Low-duty-cycle mechanisms can reduce the energy consumption in wireless sensor networks. Many related researches have been made in recent years. In the low-duty-cycle environment, the latency of sending packets from a sink to each node is much longer than traditional WSNs because nodes stay asleep most of the time. In this paper, the Centralized Cluster-based Location Finding (CCLF) algorithm is proposed to reduce the high latency in low-duty-cycle WSNs by finding a suitable position for the sink. The algorithm mainly consisted of three steps: (1) Cluster construction, (2) The fast look-up table (FLU-table) construction, and (3) Sink location decision. The simulation results show that the performance of the CCLF algorithm approaches the performance of the optimal algorithm. Moreover, the CCLF algorithm requires less operation time compared with the optimal algorithm.

The rapid development of underwater technology has opened up new possibilities and introduced new challenges for MAC and protocol designs. The packet delay reduction becomes critical issues in underwater sensor networks (USNs) since the propagation speed of water is rather slow compared with radio waves. In addition, due to the property of intermittent link connectivity in underwater, the existing message-based and synchronization-based approaches can not meet packet delivery requirements. In this paper, a tailored multihop routing protocol is designed for transmission delay reduction without packet exchanges and retransmissions. The observation of delivery analysis with different link qualities gives insights on relay nodes selection for lower delivery loss rate. To prevent excessive redundant packets, this work includes a strategy of adjusting the number of packet cloning dynamically between each hop.

In wireless sensor networks (WSNs) deployed in an outdoor environment, obstacles may occur as a result of a non-uniform distribution of the sensor nodes or the presence of barriers. These obstacles result in a degradation of the network performance, so obstacle identification is a major concern in most WSN applications. This paper develops a Decentralized Boundary Detection (DBD) algorithm for identifying sensor nodes near a hole or obstacle in the WSN. The algorithm does not require any knowledge of the node locations or distances between two nodes. The detection capability is provided even in networks where the sensor nodes have a non-unit disk communication range.

Minimizing energy consumption is a fundamental requirement when deploying wireless sensor networks. Accordingly, various topology control protocols have been proposed, which aim to conserve energy by turning off unnecessary sensors while simultaneously preserving a constant level of routing fidelity. However, although these protocols can generally be integrated with any routing scheme, few of them take specific account of the issues which arise when they are integrated with geographic routing mechanisms. Of these issues, the dead-end situation is a particular concern. The dead-end phenomenon (also known as the "local maximum” problem) poses major difficulties when performing geographic forwarding in wireless sensor networks since whenever a packet encounters a dead end, additional overheads must be paid to forward the packet to the destination via an alternative route. This paper presents a distributed dead-end free topology maintenance protocol, designated as DFTM, for the construction of dead-end free networks using a minimum number of active nodes. The performance of DFTM is compared with that of the conventional topology maintenance schemes GAF and Span, in a series of numerical simulations conducted using the ns2 simulator. The evaluation results reveal that DFTM significantly reduced the number of active nodes required in the network and thus prolonged the overall network lifetime. DFTM also successfully constructed a dead-end free topology in most of the simulated scenarios. Additionally, even when the locations of the sensors were not precisely known, DFTM still ensured that no more than a very few dead-end events occurred during packet forwarding.

In order to guarantee collision-free transmissions in TDMA-based wireless sensor networks, a substantial amount of work in literature has been done by modeling the problem into minimum graph coloring. However, our observation reveals that such approach is not effective towards low-latency transmissions due to the inherent principle of the graph coloring. This paper introduces DCLS, a distributed collision-free low-latency link scheduling scheme. The scheme considers the network snapshot at each time slot, and determines a set of collision-free transmission pairs on each snapshot. With DCLS, the delay is asymptotically smaller than that with the graph coloring model, and the running time complexity on each snapshot is O(diam), where diam is the diameter of the network graph. From the simulation result, the delay is significantly reduced in the network of maximum degree Δ ranged from 6 to 18, and the duty cycle is 0.23 in average.

Abstract As the prevalence of Wireless Sensor Networks (WSNs) grows in the military and civil domains, network security becomes an evermore critical concern. In Sybil attack, malicious nodes forge a lot of fake identities to disrupt the network's protocols. Previous work used either authentication‐based mechanisms which use shared encryption keys, or location information to verify the identities of the nodes. By contrast, this paper uses only neighboring node relation to verify the node identities. The evaluation results show that the detection rate was around 95% when the network was not very sparse. Copyright © 2010 John Wiley & Sons, Ltd.

In this study, we have investigated the effectiveness of building ¿Fault-Tolerant Backbone¿ for data dissemination in Tmote Sky sensor networks. Tmote Sky sensors provide programmable and adjustable output power for data transmission. Users can control adequate transmission power for each sensor. Based on our measurements of Tmote Sky, there is a steadily transmitted distance for every power level. For certain power level, successfully-transmitted ratio was approximately 100 percent when the distance between sender and receiver was less than the steadily-transmitted distance. In accordance with the character on Tmote Sky, the ideas of fault-tolerant backbone has been made for constructing a fault-tolerant and stable system for Tmote Sky. The fault-tolerant backbone protocol builds up a connected backbone, in which nodes are endowed with a sleep/awake schedule. Practical experimental results reveal the fast fault recovery and high successfully-transmitted ratio can be fulfilled in the realistic system. The following goals in the implementation have been reached, including self-configurable fault-tolerant groups, automatic backbone construction, automatic failure recovery, and route repair.

Mobile users are often involved in team activities and exhibit collaborative mobility in realistic ad hoc network application scenarios. Due to diverse group mobility patterns, the network may remain partitioned for extended durations. This paper describes an approach to reduce the communication disruptions by deploying forwarding nodes. The forwarding nodes explore the field periodically in orderto gather network information for countering the influence from mobility and sparseness of node deployment. By selecting the rendezvous grid pair, the forwarding nodes can move to appropriate locations to recover communication between disconnected groups. The mechanism has been implemented in the network simulator ns-2. The simulation results show that the mechanism can efficiently improve information availability and also achieve the required communication quality.

This paper presents a range-free position determination (localization) mechanism for sensors in a three-dimensional wireless sensor network based on the use of flying anchors. In the scheme, each anchor is equipped with a Global Positioning System (GPS) receiver and broadcasts its location information as it flies through the sensing space. Each sensor node in the sensing area then estimates its own location by applying basic geometry principles to the location information it receives from the flying anchors. The scheme eliminates the requirement for specific positioning hardware, avoids the need for any interaction between the individual sensor nodes, and is independent of network densities and topologies. The performance of the localization scheme is evaluated in a series of simulations performed using ns-2 software and is compared to that of the Centroid and Constraint range-free mechanisms. The simulation results demonstrate that the localization scheme outperforms both Centroid and Constraint in terms of a higher location accuracy, a reduced localization time, and a lower beacon overhead. In addition, the localization scheme is implemented on the Tmote Sky for validating the feasibility in the real environment.

A route between the sensors and the sink in opportunistic sensor networks may have never existed. Traditional routing protocols cannot be directly applied to deal with the disconnection problem. This paper develops a mobile relay infrastructure approach to supporting data delivery in opportunistic sensor networks. Mobile relays cooperate to establish a routing infrastructure by modifying their trajectories. Unlike previous mobile infrastructure schemes, mobile relays move to specific positions and forward data instead of buffering and carrying data. The approach has been successfully implemented in the network simulator ns-2. The simulation results show that our infrastructure significantly improves the packet delivery rate.

A topology control algorithm can be applied to wireless sensor networks to construct a virtual backbone for maintaining network connectivity. For handling node failure, previous algorithms typically prefer to have redundant nodes so the power efficiency was compromised. This paper describes a Power-CDS (P-CDS) mechanism that schedules active and backup sensor nodes in the backbone for tolerating failures. When node failure is detected, both active and backup nodes will adjust their transmission ranges to maintain connectivity. Simulation results show that P-CDS forms a smaller connected dominating set than the other fault- tolerant topology control algorithm. P-CDS also provides efficient broadcast with relatively low energy consumption.

Previous fault-tolerant time synchronization protocols are vulnerable to the number of misbehaved nodes in a sensor network. With the increasing number of faulty sensor nodes, more erroneous data are exchanged, so it becomes more difficult to synchronize time accurately for all working nodes. This paper develops a fault diagnosis mechanism for time synchronization, called Partitioned Sample Time Diagnosis (PSTD), that can locate potential misbehaved sensor nodes and then exclude them from future rounds of synchronization. Due to the diagnosis capability, PSTD avoids interference of the identified faulty nodes and thus achieves better performance. The simulation results reveal that PSTD algorithms maintain longer time interval for the time-synchronized network compared to the previous protocols.

The paper is about a comment on variable-range transmission power control in wireless ad hoc networks.

In order to improve lifetime of sensors, it is important to obtain a smaller partial set of working sensor nodes for full coverage in wireless sensor networks. This paper presents a decentralized self-configured method for determining active nodes required to ensure complete coverage. A two-way coverage mechanism identifies blind/overlapping areas and activates/deactivates sensor nodes as required to maintain the coverage. The mechanism was implemented successfully using the network simulator NS-2. The performance was compared to that provided by three other alternative approaches, including OTTAWA, PEAS, and OGDC. The results show that the two-way algorithm is network scalable and coverage efficient

Ad hoc networks have been extensively deployed in recent years. In such networks, obtaining a high efficiency is difficult to achieve due to mobility. Many routing approaches have been proposed to improve the performance of ad hoc networks, including table-driven and demand-driven routing protocols. Geographic routing provides a suitable solution for large-scale and dynamic networks, and reduces link failure problems during packet routing. In wireless networks, nodes movement may cause a network partition, which hinders both data transmission and information availability within the network. Several schemes have been proposed for resolving the partition problem in wireless ad hoc networks. In general, the previous solutions either relied on the use of mobile devices with known trajectories or introduced additional specific mobile nodes. By contrast, this paper proposes a scheme for overcoming the problem of network partitions using only the available devices in the environment. With the scheme, the source node searches for another mobile node, designated as the helping node, which is able to meet the destination. The selected mobile node then carries the packet toward the destination. If the helping node changes its trajectory before it forwards the packet to the destination node, a hand-off scheme is activated and the data packet is transferred to another mobile node for onward transmission

With the progress of advanced technology, people anticipate better quality in the medical health care service. Due to more robust requirements in medical environments, higher availability and reliability are expected. This paper describes a recovery scheme for disconnection failures in a Web-based medical teleconsultation system. With the support, the disconnected clients can re-enter the system to resume the unfinished discussion using the synchronization mechanisms. The experiments have shown that the mechanism performed successfully. The lazy synchronization further reduced the required time that the clients had to wait for joining the consultation after network disconnections

Location service support is required for mobile nodes using geographic ad hoc routing. The mobile nodes need location information of destinations for sending data packets. Due to mobility or node failures, queries to the location servers may not always succeed. This paper presents a distributed location service (DLS) that employs dual servers to maintain position information of mobile nodes and to achieve efficient performance. With the scheme, the number of failed queries, including no response and erroneous results, can be reduced. DLS has been implemented and evaluated using the ns2 simulator. The experimental results show that DLS not only improved service availability but also location query latency.

A mobile ad hoc network consisting of a set of mobile nodes does not have any infrastructure support or central management. Data packets are routed by the mobile nodes in the network instead of base stations. Due to node mobility or geographical limitations, the ad hoc network topology may be partitioned. Data communication and information availability are thus affected. The disconnection problem makes a critical strike on ad hoc routing protocols because most protocols typically assume that the ad hoc network is always connected. We describe an approach to recovering the disconnected mobile ad hoc networks by deploying forwarding nodes. The forwarding nodes can automatically move to appropriate locations for interconnecting network partitions. The mechanism is distributed and self-organized and can be integrated with other routing protocols. The mechanism has been implemented in the network simulator ns-2. The simulation results show that the algorithm can efficiently improve information availability and recover network partitions.

When studying [T.C. Oliveira et al. (2004)], we found an error and some ambiguous parts. In this paper, we state those errors.

OO software engineering (OOSE) has been a popular methodology for years; however, there are still some issues remaining unsolved: a generic mechanism for checking consistency of designs is still lacking; software has some problems resulting from process issue; and imperative engineer have a huge gap to adopt OOSE. These three issues intersect in the early analysis phase, therefore we represent a new methodology that provides a complete global view of OO software system that solves issues we identified, and uses requirements document and some analysis documents as foundation. Rules and a case study are also presented to exemplify the result of applying our methodology on both OOSE and imperative software engineering.

Many software systems are established using client-server models. If a program on the server needs to be updated due to software maintenance, the service provided by the program will be required to terminate. The clients cannot be served until the update process is completed. The update process obviously reduces commercial profit especially for e-commerce systems. Therefore, a nonstop software update scheme is needed for high system availability. This paper develops an NSU system to update general server programs without stopping services. The NSU system can take over the task of old version using a redundant server for updating. The system supports applications with shared variables and maintains data consistency during software update. The current implementation is designed for Java programs running on the Linux operating system. Experimental results show that the NSU system incurred less than 1% overhead during normal execution.

Heterogeneous computing environments, where computers may have different instruction set architectures, data representations, and operating systems, complicate checkpointing and recovery of processes. This paper describes an approach to recovery and an implementation, PREACHES, that provides portable checkpointing of single-process applications in heterogeneous systems using checkpoint propagation. The checkpoint propagation mechanism creates machine-dependent checkpoints for different architectures in the heterogeneous environment. A process is restored on a specific machine with the checkpoint that is appropriate for the architecture. An implementation of PREACHES has been evaluated on a heterogeneous network of workstations, including Sun, HP, and Pentium machines. The experimental results show that PREACHES achieves efficient checkpointing and rapid recovery.

Software updating is an inevitable process in the lifetime of a computer system. Numerous update mechanisms have been applied to software products. However, only little attention has been devoted to online nonstop software updating. Traditional update utilities typically require programs to stop execution during modifications, and this decreases the availability of the systems. For e-commerce (electronic commerce) companies, improving system availability implies increasing opportunities for greater business profit. Therefore, minimizing the system maintenance time for necessary software updates becomes a critical issue. This paper describes an approach to nonstop software updating. The approach composes a program using two replicated execution blocks. By switching between the execution blocks, the program can be modified without terminating its service. The paper also discusses the system environments that are required in order to support the approach.

The growing importance of VLSI is gaining it much attention, but the increasing complexity of VLSI also creates lots of problems for computer scientists to work on. If no other new way to help designers is developed, then the whole design process will become more difficult to maintain. That's why there is a need for an object-oriented (OO) design framework for the CAD world. Not only is it necessary to develop new methods for CAD, but the extension of reuse technology into the design process will also become a main key to success. We propose an OO-based framework with an object specification language (OSL) for reuse. In our OO design framework, CAD tools are reverse-engineered into a toolbox in a reuse library. During the design (reuse) process, toolboxes are selected automatically for the design process. Thus, tool selection, integration and adoption problems are merged into the design process, together with the OSL specification. With this OO design framework, tool reuse and design data reuse can be automatically integrated into the design process.

Numerous mathematical approaches have been proposed to determine the optimal checkpoint interval for minimizing total execution time of an application in the presence of failures. These solutions are often not applicable due to the lack of accurate data on the probability distribution of failures. Most current checkpoint libraries require application users to define a fixed time interval for checkpointing. The checkpoint interval usually implies the approximate maximum recovery time for single process applications. However, actual recovery time can be much smaller when message logging is used. Due to this faster recovery, checkpointing may be more frequent than needed and thus unnecessary execution overhead is introduced. In this paper, an adaptive checkpointing protocol is developed to accurately enforce the user-defined recovery time and to reduce excessive checkpoints. An adaptive protocol has been implemented and evaluated using a receiver-based message logging algorithm on wired and wireless mobile networks. The results show that the protocol precisely maintains the user-defined maximum recovery times for several traces with varying message exchange rates. The mechanism incurs lour overhead, avoids unnecessary checkpointing, and reduces failure free execution time.