Wireless Ad Hoc Networks

Wireless Ad Hoc Networks

A mobile ad hoc network is a relatively new term for an old technology : a network that does not rely on pre-existing infrastructure. Roots of this technology could be traced back to the early 1970s with the DARPA PRNet and the SURAN projects. The new twitch is the application of this technology in the non-military communication environments. Additionally, the research community has also recently addressed some extended features of this technology, such as multicasting and security. Also numerous new solutions to the old problems of routing and medium access control have been proposed. This survey attempts to summarize the state-ofthe-art of the ad hoc networking technology in four areas: routing, medium access control, multicasting, and security. Where possible, comparison between the proposed protocols is also discussed. A Mobile Ad Hoc Network (MANET) is a network architecture that can be rapidly deployed without relying on pre-existing fixed network infrastructure. The nodes in a MANET can dynamically join and leave the network, frequently, often without warning, and possibly without disruption to other nodes’ communication. Finally, the nodes in the network can be highly mobile, thus rapidly changing the node constellation and the presence or absence of links. Examples of the use of the MANETs are: • tactical operation : for fast establishment of military communication during the deployment of forces in unknown and hostile terrain; • rescue missions : for communication in areas without adequate wireless coverage; • national security : for communication in times of national crisis, where the existing communication infrastructure is non-operational due to a natural disaster or a global war; • law enforcement : for fast establishment of communication infrastructure during law enforcement operations; • commercial use : for setting up communication in exhibitions, conferences, or sales presentations; • education : for operation of wall-free (virtual) classrooms; and • sensor networks : for communication between intelligent sensors (e.g., MEMS mounted on mobile platforms. Nodes in the MANET exhibit nomadic behavior by freely migrating within some area, dynamically creating and tearing down associations with other nodes. Groups of nodes that have a common goal can create formations (clusters) and migrate together, similarly to military units on missions or to guided tours on excursions. Nodes can communicate with each other at any time and without restrictions, except for connectivity limitations and subject to security provisions. Examples of network nodes are pedestrians, soldiers, or unmanned robots. Examples of mobile platforms on which the network nodes might reside are cars, trucks, buses, tanks, trains, planes, helicopters or ships. MANETs are intended to provide a data network that is immediately deployable in arbitrary communication environments and is responsive to changes in network topology. Because adhoc networks are intended to be deployable anywhere, existing infrastructure may not be present. The mobile nodes are thus likely to be the sole elements of the network. Differing mobility patterns and radio propagation conditions that vary with time and position can result in intermittent and sporadic connectivity between adjacent nodes. The result is a time-varying network topology. MANETs are distinguished from other ad-hoc networks by rapidly changing network topologies, influenced by the network size and node mobility. Such networks typically have a large span and contain hundreds to thousands of nodes. The MANET nodes exist on top of diverse platforms that exhibit quite different mobility patterns. Within a MANET, there can be significant variations in nodal speed (from stationary nodes to high-speed aircraft), direction of movement, acceleration/deceleration or restrictions on paths (e.g., a car must drive on a road, but a tank does not). A pedestrian is restricted by built objects while airborne platforms can exist anywhere in some range of altitudes. In spite of such volatility, the MANET is expected to deliver diverse traffic types, ranging from pure voice to integrated voice and image, and even possibly some limited video. 1.2. The Communication Environment and the MANET Model The following are a number of assumptions about the communication parameters, the network architecture, and the network traffic in a MANET. • Nodes are equipped with portable communication devices. Lightweight batteries may power these devices. Limited battery life can impose restrictions on the transmission range, communication activity (both transmitting and receiving) and computational power of these devices. • Connectivity between nodes is not a transitive relation; i.e., if node A can communicate directly with node B and node B can communicate directly with node C, then node A may Micro-Electro-Mechanical-Systems3 not, necessarily, be able to communicate directly with node C. This leads to the hidden terminal problem . • A hierarchy in the network routing and mobility management procedures could improve network performance measures, such as the latency in locating a mobile. However, a physical hierarchy may lead to areas of congestion and is very vulnerable to frequent topological reconfigurations. • We assume that nodes are identified by fixed IDs (based on IP [Pos81] addresses, for example). • All the network nodes have equal capabilities. This means that all nodes are equipped with identical communication devices and are capable of performing functions from a common set of networking services. However, all nodes do not necessarily perform the same functions at the same time. In particular, nodes may be assigned specific functions in the network, and these roles may change over time. • Although the network should allow communication between any two nodes, it is envisioned that a large portion of the traffic will be between geographically close nodes. This assumption is clearly justified in a hierarchical organization. For example, it is much more likely that communication will take place between two soldiers in the same unit, rather than between two soldiers in two different brigades. A MANET is a peer-to-peer network that allows direct communication between any two nodes, when adequate radio propagation conditions exist between these two nodes and subject to transmission power limitations of the nodes. If there is no direct link between the source and the destination nodes, multi-hop routing is used. In multi-hop routing, a packet is forwarded from one node to another, until it reaches the destination. Of course, appropriate routing protocols are necessary to discover routes between the source and the destination, or even to determine the presence or absence of a path to the destination node. Because of the lack of central elements, distributed protocols have to be used. The main challenges in the design and operation of the MANETs, compared to more traditional wireless networks, stem from the lack of a centralized entity, the potential for rapid node movement, and the fact that all communication is carried over the wireless medium. In standard cellular wireless networks, there are a number of centralized entities (e.g., the basestations, the Mobile Switching Centers (MSCs), the Home Location Register (HLR), and the Visitor Location Register (VLR)). In ad-hoc networks, there is no preexisting infrastructure, and these centralized entities do not exist. The centralized entities in the cellular networks perform the function of coordination. The lack of these entities in the MANETs requires distributed algorithms to perform these functions. In particular, the traditional algorithms for mobility management, which rely on a centralized HLR/VLR, and the medium access control schemes, which rely on the base-station/MSC support, become inappropriate. All communications between all network entities in ad-hoc networks are carried over the wireless medium. Due to the radio communications being vulnerable to propagation impairments, connectivity between network nodes is not guaranteed. In fact, intermittent and sporadic connectivity may be quite common. Additionally, as the wireless bandwidth is limited, its use should be minimized. Finally, as some of the mobile devices are expected to be handheld with limited power sources, the required transmission power should be minimized as well. Therefore, the transmission radius of each mobile is limited, and channels assigned to mobiles are typically spatially reused. Consequently, since the transmission radius is much smaller than the network span, communication between two

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