc networks. Each mobile node determines its own transmission power based only on its local information. The proposed algorithm first constructs local SPTs from the initial graph, after which the total power consumption is further reduced by allowing each mobile node to search the replaceable links individually. The constructed topology ensures network connectivity, and possesses the following desirable energy-efficient features: (i) the power stretch factor is bounded and can be predetermined, (ii) the power consumption is evenly distributed among the mobile nodes, and (iii) the total power consumption is lower than that obtained by the best known algorithms. The performance improvements of the proposed algorithm are demonstrated through extensive simulations. We conclude our work with a discussion of future research directions toward more integrated mobile network architectures.

High-performance obstacle-avoiding rectilinear steiner tree construction

Rectilinear Steiner trees are used to route signal nets by global and detail routers in VLSI design for a long time. However, in current IC industry, there are significantly increasing obstacles to be considered, such as large-scale power networks, pre-routed nets, IP blocks, and antenna jumpers. Accordingly, the obstacle-avoiding rectilinear Steiner minimal tree (OARSMT) problem has become more important. In this article, we propose a new routing graph, obstacle-avoiding routing graph (OARG), for the OARSMT problem. Due to the important properties of OARG, we construct a 3-step algorithm and a local refinement scheme, which both can take advantage of these properties, to find a suboptimal solution efficiently. Furthermore, each step of our 3-step algorithm as well as the local refinement scheme has theoretical or practical benefits. Therefore, each of them can be applicable to other existing works for general or specific considerations such as efficiency or effectiveness. Extensive experimental results show that our method outperforms all existing works in terms of wirelength and achieves the best speed performance.

SPT-based power-efficient topology control for wireless ad hoc networks

This paper presents a localized shortest-path-tree (SPT) based algorithm to cope with the topology control problem in wireless ad hoc networks. Each mobile node determines its own transmission power based only on its local information. The proposed algorithm first constructs local SPTs from the initial graph, and then the total power consumption is further reduced by allowing each mobile node to search and excise the replaceable links individually. The constructed topology ensures network connectivity, and possesses several desirable energy-efficient features: 1) its power stretch factor is bounded and can be predetermined, 2) the power consumption is evenly distributed among the mobile nodes, and 3) its total power consumption is lower than that obtained from the best known algorithms. The performance improvements of the proposed algorithm are demonstrated through extensive simulations.

An efficient time-based checkpointing protocol for mobile computing systems over mobile IP

Time-based coordinated checkpointing protocols are well suited for mobile computing systems because no explicit coordination message is needed while the advantages of coordinated checkpointing are kept. However, without coordination, every process has to take a checkpoint during a checkpointing process. In this paper, an efficient time-based coordinated checkpointing protocol for mobile computing systems over Mobile IP is proposed. The protocol reduces the number of checkpoints per checkpointing process to nearly minimum, so that fewer checkpoints need to be transmitted through the costly wireless link. Our protocol also performs very well in the aspects of minimizing the number and size of messages transmitted in the wireless network. In addition, the protocol is nonblocking because inconsistencies can be avoided by the piggybacked information in every message. Therefore, the protocol brings very little overhead to a mobile host with limited resource. Additionally, by taking advantage of reliable timers in mobile support stations, the time-based checkpointing protocol can adapt to wide area networks.

SPT-based topology algorithm for constructing power efficient wireless ad hoc networks

In this paper, we present a localized Shortest Path Tree (SPT) based algorithm for constructing a sub-network with the minimum-energy property for a given wireless ad hoc network. Each mobile node determines its own transmission power based only on its local information. The proposed algorithm constructs local shortest path trees from the unit disk graph. The performance improvements of our algorithm are demonstrated through simulations.

A low overhead checkpointing protocol for mobile computing systems

Checkpointing protocols for distributed computing systems can also be applied to mobile computing systems, but the unique characteristics of the mobile environment need to be taken into account. In this paper, an improved time-based checkpointing protocol is proposed, which is suitable for mobile computing systems based on Mobile IP. The main improvement over a traditional time-based protocol is that our protocol reduces the number of checkpoints per checkpointing process to nearly minimum, so that fewer checkpoints need to be transmitted through the bandwidth-limited wireless links. The proposed protocol also performs very well in the aspects of minimizing the number and the size of messages transmitted in the wireless network. Therefore, the protocol brings very little overhead to a mobile host which has limited resource. Additionally, by integrating the improved timer synchronization technique, our protocol can also be applied to wide area networks.

A Randomized Distributed Algorithm for Peer-to-Peer Data Replication in Wireless Ad Hoc Networks

In this paper, we focus on enhancing the data accessibility of ad hoc networks, with emphasis on peer-to-peer communications. To achieve this goal, we propose a randomized distributed algorithm for data replication. Furthermore, a probabilistic approach is presented to derive the upper bound of convergence by a novel technique, called path coupling, which gives more insight into factors determining system performance. Our analysis demonstrates that data accessibility can be improved by the proposed approach, with very limited memory consumption.

Randomised and distributed methods for reliable peer-to-peer data communication in wireless ad hoc networks

Peer-to-peer (P2P) communications have attracted a great deal of attention from the network research community in recent years. However, due to the fundamental limitations of wireless environments, providing reliable data availability for P2P applications over wireless ad hoc networks is still a major challenge. To address the problem, a distributed and randomised scheme based on self-avoiding walks is proposed. The scheme concatenates disparate network layers, with the goal of recovering from routing failures that disrupt P2P data accessibility. In addition, a probabilistic approach is presented that explores the tradeoffs between several system parameters. Some new analysis tools, such as path coupling, are utilised which provide a better understanding of the systems operations. That the proposed concepts and techniques make a significant contribution to the design of effective and efficient P2P applications in wireless ad hoc networks is believed.