This chapter offers a state-of-the-art review in mobile ad hoc networks (MANETs). It first introduces the history of ad hoc networks, explaining the ad hoc network concept and referring to the main characteristics of these networks and their fields of application. It then focuses on technologies and protocols specific to ad hoc networks. Firstly, it refers to relevant proposals targeting the PHY/MAC layers. Secondly, it discusses the different routing protocol proposals for ad hoc networks according to the category to which they belong. Finally, it includes an overview of the different protocols proposed for ad hoc networks at the transport layer. The chapter concludes with some remarks on future trends in these networks.
The history of wireless networks dates from the 1970s. In fact, radio communications and computer networks were first combined by the University of Hawaii, in 1971, in an experimental network named ALOHANET. That network offered bidirectional communications following a star topology, and its purpose was to allow communicating with US mainland. During the 1980s, the technology was improved, and towards the end of the 1990s, interest on wireless networks reached a peak, mainly due to the fast growth of the Internet.
Nowadays we can split existing wireless networks into different categories according to their scope and size. Wireless wide area networks (WWANs), such as GSM and UMTS (Ojanpera, T. & Prasad, R., 1998), usually cover hundreds of kilometers and use private frequency bands. Such networks are usually owned and maintained by telecommunications providers, and their purpose is to offer services in a country or a region of it. Wireless metropolitan area networks (WMANs), such as WiMax (IEEE 802.16 WG, 2004), typically have a range of a few kilometers, and can operate over both private and public frequency bands, so that both telecommunication companies and private users can take advantage of them. Wireless local area networks (WLANs), such as WiFi (IEEE 802.11 WG, 1999), usually cover areas between a few tens of meters up to a kilometer. They typically use public frequency bands so that users can freely install and use them. At the lower end, we have wireless personal area networks (WPANs), such as Bluetooth (IEEE 802.15 WG, 2005), which also use free frequency bands that are used to replace cables within a very limited area around a single user (few meters).
This chapter focuses on recent developments in terms of infrastructure-less wireless networks, more commonly known as ad hoc networks, that extend WLAN technologies to offer more flexible solutions. All nodes within an ad hoc network provide a peer-level multihopping routing service to allow out-of-range nodes to be connected. Unlike a wired network, nodes in an ad hoc network can move freely, thus giving rise to frequent topology changes.
Such a network may operate in a stand-alone fashion or be connected to the larger Internet. An ad hoc architecture has many benefits, such as self-reconfiguration and adaptability to highly variable characteristics, namely, power and transmission conditions, traffic distribution variations, and load balancing. However, those benefits come with many challenges. New algorithms, protocols, and middleware have to be designed and developed to create a truly flexible and decentralized network.
In terms of applications, ad hoc networks offer the required flexibility to adapt to situations where no sort of infrastructure is available. Examples of such situations are army units moving inside hostile territories, or organized teams, such as firemen, performing rescue tasks. In general, mobile ad hoc networks can be used on all those situations characterized by lack of fixed infrastructure, peer-to-peer communication, and mobility support.
Key Terms in this Chapter
VANET: Vehicular ad hoc network, consisting of a network of vehicles, moving at a relatively high speed, that communicate among themselves with different purposes, being the main purpose that of improving security on the road.
Link State: Routing protocols, based on this technique, maintain a routing table with the full topology. The topology is built by finding the shortest path in terms of link cost, cost that is periodically exchanged among all the nodes through a flooding technique.
Distance Vector: Routing technique that maintains a table for the communication taking place, and employs diffusion (not flooding) for information exchange between neighbors. All the nodes must calculate the shortest path towards the destination using the routing information of their neighbors.
MAC Layer: The medium access control layer is a protocol layer embedded within the link layer that is responsible for coordinating the access to a shared medium according to a set of rules.
Node: In the context of mobile ad hoc networks (MANETs), it usually refers to a mobile terminal, such as a PDA, laptop, smartphone, or other device with wireless communication capabilities that participates in the networks both as a traffic generator and traffic forwarder.
Source Routing: Technique where all the data packets have the routing information on their headers. The route decision is made on the source node, which avoids routing loops entirely.
SSH: Secure shell is a protocol that allows accessing a remote computer in a secure manner by employing cryptographic techniques. Usually, the term refers also to the client/server tools that support this protocol.