Middleware has become a key enabler for the development of distributed applications. Unfortunately, conventional middleware technologies do not yet offer sufficient functionality to make them suitable for mobile environments. This dissertation proposes a novel middleware approach termed opportunistic overlays and its dynamically reconfigurable support framework for building efficient mobile applications. Specifically, we address the inefficiency of content delivery introduced by node mobility and by dynamically changing system loads, in the context of publish/subscribe systems. In response to changes in physical network topology, in nodes' physical locations, and in network node behaviors, the opportunistic overlay approach dynamically adapts event dissemination structures (i.e., broker overlays) with the goal of optimizing end-to-end delays in event delivery. Adaptation techniques include the dynamic construction of broker overlay networks, runtime changes of mobile clients' assignments to brokers, and dynamic broker load balancing. Essentially, opportunistic overlays implement a middleware-level analogue of the networking routing protocols used in wireless communications (i.e., Mobile IP, AODV, DSR and DSDV). By thus coordinating network- with middleware-level routing, opportunistic overlays can attain substantial performance improvements over non-adaptive event systems. Such improvements are due to their use of shorter network paths and better balancing of loads across event brokers. Opportunistic overlays and the adaptive methods they use are realized by a set of distributed protocols implemented in a Java-based publish/subscribe infrastructure. Comprehensive performance evaluations are performed via simulation, emulation, and with two representative applications on actual networks. Experimental results demonstrate that the opportunistic overlay approach is practically applicable and that the performance advantages attained from the use of opportunistic overlays can be substantial, in both infrastructure-based mobile environments and mobile ad hoc networks.

A mobile ad hoc network (MANET) consists of a group of autonomous mobile nodes with no infrastructure support. In this research, we develop a distributed intrusion detection and response system for MANET, and we believe it presents a second line of defense that cannot be replaced by prevention schemes. We based our detection framework on the study of attack taxonomy. We then propose a set of detection methods suitable of detecting different attack categories. Our approaches are based on protocol specification analysis with categorical and statistical measures. Node-based approaches may be too restrictive in scenarios where attack patterns cannot be observed by any isolated node. Therefore, we have developed cooperative detection approaches for a more effective detection model. One approach is to form IDS clusters by grouping nearby nodes, and information can be exchanged within clusters. The cluster-based scheme is more efficient in terms of power consumption and resource utilization, it is also proved resilient against common security compromises without changing the decentralized assumption. We further address two response techniques, traceback and filtering. Existing traceback systems are not suitable for MANET because they rely on incompatible assumptions such as trustworthy routers and static route topology. Our solution, instead, adapts to dynamic topology with no infrastructure requirement. Our solution is also resilient in the face of arbitrary number of collaborative adversaries. We also develop smart filtering schemes to maximize the dropping rate of attack packets while minimizing the dropping rate of normal packets with real-time guarantee. To validate our research, we present case study using both ns-2 simulation and MobiEmu emulation platform with three ad hoc routing protocols: AODV, DSR and OLSR. We implemented various representative attacks based on the attack taxonomy. Our experiments show very promising results using node-based and cluster-based approaches.

In this work, we consider the channel assignment problem in single radio multi-channel mobile ad-hoc networks. Specifically, we investigate the granularity of channel assignment decisions that gives the best trade-off in terms of performance and complexity. We present a new granularity for channel assignment that we refer to as component level channel assignment. The strategy is relatively simple, and is characterized by several impressive practical advantages. We also show that the theoretical performance of the component based channel assignment strategy does not lag significantly behind the optimal possible performance, and perhaps more importantly we show that when coupled with its several practical advantages, it significantly outperforms other strategies under most network conditions.

Ad hoc network is a new tendency of data transmission in the future. Because of the convenience and necessity of mobile phone and/or portable computer coupled with wireless data services, TCP/IP has become an important topic for the study in wireless networks. However, there were a few difficulties in data transmission that must be overcome due to ad hoc environments and the characteristics of the IEEE 802.11 protocols. In these protocols, MAC layer is our primary research topic. Based on the RTS/CTS signal of dynamic retransmission [4], this study presented an improvement to solve the problems: (1) media resources was easily robbed due to the RTS/CTS signal competition, (2) data frame would be dropped by the IEEE 802.11 protocol due to too many times collisions. In addition, this study modifies the CWND in TCP layer in accordance with congestion conditions. Sender can transmit data packets to the network, in which resources can be completely utilized without any waste or loss. We also use the related parameters from the IP and TCP header to calculate flow numbers. The calculated parameters are recorded in IP and TCP header, and then instantly forwarded to the receiver via routers. The receiver can forward these parameters back to the sender by using back transmission method. The simulation result shows that the proposed methods can effectively improve TCP performance, such as packet loss rate, and fastly increase the CWND, the buffer utilization, and so forth. Thus, the network can perform more effectively while using the MAC-layer RTS/CTS signal.

In this thesis, we proposed a delay limited routing scheme in wireless ad hoc networks. When nodes transmit packets in wireless ad hoc networks, most people think the one-hop way is better than the multi-hop way in reducing the delay time. Since most cases in wireless ad hoc networks are not single sources, we should consider at least two sources transmitting packets at the same time and then use the multi-hop way in order to reduce the energy consumption. We want to maximize the throughput with limited delay. Our contribution is to transform the optimal scheduling problem in wireless ad hoc networks to the classic maximum flow problem. The maximum flow approach does maximize the throughput and can get the optimal solution.

In this thesis, we consider how to reduce the communication cost of the key exchange procedures as many as possible, while the secure group communication can still be achieved. Due to the energy consumption is usually proportional to the distance, we use the shortest paths algorithm to find the shortest communication paths between any pair of the secure group members. We first propose a straightforward heuristic named Minimum-Energy First-Selected ( MEFS ). MEFS tries to select the pair of group members which has less communication cost than all other pairs have at every time. Though MEFS performs better than random selecting, it still has some weakness in solving the energy-aware key management problem. So we use the concept of the minimum cost flow problem, and by appropriate transformation, then we get the optimal solution of the energy-aware key management problem under some constraints. At last, the simulation results proves that the minimum cost flow approach actually works better than MEFS does.

Broadcasting is a fundamental primitive in local area networks (LANs).Operations of many data link protocols, for example, ARP (Address Resolution Protocol) and IGMP (Internet Group Management Protocol), must rely on this LAN primitive. To develop the broadcasting service in mobile ad hoc wireless LANs (WLANs) is a challenge. This is because a mobile ad hoc WLAN is a multi-hop wireless network in which messages may travel along several links from the source to the destination via a certain path. Additionally, there is no fixed network topology because of host moving. Furthermore, the broadcast nature of a radio channel makes a packet be transmitted by a node to be able to reach all neighbors. Therefore, the total number of transmissions (forward nodes) is generally used as the cost criterion for broadcasting. The problem of finding the minimum number of forward nodes in a static radio network is NP-complete. Almost all previous works, therefore, for broadcasting in the WLAN are focusing on finding approximation approaches in a, rather than, environment. In this paper, we propose a novel distributed protocol in WLANs to significantly reduce or eliminate the communication overhead in addition to maintaining positions of neighboring nodes. The important features of our proposed protocol are the adaptability to dynamic network topology change and the utilization of the existing routing protocol. The reduction in communication overhead for broadcasting operation is measured experimentally. From the simulation results, our protocol not only has the similar performance as the approximation approaches in the static network, but also outperforms existing ones in the adaptability to host moving.

We develop two algorithms for important problems in mobile ad hoc networks (MANETs). A MANET is a collection of mobile processors (“nodes”) which communicate via message passing over wireless links. Each node can communicate directly with other nodes within a specified transmission radius; other communication is accomplished via message relay. Communication links may go up and down in a MANET (as nodes move toward or away from each other); thus, the MANET can consist of multiple connected components, and connected components can split and merge over time. We first present a deterministic leader election algorithm for asynchronous MANETs along with a correctness proof for it. Our work involves substantial modifications of an existing algorithm and its proof, and we adapt the existing algorithm to the asynchronous environment. Our algorithm’s running time and message complexity compare favorably with existing algorithms for leader election in MANETs. Second, many algorithms for MANETs require or can benefit from knowledge about the size of the network in terms of the number of processors. As such, we present an algorithm to approximately determine the size of a MANET. While the algorithm’s approximations of network size are only rough ones, the algorithm has the important qualities of requiring little communication overhead and being tolerant of link failures.

This thesis presents an intelligent early packet discard (I-EPD) for real-time video streaming over a multi-hop ad hoc network. In a multi-hop ad hoc network, the quality of transferring real-time video streams could be seriously degraded, since every intermediate node (IN) functionally like forwarding device does not possess large buffer and sufficient bandwidth. Even worse, a selected forwarding node could leave or power off unexpectedly which breaks the route to destination. Thus, a video packet temporarily buffered in intermediate nodes may exceed its time constraint when either a congested or failed link occurs; a stale video packet is useless even if it can reach destination after network traffic becomes smooth or failed route is reconfigured. In the proposed I-EPD, an IN can intelligently determine whether a buffered video packet should be discarded based on an estimated time constraint which is calculated from the RTP timestamps and the round trip time (RTT) measured by RTCP. For the purpose of validation, we implement the I-EPD scheme on a Linux-based embedded system. We compare the quality of video streams under different bit rates and different route repair time. In addition, we use PSNR to validate the quality of pictures from the aspect of application layer. The experimental results demonstrate that with I-EPD buffer utilization on IN can be more effectively used and unnecessary bandwidth wastage can be avoided.

This paper studies the integrated mobile ad-hoc networks (MANETs) and Internet network which the Mobile IP and AODV routing protocols are integrated to offer MANET nodes to set up multi-hop routes to the Internet. Gateways act as bridges for forwarding data packets between integrated MANETs and the Internet. Mobile nodes in MANETs must discover available gateways to gain access to the Internet. This study presents an adaptive gateway discovery scheme that balances efficiency and overhead by limiting the flooding scope of gateway advertisement messages. The proposed scheme dynamically adjusts the ADV_TTL according to the average delay time of network. The simulation results showed that the proposed scheme obtains the shortest average delay time while keeping the network a relatively lower overhead.