Topology and mobility considerations in mobile ad hoc networks
Introduction
The mobile devices in a mobile ad hoc network (MANET) play a very different role than in a conventional wireless LAN (WLAN). In a conventional WLAN, communications are centered on the base station or access point; the infrastructure up to the base station is mostly fixed, so the topology is stable. In a MANET, mobile nodes act not only as end systems, but also as routing devices. The topology of the network is dependent on the relative locations and connections of nodes within the network.
This results in a topology that is potentially extremely dynamic. This effects all aspects of an ad hoc network, including the medium access control (MAC) layer and routing protocols. In order to achieve acceptable performance, the MANET as a whole must find effective ways of managing the side caused by the changing topology. In order to achieve this, those effects must first be understood.
To date, most research has focused on practical aspects––development of mechanisms and protocols for use in ad hoc networking, particularly for routing. This has reached limited success. Although the concept of a MANET has been shown to be workable, in practice the performance has been inadequate to gain widespread acceptance or commercial feasibility. In fact, even the understanding of the resulting protocols is questionable, due to the difficulty of comparing evaluation results.
A better knowledge of the effects of various MANET parameters and characteristics will greatly aid in the development of new ideas. A better understanding will:
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Allow a wiser selection of scenarios for testing and evaluation.
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Reduce or eliminate design decisions based on faulty assumptions.
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Promote optimization of parameters for real network conditions.
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Reveal characteristics to be used in developing future protocols.
With these potential benefits in mind, a number of experiments have been performed in order to investigate the effects of several parameters, including the number of nodes, their behaviour, and the environment in which they exist. Several aspects have been explored, including the overall connection characteristics in the network, the severity of the topology instability, as well as the resulting effects on routing in a MANET.
Section snippets
Mobile ad hoc networks
A mobile ad hoc network consists of a collection of wireless-enabled devices. Links are formed between node-pairs that are within direct communication range, and the nodes and links combine to create the network topology. During the lifetime of the network, nodes may move around within the network, altering the topology by creating or breaking links between nodes.
Nodes may also enter or leave the network. This may be due to mobility, if a node moves out of range of all other nodes in the
Network scenarios
The network scenarios to be examined were chosen to mimic the common scenarios chosen to test MANET routing protocols. Three different variables were provided to examine static network scenarios, with two additional variables added for mobile network scenarios.
Results and analysis
In the following experiments, two aspects of MANETs were investigated. First, the overall connectivity of the network was explored. This included the properties of the nodes, their links and the links’ lifetimes. Second, these links were put together to form routes within the network. Path lengths, lifetimes, the existence of multiple paths, and the importance of individual nodes were each addressed.
Conclusions
In this paper, a number of statistics were collected from the topologies and mobility patterns of mobile ad hoc networks. Connectivity, node degrees, and path lengths were presented, along with link lifetimes and times to route failures. Overall, the results are cause for concern. Not only do many links break after a relatively short time period, but their short-lifespan is also propagated and exacerbated in the life spans of the routes.
The shortness of the route life spans is a problem. With
Brent Ishibashi received the B.Sc. degree from the University of Guelph (Canada) in 2000. He recently completed his M.Math degree at the School of Computer Science of the University of Waterloo (Canada), and is now working towards a Ph.D. degree, also at the University of Waterloo. His research focuses on resource management in ad hoc network environments. Recent work has involved investigating the interactions between the link and network layers, particularly with finding approaches for better
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Brent Ishibashi received the B.Sc. degree from the University of Guelph (Canada) in 2000. He recently completed his M.Math degree at the School of Computer Science of the University of Waterloo (Canada), and is now working towards a Ph.D. degree, also at the University of Waterloo. His research focuses on resource management in ad hoc network environments. Recent work has involved investigating the interactions between the link and network layers, particularly with finding approaches for better integrating medium access control with ad hoc routing.
Raouf Boutaba is currently an Associate Professor in the School of Computer Science of the University of Waterloo. Before that he was with the Department of Electrical and Computer Engineering of the University of Toronto. Before joining academia, he founded and was the director of the telecommunications and distributed systems division of the Computer Science Research Institute of Montreal (CRIM). He conducts research in the areas of network and distributed systems management and resource management in multimedia wired and wireless networks. He has published more than a hundred papers in refereed journals and conference proceedings. He is the recipient of the Premier’s Research Excellence Award, a fellow of the Faculty of Mathematics of the University of Waterloo, and a distinguished lecturer of the IEEE Computer Society. He is the Chairman of the IFIP Working Group on Networks and Distributed Systems, the Vice Chair of the IEEE Communications Society Technical Committee on Information Infrastructure, and the Chair of the IEEE Communications Society Committee on Standards. He is on the advisory editorial board of JNSM, the editorial board of JCN, and the editorial board of Computer Networks.