Fault-tolerance is an important design issue in building a reliable mobile computing system. This paper considers checkpointing recovery services for a mobile computing system based on the ad-hoc network environment. Since potential problems of this new environment are insufficient power and limited storage capacity, the proposed scheme tries to reduce disk access frequency for saving recovery information, and also the amount of information saved for recovery. A brief simulation study has been performed and the results show that the proposed scheme takes advantage of the existing checkpointing recovery schemes.

In this paper, we investigate the critical low coverage problem of position aware localized efficient broadcast in mobile ad hoc ubiquitous sensor networks and propose a generic framework for it. The framework is to determine a small subset of nodes and minimum transmission radiuses based on snapshots of network state (local views) along the broadcast process. To guarantee the accuracy of forward decisions, based on historical location information nodes will predict neighbors' positions at future actual transmission time and then construct predicted and synchronized local views rather than simply collect received "Hello" messages. Several enhancement technologies are also proposed to compensate the inaccuracy of prediction and forward decisions. To verify the effectiveness of our framework we apply existing efficient broadcast algorithms to it. Simulation results show that new algorithms, which are derived from the generic framework, can greatly increase the broadcast coverage ratio.

Recent advances in wireless technologies and electronic devices have enabled the development of low-cost and large-scale wireless sensor networks. Many recent studies have targeted wireless sensor networks, but forwarding messages while satisfying both delay and reliability constraints remains an unsatisfied goal. Although most existing proposals aim at resiliency in multi-hop networks, improvement of network performance such as throughput and latency, and load balancing, supporting multiple forwarding strategies satisfying various demands is not discussed. We propose a forwarding method for messages with delay and reliability constraints on a single wireless sensor network. We evaluate message loss rate by using computer simulation and by implementing the proposed method on actual sensor nodes. Our results show that the proposed method achieves lower message loss rate for messages with delay and reliability constraints than that for best-efforts messages on a network with error-prone wireless links.

A mobile ad hoc network is an autonomous wireless network which consists of mobile nodes without any base stations. Many routing schemes and services have been proposed for mobile ad hoc networks. However, since these schemes tend to be evaluated through simulation experiments, it is not known whether they work effectively in real environments or not. Therefore, in order to verify their practical use in mobile ad hoc networks, it is necessary to perform field experiments using actual mobile nodes. If the network size is large, it is difficult to perform field experiments due to problems with limited battery, difficulty of topology control and so on. Realization of rapid topology change of the ad hoc networks topology is especially difficult. In order to solve this problem, this paper proposes a testing environment for mobile ad hoc network software, which emulates field experiments in wired networks.

The directional MAC protocols improve spatial reuse, but require the exact location of destination and have the problem of deafness. In this paper, we propose a dual-channel MAC protocol with directional antennas for mobile ad-hoc networks. In the proposed MAC protocol, RTS/CTS are sent omnidirectionally as nodes do not have the exact location of the destination in mobile environments. Omnidirectional transmissions on control channel overcome deafness, but have low spatial reuse. We propose a new blocking algorithm to improve spatial reuse on control channel. We use the negative CTS (NCTS) to solve the exposed terminal problem. We confirm throughput of the proposed MAC protocol by simulations using Qualnet ver. 3.8 simulator.

This paper presents an active quality of service (QoS) aware service composition protocol for mobile ad hoc networks (MANETs), with the goal of conserving resources subject to QoS requirements. A problem of QoS based service composition in MANETs is transformed into a problem of the service path discovery. We extend Dynamic Source Routing protocol to discover and compose elementary services across the network. Some message processing measures are taken to effectively reduce control overhead. Simulation results demonstrate the effectiveness of the proposed protocol.

In mobile ad hoc networks, as the communications have to be achieved autonomously and via third parties, a variety of risks against stable communication and session completion can be predicted. Many researchers have been studying these issues, i.e., mobility, broadcast storm, selfish behavior on data forwarding, security, and so forth. However, availability of routes against selfish power off is still to be addressed. This paper defines a new, realistic and practical problem beyond the scope of malicious program modification, called selfish power off, and evaluates the influence of it. Here, a selfish user turns his terminal on only when it has some data to send or receive and decline to work as an intermediate user by turning its terminal power off. In addition, this paper proposes a solution, called Proactive COoperation Mechanism (PCOM) against selfish power off in order to eliminate the influence of selfish power off. In PCOM, each user holds the cooperation records of its adjacent users, and forwards only those packets that are generated by users with good cooperation records. PCOM thus prevents SUs from joining the network. Extensive simulation shows the efficiency of our proposal in maintaining network connectivity and fairness in resource consumption.

Most wireless networks consist of heterogeneous nodes with diverse characteristics. These heterogeneous nodes have various moving characteristics such as speed and pausing time. Since conventional wireless routing schemes are designed for networks with homogeneous mobility, it is difficult to accomplish communication without degrading its quality, e.g., packet reachability and delay, in networks with heterogeneous mobility. In this paper, we propose efficient extensions of a proactive routing protocol to achieve sufficient communication quality in networks with heterogeneous mobility. The proposed extensions consist of three features, i.e., differential topology update, unidirectional movement notification and link quality based route calculation. Complementary actions among these functions can improve communication quality with acceptable control overhead. Simulation results reveal that the proposed scheme can achieve higher packet reachability and lower delay with low control overhead compared with existing routing schemes.

In this paper, to realize the low delay and high throughput route discovery in multi-rate ad hoc networks, we propose a novel on-demand routing using signal strength, called signal strength aware routing (SSR). SSR is based on the on-demand routing with the route request (RREQ) and route reply (RREP) procedure. In SSR, a node measures the signal strength of a received RREQ, and calculate the appropriate data transmission rate. Nodes also calculate the standby time for the RREQ forwarding proportionally to the medium time at the data transmission rate. A RREQ through higher data rate links arrives at the destination earlier, and the destination can select a low delay and high throughput route easily. We evaluate the performance of SSR in terms of delay, throughput and route discovery delay by means of QualNet network simulator. As a result, we show that SSR can discover the lower delay and higher throughput route than the conventional shortest hop routing without increasing control overhead in multi-rate ad hoc networks.

It is predicted that there will be a high demand for video applications in future wireless networks including wireless ad hoc networks. However, supporting video applications over mobile ad hoc networks is more complicated than with other networks due to the lack of support from a preinstalled infrastructure. In this paper, we tackle this problem by adopting the concept of multipoint-to-point video transmission used in wire-line networks. A novel framework designed with features to accommodate the characteristics of ad hoc networks is presented. There are two key features in our proposal. First, Multiple Description Coding (MDC) scheme is used for video coding to reduce the redundancy by avoiding the transmission of duplicate video frames. Second, the routing protocol is expanded to include finding disjoint routes from video sources to the receiver so that a single link breakage or a single intermediate node failure affects transmission from only the minimum number of nodes. Furthermore, the use of disjoint routes also enables the workload to be distributed more evenly within the network. A simulation study was carried out using NS-2 to demonstrate the performance of the proposed mechanism. We show that the proposed mechanism outperforms conventional point-to-point transmission, especially under conditions of high mobility.

Probability distribution of the node degree has long been regarded as one of the important metrics to characterize the wireless ad hoc networks. However, there is no exact formulation for this probability distribution when shadow fading effects are considered. In this letter, by deriving the conditional probability of the node degree and the joint distribution of the distances between nodes and a randomly selected reference node (RSRN), we show that the probability distribution of the node degree for the wireless ad hoc networks in the shadow fading environments is binomial.

Owing to the large amount of speckle noise and ill-defined edges present in echocardiographic images, computer-based boundary detection of the left ventricle has proved to be a challenging problem. In this paper, a Markovian level set method for boundary detection in long-axis echocardiographic images is proposed. It combines Markov random field (MRF) model, which makes use of local statistics with level set method that handles topological changes, to detect a continuous and smooth boundary. Experimental results show that higher accuracy can be achieved with the proposed method compared with two related MRF-based methods.

In ad-hoc networks, mobile nodes are limited by a range of radio coverage and have an irregular source of power due to their battery. In ad-hoc networks, there are a lot of situations that all mobile nodes need to agree on their key not at the same time but in part and then merge themselves subsequently. This is because ad-hoc networks have specific features such as mobility and allow various conditions during configuration. In this thesis, we propose MCP (Merging Clusters Protocol), a simple key agreement scheme that can effectively deal with merging different adjacent clusters in mobile ad-hoc networks. When nodes of each cluster have already agreed on their own group keys and intend to merge themselves for further secure communications, MCP can be used in an efficient and secure way. In addition, it can be utilized for efficient group key agreement in a large ad-hoc network. We analyze the security and efficiency of MCP and discuss the experimental results according to practical implementation scenarios.

The present paper proposes two novel and practical schemes for distributed and asynchronous power control in wireless ad hoc networks, in which users dynamically share several frequency bands as in "cognitive radio" networks. These schemes iteratively adjust transmit powers of individual network transmitters with respect to mutually caused interference in the shared bands. Their most attractive feature is that they find network-wide acceptable trade-offs to diverse signal-to-noise and interference (SINR) requirements and efficiently use techniques of stochastic approximation and time-averaging to guarantee a robust performance in random channels. Advantageously, both proposed algorithms do not assume any particular modulation, coding, QoS measure definition or network architecture, which assures their high applicability in the industry and research. Moreover, the broad definition and non-linear nature of these schemes mathematically generalize and thus encompass as a special case many widely deployed power control schemes such as e.g. those for achieving fixed SINR targets or using game-theoretic utility maximization. Simulations are provided to illustrate our approach and its better performance compared to standard algorithms.

In wireless ad hoc networks, network services are provided through the cooperation of all nodes. Albeit that good teamwork could smoothly run a mobile network, selfish node behaviors would probably cause it to break down. Some examples of these selfish node behaviors would include, "listening only" for saving energy or "receiving the valuable" without forwarding the packets to others. To cope with this problem, we propose PDM, a price-demand function based pricing model, to restrains the selfish behaviors of mobile nodes. PDM is based on the packet sending requirements of the source nodes and the forwarding cost of relay nods. Using this pricing methodology, the packet forwarding activities will be profitable for the relay node and further stimulate cooperation in the network. In particular, the new model enjoys the merit of giving relay nodes no reason to dishonestly report their forwarding costs, because an honest cost claim has proven to be an optimal strategy for relay nodes. Furthermore, our new model uses a price-demand function to reflect the relationship between the service demand of the source nodes and the service supply of the relay nodes. As a consequence, our approach reduces the source nodes' payments to send packets, and at the same time guarantees that the packets sent by the source nodes are delivered to the destination.

We present an effective method of collision recovery for orthogonal frequency division multiplexing (OFDM)-based communications. For the OFDM system, the modulated message data can be demodulated using the partial time-domain OFDM signal. Therefore, the partial time-domain signal can be adopted to reconstruct the whole OFDM time-domain signal with estimated channel information. This property can be utilized to recover packets from the collisions. Since most collisions are cases in which a long packet collides with a short packet, the collided part is assumed to be short. The simulated results show that the method can recover the two collided packets with a certain probability and can be developed to solve the problem of hidden terminals. This method will dramatically benefit the protocol design of wireless networks, including ad hoc and sensor networks.

In this paper, we propose a novel highly reliable packet transmission protocol for ad-hoc wireless networks without deciding the route from a source node to a destination node in advance. The proposed protocol uses distributed surrounding nodes as transmitters of the retransmission packet when the transmitted packet contains an error. In this protocol, when a packet is not correctly received at the destination node, the source node and the surrounding nodes that have correctly received the packet simultaneously retransmit the same data packet to the destination node. The transmitting timing is triggered by the control packet transmitted from the source node. These operations are repeated until the packet reaches to the destination node like automatic repeat request (ARQ). Moreover, the retransmitted packet is encoded by one branch of the space time block code (STBC) for improving the performance with the network diversity effect. We call this method as STBC Distributed ARQ scheme. The performance of the proposed scheme is evaluated by the computer simulations.

A proactive cluster-based distance vector routing protocol based on DSDV protocol is proposed for mobile ad hoc networks. A network is divided into a number of clusters, each cluster having a clusterhead that directly connects the other nodes in the same cluster. Each clusterhead broadcasts update request (UREQ) messages at reqular intervals or in an event driven manner to its neighbor clusterheads. In this way, clusterheads update their own global routing tables that give a path among all clusterheads. In this process, multiple local paths from source clusterhead to its neighbor clusterheads are established as well. A node having a packet to send forwards the packet to its own clusterhead. The clusterhead determines next clusterhead with respect to the destination clusterhead by looking up its own global routing table. Then, the packet is delivered to the next clusterhead along a local path. The advantages of this protocol are threefold. The size of a global routing table is small since it has entries only for clusterheads. A UREQ message travels up from the initiating clusterhead to neighbor clusterheads. Hence, the convergence range of a update request is at least nine times as wide as that of DSDV or CGSR, increasing the correctness of routing. Lastly, a technique to bypass clusterheads, whenever possible, on the fly during packet transmission is presented, to reduce route length as well as to prevent clusterheads from becoming congested. Simulation results show that PCDV outperforms some key protocols of the same category greatly.

In this paper a multi-channel MAC protocol with dynamic channel allocation (MMAC-DCA) in CDMA Ad Hoc networks is proposed. Under MMAC-DCA, the service sub-channels are dynamically allocated by the RTS/CTS dialogue on the common sub-channel, only when a node has a packet to transmit. In addition, a Markov mode is presented to analyze the performance of MMAC-DCA.

Ad-hoc networks do not need any preexisting network infrastructure and have been developed as temporal networks in various fields. An infostation is a system to relay information that is not delay-sensitive. In this paper, we propose an early disaster warning system built on infostations and an adhoc network composed of sensors attached to moving animals. We analyze the performance of the proposed system by simulation. The analysis results show that the proposed system will be very useful in the early detection of big forest fires which occur frequently in Korea mountain areas.