Ad hoc networks are gaining increasing popularity in recent years because of their ease of deployment. No wired base station or infrastructure is supported, and each host communicasts one another via packet radios. In ad hoc networks, routing protocols are challenged with establishing and maintaining multihop routes in the face of mobility, bandwidth limitation and power constraints. In this dissertation, we study the routing strategies for ad hoc networks. On-demand routing protocols and table-driven algorithms are analyzed and compared against each other. Our study shows that on-demand protocols are better suited for mobile networks because they generate less control overhead and manage the mobility in a more efficient manner. Simulation experiments also indicate that providing multiple routes is beneficial in increasing the robustness against mobility.

As mobile computing gains popularity, the need for ad hoc routing protocols will continue to grow. There have been numerous simulations comparing the performance of these protocols under varying conditions and constraints. One question that arises is whether the choice of MAC protocol affects the relative performance of the routing protocols being studied. This paper investigates the answer to that question by simulating the performance of three ad hoc routing protocols when run over different MAC protocols. It is determined that the choice of MAC layer protocol does, in fact, affect the relative performance of the routing protocols. I. INTRODUCTION AND MOTIVATION The number and variety of wireless devices and applications has dramatically increased within the past few years. As these products begin to permeate the marketplace, the need to provide communication between them is becoming increasingly important. In an effort to establish and maintain routing paths in these ad hoc mobile...

A multiple access protocol that is particularly suitable for cellular Internet access and satellite-based networks with on-board processing is developed in this paper. The basic idea is that when a user wishes to send a message, it transmits with a probability paccess that depends on the load on the channel. Under conditions of low load, the probability paccess approaches 1, while at high load paccess is relatively low. This media access control protocol guarantees high channel utilization at high load, as well as low delay at low load periods. Using the statistical usage of the shared channel, the load is estimated with certain uncertainty. Our analysis shows that using the statistical usage of the shared channel, the optimal access probability can be well estimated for a broad class of load distribution patterns. In addition, we propose to use a central station to broadcast the value of paccess in networks with poor collision detection capability, or long feedback delay. The proposed method is particularly suitable for shared channels with poor collision detection capability, under conditions of bursty traffic and a large number of users. Examples for such channels are the reservation channel in satellite-based networks with on-board processing, and the control channel in cellular networks. Hence, the proposed method can be used for cellular Internet access and for accessing public satellite-based networks. The broadcast mechanism that already exists in such networks can be used to inform the users the dynamic access probability. Keywords---wireless networks, multiple access, MAC I.

Abstract—With the rapid growth of WLAN capability for mobile devices such as laptops, handhelds, mobile phones and vehicles, we will witness WLANs with very large numbers of active nodes for which very efficient medium access control techniques will be needed to cope with high loads and mobility. We propose a high performance solution based on an innovative node elimination algorithm that uses short and unmodulated bursts of energy during contention – no data is exchanged. We also present a modified OFDM PHY layer, based on IEEE 802.11a, which allows sensing and bursting on individual subcarriers. We show that the protocol maintains a very low overhead and collision probability which lead to high and virtually constant network throughput at all analyzed network loads, even beyond 500 nodes. The protocol is validated by extensive simulation, comparing it against the IEEE 802.11a and SYN-MAC protocols. I.

Small battery powered hand held computers are becoming more and more important for the daily work. The growing number of these mobile units demand for communication structures which are specially tailored for their needs. A lot of research has been done, especially in the area of cellular networks, leading to the construction of highly reliable, wireless networks offering acceptable bandwidth for data transfer. However, cellular networks require a well organized infrastructure and cause considerable costs for setup and operation. In order to gain more flexibility ad-hoc networks have been developed which do not require any external infrastructure at all. The basic idea is that all mobile units act as routers and cooperate in transmitting messages. Unfortunately, coping with such a highly dynamic environment is a difficult task. Due to the lack of a central point of administration, the management must be distributed among all the nodes. This causes communication overhead which limits the number of supportable nodes and the lifetime of battery powered machines. Hence, the few existing prototypes support only a small number of users within a small area. New approaches must be found which allow the construction of larger ad-hoc networks. This paper presents part of our research on this topic. In particular, we describe the architecture of a simulation system for evaluating ad-hoc networking protocols and present example simulations analyzing the throughput of some medium access and routing protocols.

A method for efficient short messages transmission in cellular networks is developed in this study. The number of Short Message Service (SMS) messages is growing very rapidly, and existing cellular networks have to support a large number of SMS and signaling messages. The rate of SMS messages has already exceeded the rate of E-mail messages transmitted through the Internet, and their number keeps growing. SMS messages are characterized by a burst of a short, single packet message. Unfortunately, the protocols currently used by existing cellular networks cannot support this type of traffic efficiently. For this reason, SMS messages are currently transmitted through the signaling channel. Clearly, this usage of the signaling channel is far from being optimal, in terms of network resources utilization. Under certain conditions, it may even block the signaling channel. This study proposes to use a Load-Adaptive Multiple Access with Congestion Avoidance (LAMA/CA) for efficient short messages transmission in cellular networks. The LAMA/CA protocol is more stable, and much more efficient for transmitting SMS messages and signaling messages, than the existing method currently used in cellular networks. Moreover, the proposed method is superior to a collision-free protocol that uses a Request To Send (RTS) and Clear To Send (CTS) mechanism, over the range of packet lengths used for SMS messages and signaling messages.

This chapter deals with the problem of designing and effectively utilizing wireless communication channels. Since the wireless medium is inherently a shared resource, controlling channel access becomes a central theme that determines the fundamental capacity of the wireless network and has a dramatic impact on system complexity and cost. Therefore, our primary focus will be the design and implementation of Media Access Control (MAC) protocols for mobile wireless networks.

The design of effective Medium Access Control protocol for ad hoc network is a highly challenging task due to the characteristics of such networks. The medium is shared and unreliable by definition, thus the access mechanism should be robust to channel errors and collisions. Furthermore, all the coordination functions must be totally distributed in a pure ad hoc environment. In this paper we address the issue of designing an effective MAC protocol for Inter Vehicular Communications (IVC) networks with a particular focus on broadcast services and topology control capabilities. 1

Abstract. A wireless ad hoc network is a collection of wireless nodes that can dynamically self-organize into an arbitrary and temporary topology to form a network without necessarily using any pre-existing infrastructure. These characteristics make ad hoc networks well suited for military activities, emergency operations, and disaster recoveries. Nevertheless, as electronic devices are getting smaller, cheaper, and more powerful, the mobile market is rapidly growing and, as a consequence, the need of seamlessly internetworking people and devices becomes mandatory. New wireless technologies enable easy deployment of commercial applications for ad hoc networks. The design of an ad hoc network has to take into account several interesting and difficult problems due to noisy, limited-range, and insecure wireless transmissions added to mobility and energy constraints. This paper presents an overview of issues related to medium access control (MAC), routing, and transport in wireless ad hoc networks and techniques proposed to improve the performance of protocols. Research activities and problems requiring further work are also presented. Finally, the paper presents a project concerning an ad hoc network to easily deploy Internet services on low-income habitations fostering digital inclusion. 1

2.1 Carrier Sense Multiple Access and IEEE 802.11.......... 4