A mobile ad hoc network is a collection of mobile nodes which dynamically forms a temporary network without using a centralized administration and maintain itself continuously to topology changes. The connections in the network can be stretched over multiple nodes (multi-hop) and the physical interconnection of the nodes among each other is wireless according to the mobile nature of the nodes [1].
Mobile ad hoc networks typically have highly dynamic topologies, not only in terms of frequent membership changes (node entering and leaving
the network), but also in terms of node mobility (nodes change the physical location and their relations to other nodes in the network). In addition, these networks often consist of a mix of different devices (e.g. handhelds, mobile phones, embedded devices, laptops) that use different physical layers (such as Bluetooth, IrDA, UMTS, Wireless LAN (IEEE 802.11, WLAN), Ethernet) with different protocols. Furthermore, participating nodes have often limited resources (such as CPU capacity, storage capacity, power and communication bandwidth). Figure 3.1 illustrates such a kind of network.
Figure 3.1: Mobile Ad hoc Network
(taken from http://www.spemaus.de/studium/diplomarbeit/html/manets.xhtml)
Concerning the highly dynamic topology, a mobile ad hoc network can-not rely on a permanent backbone infrastructure, therewith each node par-ticipating in the network should be willing to act as host and as router simultaneously and forward packets for order nodes.
Further, the communication between wireless nodes is more difficult than between hardwired nodes. Wireless links show a strong time varying statis-tical behaviour caused by many factors, such as physics of the propagation medium, interferences, noise, fading characteristics, shadowing, potential power control and multiple medium access with the hidden and exposed
3.1 Mobile Ad hoc Networks (MANET) Chapter 3
terminal problems [2].
Consequently, new network protocols and applications adapted for mo-bile ad hoc networks are required, due to its dynamic topology and limited resources. Currently, significant research is done in this area. For exam-ple, the IETF has created a working group named MANET (Mobile Ad-hoc Networks) to standardize the IP routing protocol functionality suitable for wireless routing application within both static and dynamic topologies. Ap-pendix A.4 describes various routing protocols suited for mobile ad hoc networks.
Many different applications for mobile ad hoc networks with various number of participating nodes are imaginable. Such networks could be useful in the office to connect the laptops and PDAs (Personal Digital Assistants) of the attendant workers among each other and with the existing infrastruc-ture (e.g. printers). These networks can also be used to interconnect people in lectures, meetings, trains or leisure activities, to exchange information or to play games. Automobiles with corresponding in-vehicle devices can form a wide mobile ad hoc network on the roads to inform each other about the volume of traffic, accidents and traffic jams, or to exchange more universal information. Furthermore, mobile ad hoc networks can be applied in hos-pital, battlefield, rescue, sensoring and monitoring scenarios or everywhere where a permanent infrastructure is either unavailable or destroyed.
As example in [3] the ad hoc city, a multi-tier wireless ad hoc network routing architecture for general purpose wide-area communication in cities, is proposed. The backbone network in this architecture is itself also a mobile multi-hop network, composed of wireless devices mounted on mobile fleets such as city buses or delivery vehicles. Figure 3.2 illustrates the system.
Furthermore, in [4] a system called Sphinx is proposed, which uses ad hoc routing in tandem with the cellular network model to achieve higher throughput and lower power consumption.
There is also an interest to connect mobile ad hoc networks with other networks, e.g. the Internet [5] [6] and to enable roaming of nodes between different ad hoc networks and between an ad hoc network and the Internet.
Furthermore, a particular mobile node should be accessible, even if it is not known to which network he is currently attached.
To support mobile hosts in the Internet there exists the Mobile IP [7]
technology, so that a mobile can be connected elsewhere than its well known
Internet
Base Stations
Figure 3.2: Example of the Ad Hoc City Architecture
(taken from [3])
fixed-address domain space. For this purpose a fixed home agent is used, which redirects the packet for the mobile host to its current position in the Internet. In a mobile ad hoc network such a fixed home agent cannot be used, as no fixed infrastructure can be assumed. Therefore, other solutions have to be developed to address a particular node in a mobile ad hoc network.
To deliver Internet and mobile computing applications to thin-client de-vices, WAP and mobile Java (J2ME) have been developed. The Wireless application protocol (WAP) [8] was designed to provide users of mobile de-vices access to the Internet via an optimized protocol for wireless communi-cation. It was declared as de facto standard in mobile communications, but has not become very popular until now. The micro edition of Java (J2ME) [9] provides the Java programming language and execution environment on resource-constrained mobile devices. With mobile Java, applications can be deployed that run independent of the underlying device hardware and soft-ware. Java provides a rich user interface, security, and the ability to perform
3.1 Mobile Ad hoc Networks (MANET) Chapter 3
off-line operations. Java seems to become popular for mobile infotainment applications.
Security considerations are also important. A network may require pri-vacy to communicate important information or a network device may be improperly configured or intentional malicious, so that the information it exchanges is incorrect and can disrupt the network. Also, wireless links are more vulnerable to eavesdropping, spoofing and denial-of-service attacks.
A special characteristic of wireless transmission is that alsounidirectional connections can exist. It is possible that a nodeA is able to ”hear” nodeB, butB cannot ”hear” A, because nodeA and B have different transmission ranges. It makes operational sense to allow a unidirectional connection B →A as a forwarding link. If necessaryA may communicate withB over other nodes, which makes the overall communication again bidirectional.
In a nutshell a mobile ad hoc network yields special challenges in:
• energy efficiency: devices have limited power
• security: wireless links are more vulnerable to eavesdropping, spoofing and denial-of-service attacks
• routing convergence: highly dynamic network topology
• protocol efficiency: bandwidth constrained, variable capacity links
• multicasting: no fixed infrastructure
• service discovery: no fixed directory agents
• media access control: collision prevention
• scalability: number of participants is not predetermined
A good overview about mobile ad hoc networks can be found in [1] and [10].