In wireless ad hoc networks, various medium access control (MAC) protocols have been developed to avoid data packet collision and improve channel utilization in the presence of hidden terminals. For distributed MAC protocols, exchange of messages (handshake), are usually required before data transmission. Based on the initiator of the handshake, MAC protocols can be categorized as sender-initiated or receiver-initiated protocols. Theoretically, the latter outperforms the former since less control overhead is required. However, fundamental assumptions made in the receiver-initiated protocols are very vulnerable. In this paper, we propose a new MAC protocol known as multiple access with reduced handshake (MARCH). MARCH utilizes the broadcast characteristics of an omni-directional antenna to reduce the number of handshakes required to initiate a data transmission. Simulation results demonstrate that this low-overhead MAC protocol results in a 65% increase in network throughput.

A mobile ad hoc network (MANET) is characterized by multi-hop wireless links in the absence of any cellular infrastructure as well as by frequent host mobility. This paper proposes a SMPQ: Spiral-Multi-Path QoS routing protocol in a MANET, while the MAC sub-layer adopts the CDMA-over-TDMA channel model. This work investigates the bandwidth reservation problem of on-demand QoS routing in a wireless mobile ad-hoc network. The proposed approach increases the ability of a route to identify a robust path, namely a spiral-multi-path, from source host to destination host, in a MANET to satisfy certain bandwidth requirements. Two important contributions of the proposed spiral-multi-path are: (1) the spiral-multi-path strengthens route-robustness and route-stability properties and (2) the spiral-multi-path increases the success rate of finding the QoS route. Performance analysis results demonstrate that our SMPQ protocol outperforms other protocols.

We propose a new protocol, Clustered Group Multicast (CGM), for multicasting in ad-hoc mobile networks. In CGM, there is a set of forwarding nodes (called multicast backbone) which are responsible for forwarding multicast datagrams. Unlike the multicasting protocols in wired networks (e.g., Internet) which construct and maintain a shortest path tree for every multicast {source, group} pair, CGM is a mesh-based multicasting protocol in which the connectivity among the nodes in the backbone is of no longer importance. Thus, there is no tree maintenance overhead, but there are more connectivity than trees and yet it can prevent long-term or permanent routing loops from occurring. A key feature of CGM is the use of the advertising agent to reduce advertising traffic to the system. An advertising agent acts as both a server and a client for the purpose of advertising join requests on behalf of its local clients. Because in CGM multicast traffic is only allowed to be delivered over the backbone, CGM restricts the amount of hosts participating in the backbone to decrease the impact of multicast traffic to the system. From the simulation results, the multicast group management traffic and multicast datagram traffic are much less than the other protocols. This is particularly important for wireless networks which lacks bandwidth.

In ad hoc wireless networks, wireless terminals can autonomously construct and can maintain the network. They communicate with some neighbor terminals, exchange network information and determine routes for packets on the multi-hop wireless network. Flexible Radio Network (FRN), one of the ad hoc wireless network systems, adopts a proprietary protocol that provides a multiple routes management and a packet retransmission mechanism against packet transmission errors. This system is a commercial product that has been in use in a recent few years. In this paper, we first evaluate the performance through simulations for data-link protocol and routing protocol of the FRN to clarify its basic properties. Furthermore, we propose some techniques that enhance its performance and solve problems on the protocols. We show how they improve the system performance through simulations and analyses.

The performance of a mobile ad hoc network (MANET) is related to the efficiency of the routing protocol in adapting to changes in the network topology and the link status. However, the use of many different mobility models without a unified quantitative "measure" of the mobility has made it very difficult to compare the results of independent performance studies of routing protocols. In this paper, a mobility measure for MANETs is proposed that is flexible and consistent. It is flexible because one can customize the definition of mobility using a remoteness function. It is consistent because it has a linear relationship with the rate at which links are established or broken for a wide range of network scenarios. This consistency is the strength of the proposed mobility measure because the mobility measure reliably represents the link change rate regardless of network scenarios.

Recently, in wired networks, a hierarchical structure has been introduced to improve management and routing. In ad hoc networks, we introduce a hierarchical structure to achieve the same goal. However, it is difficult to introduce the hierarchical structure because all mobile hosts are always moving around the network. So, we proposed the clustering scheme to construct the hierarchical structure before. In this paper, we propose a new hierarchical routing protocol called Hi-TORA based on the clustering scheme. And we show the experimental evaluation of Hi-TORA with respect to the number of control packets, accuracy of packet delivery and hop counts in comparison with TORA.

The conventional clustering method has the unique potential to be the framework for power-conserving ad hoc networks. In this environment, studies on energy-efficient strategies such as sleeping mode and redirection have been reported, and recently some have even been adopted by some standards like Bluetooth and IEEE 802.11. However, consider wireless sensor networks. The devices employed are power-limited in nature, introducing the conventional clustering approach to the sensor networks provides a unique challenge due to the fact that cluster-heads, which are communication centers by default, tend to be heavily utilized and thus drained of their battery power rapidly. In this paper, we introduce a re-clustering strategy and a power-limit constraint for cluster-based wireless sensor networks in order to address the power-conserving issues in such networks, while maintaining the merits of a clustering approach. Based on a practical energy model, simulation results show that the improved clustering method can achieve a lifetime nearly 3 times that of a conventional one.

The software rejuvenation is a proactive fault management technique for operational software systems which age due to the error conditions that accrue with time and/or load, and is important for high assurance systems design. In this paper, fine-grained shock models are developed to determine the optimal rejuvenation policies which maximize the system availability. We introduce three kinds of rejuvenation schemes and calculate the optimal software rejuvenation schedules maximizing the system availability for respective schemes. The stochastic models with three rejuvenation policies are extentions of Bobbio et al. (1998, 2001) and represent the failure phenomenon due to the exhaustion of the software resources caused by the memory leak, the fragmentation, etc. Numerical examples are devoted to compare three control schemes quantitatively.

In a fixed-channel-allocation (FCA) cellular network, a fixed number of channels are assigned to each cell. However, under this scheme, the channel usage may not be efficient because of the variability in the offered traffic. Different approaches such as channel borrowing (CB) and dynamic channel allocation (DCA) have been proposed to accommodate variable traffic. Our work expands on the CB scheme and proposes a new channel-allocation scheme--called mobile-assisted connection-admission (MACA) algorithm--to achieve load balancing in a cellular network, so as to assure network communication. In this scheme, some special channels are used to directly connect mobile units from different cells; thus, a mobile unit, which is unable to connect to its own base station because it is in a heavily-loaded "hot" cell, may be able to get connected to its neighboring lightly-loaded cold cell's base station through a two-hop link. Research results show that MACA can greatly improve the performance of a cellular network by reducing blocking probabilities.

Multicasting is an efficient communication tool for use in multi-point applications such as conferencing and information distribution. In ad hoc networks, node mobility causes frequent changes of network topology, and re-construction of the multicast tree in an efficient and effective manner becomes a critical issues. In case of link breakage, most of the multicast tree construction protocols available presently require either a total re-build of the tree or to reconnect a disjoined node back to the multicast tree via the shortest path which may disrupt the optimising factors, such as energy consumption, delay or cost, used in the building of the original tree. In this paper, we introduce a computationally efficient recovery algorithm which will also minimise the power consumption on the tree.

A clustering scheme for ad hoc networks is aimed at managing a number of mobile devices by utilizing hierarchical structure of the networks. In order to construct and maintain an effective hierarchical structure in ad hoc networks where mobile devices may move at high mobility, the following requirements must be satisfied. (1) The role of each mobile device for the hierarchical structure is adaptive to dynamic change of the topology of the ad hoc networks. The role of each mobile device should thus change autonomously based on local information in each mobile device. (2) The overhead for management of the hierarchical structure is small. The number of mobile devices in each cluster should thus be almost equivalent. This paper proposes an adaptive multihop clustering scheme for highly mobile ad hoc networks. The results obtained by extensive simulation experiments show that the proposed scheme does not depend on mobility and node degree of mobile devices in the network, which satisfy the above requirements.

We propose a new resource management scheme, Distributed Channel Allocation Protocol (DCAP), for multimedia Ad hoc Wireless LANs (AWLANs). This scheme implements a Quality-of-Service (QoS) providing distributed resource management on the Orthogonal Frequency Division Multiplexing-Code Division Multiple Access (OFDM-CDMA) channel architecture. According to the performance evaluation results for MPEG traffic sources, DCAP can be a good choice of resource management scheme for AWLANs supporting multimedia services on the Broadband Wireless Access (BWA)-type physical layer.

In Ad Hoc networks, communication between a pair of hosts uses channel resources, such that the channel cannot be used by the neighboring hosts. A channel used by one pair of hosts can be reused by another pair of hosts only if their communication ranges do not overlap. Channels are limited resources, accounting for why exploiting channel reuse opportunities and enhancing the channel utilization is essential to increasing system capacity. However, exploiting channel reuse opportunities may cause a co-channel interference problem. Two pairs of communicating hosts that use the same channel may gradually move toward to each other. A channel reassignment operation must be applied to these hosts to maintain their communication. This investigation presents a channel assignment protocol that enables the channel resources to be highly utilized. Following this protocol, a channel reassignment protocol is also proposed to protect the communicating hosts from co-channel interference caused by mobility. The proposed reassignment protocol efficiently reassigns a new available channel to a pair of hosts that suffers from co-channel interference. The performance of the proposed protocols is also examined. Experimental results reveal that the proposed protocols enable more hosts to communicate simultaneously and prevent their communication from failing.

Existing routing protocols for mobile ad hoc networks assume that all nodes have the same transmission range. In other words, the mobile ad hoc network has symmetric links, which means that two neighboring nodes A and B are within the transmission range of one another. However, since nodes consume battery power independently according to their computing and communication load, there exist asymmetric links, which means that node A is within node B's transmission range, but not vice versa. In this paper, two approaches are presented to support routing in the existence of asymmetric links: GAHA (GPS-based Hop-by-hop Acknowledgment) and GAPA (GPS-based Passive Acknowledgment) schemes. Both GAHA and GAPA can be applied to any routing protocols by utilizing GPS (Global Positioning System) location information. Simulation results reveal that both GAHA and GAPA protocols cope well in the presence of asymmetric wireless links and nodes' mobility.

This paper deals with contour extraction of fetus' head from echocardiogram and its application to diagnosis in obstetrics. Active contour model "SNAKES" is modified and used for contour extraction. After contour extraction we automatically obtained the biparietal diameter (BPD) and the occipitofrontal diameter (OFD) from the contour.

Access control involves a check to see if a user has an access right to a resource and then a decision is made as to whether his/her access to the resource is to be allowed or denied. Typical access control models are the Discretionary Access Control Model, the Mandatory Access Control Model, and the Role-Based Access Control Model. Today, the Role-Based Access Control Model has become popular and is recognized as an effective method. However, until now, the Role-Based Access Control Model was adequate only for bureaucracy organizations, in which some roles are standardized and organizational hierarchy is stable. Team-Based Access Control models that were designed for team-based organizations have been proposed, but they do not reflect some features of an adhocracy organization, which are organic, temporary, not standardized, changeable, and obscure in terms of hierarchical relationship, such as a Task Force Team in the company. This study shows the characteristics of an adhocracy organization that differ from the existing bureaucracy organization, and then shows why existing access control models have caused some problems. Finally, a revised Role-Based Access Control model is proposed to solve those problems and is analyzed according to main evaluation standards.

A new distributed medium access control (MAC) protocol--Soft Reservation Multiple Access with Priority Assignment (SRMA/PA) protocol--is introduced for supporting the integrated services of real-time and non-real-time applications in mobile ad-hoc networks. The SRMA/PA protocol allows the distributed nodes to contend for and reserve time slots with RTS/CTS-like "collision-avoidance" handshake and "soft reservation" mechanism augmented with distributed and dynamic access priority control. The SRMA/PA protocol realizes distributed scheduling for guaranteeing QoS requirements of integrated services and maximizes statistical multiplexing gain. We have demonstrated by simulation studies that the multiplexing gain can be improved significantly without unduly compromising on system complexity. Moreover, we have shown that the proposed back-off mechanism designed for delay-constrained services is useful for further improving utilization of the channel.

To reduce the bandwidth needed for data transmission in an ad-hoc communication network, such as an Intelligent Transportation System (ITS) inter-vehicle communication network, a broadcast scheme is proposed where the data to be transmitted is arranged into several classes. Each class contains more specific and detailed information. Since information to be transmitted often has a geographical relevance, the classes can be structured to represent this relationship. As data is routed through the ad-hoc network, the total amount of transmitted data is reduced by removing the data contained in one class on each hop. The class structure is adaptive so that in unforeseen situations the relative importance of transmitted data can be dynamically adjusted. Furthermore different manufacturers can implement different classes structures, and total length of data may be different. By computer simulation it was shown that in the proposed system the required bandwidth for transmission to achieve similar data reception rates to conventional non-structured data schemes can be reduced to less than one third, resulting in a more efficient transmission scheme. In addition a packet structure similar to IP packets is proposed which will enable easily integration of multimedia transmissions into vehicle to vehicle communications.

In this paper, the performance of HIPERLAN (HIgh PErformance Radio Local Area Networks) CAC (Channel Access Control) of ETSI (European Telecommunication Standards Institute) in Europe, as High speed wireless LAN, is analyzed mathematically. The CAC protocol of HIPERLAN is the EY-NPMA (Elimination Yield-Nonpreemptive Priority Multiple Access) in which data is transmitted after prioritization, elimination and yield phase. We analyzed channel contention phase composed of elimination and yield phase and then throughput is inspected by simulation. This result is useful to design and implement of Ad hoc wireless networks.

Recently, demand for multimedia service applicable with wireless ad hoc networks is increasing. Many protocols which support QoS (Quality of Service) defined by ITU H.323 are suitable for the system with a central coordinator (ex. access point), and are not applied to ad hoc networks. Since each mobile station sends a packet randomly in a wireless network, it is necessary to avoid a packet collision which results in the decrement of the channel throughput. IEEE 802.11 is a standard for wireless LANs. The basic access mechanism in its media access control (MAC) protocol is the distributed coordination function (DCF) based on carrier sense multiple access with collision avoidance (CSMA/CA). However, this mechanism does not have QoS control, and packet collision may occur frequently with the increase of the number of mobile stations. In this paper, the two queues, voice over IP (VoIP) queue and DATA queue, are set in each mobile station, and we propose a novel MAC protocol using multiple queues for QoS. Our scheme consists of carrier sensing, packet scheduling and collision avoidance, and improves the contention window in DCF. We use two kinds of packet scheduling. One is the scheduling in a mobile station, and the other is the scheduling among mobile stations. We called the proposed scheduling among mobile stations D2VTS (Distributed Double Virtual Time Scheduling). Simulations are conducted to analyze the proposed scheme. The simulation results show that our scheme is able to carry multimedia traffic with a higher performance compared with the conventional DCF.