[Book Title], Edited by [Editor’s Name].
ISBN 0-471-XXXXX-X Copyright © 2000 Wiley[Imprint], Inc.
Chapter 0
Mobile Ad-Hoc Networks
Silvia Giordano
Abstract
Ad-hoc networks are a key in the evolution of wireless networks. Ad-hoc networks are
typically composed of equal nodes, which communicate over wireless links without any
central control. Ad-hoc wireless networks inherit the traditional problems of wireless and
mobile communications, such as bandwidth optimisation, power control and transmission
quality enhancement. In addition, the multi-hop nature and the lack of fixed
infrastructure brings new research problems such as configuration advertising, discovery
and maintenance, as well as ad-hoc addressing and self-routing. Many different
approaches and protocols have been proposed and there are even multiple
standardization efforts within the Internet Engineering Task Force, as well as academic
and industrial projects. This chapter focuses on the state of the art in mobile ad-hoc
networks. It highlights some of the emerging technologies, protocols, and approaches (at
different layers) for realizing network services for users on the move in areas with
possibly no pre-existing communications infrastructure.
INTRODUCTION
Future information technology will be mainly based on wireless technology ([51], [57],
[50]). Traditional cellular and mobile networks are still, in some sense, limited by their
need for infrastructure (i.e., base stations, routers). For mobile ad-hoc networks, this final
limitation is eliminated.
Ad-hoc networks are a key in the evolution of wireless networks [49]. Ad-hoc
networks are typically composed of equal nodes, which communicate over wireless links
without any central control. Although military tactical communication is still considered
as the primary application for ad-hoc networks, commercial interest in this type of
networks continues to grow. Applications such as rescue missions in times of natural
disasters, law enforcement operation, commercial and educational use, and sensor
networks are just few possible commercial examples.
Ad-hoc wireless networks inherit the traditional problems of wireless and mobile
communications, such as bandwidth optimisation, power control and transmission quality
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enhancement. In addition, the multi-hop nature and the lack of fixed infrastructure
generates new research problems such as configuration advertising, discovery and
maintenance, as well as ad-hoc addressing and self-routing.
Figure 1 A mobile ad-hoc network
In mobile ad-hoc networks, topology is highly dynamic and random. In addition,
the distribution of nodes, and, eventually, their capability of self-organising play an
important role. The main characteristics can be summarized as follows:
• The topology is highly dynamic and frequent changes in the topology may be hard to
predict.
• Mobile ad-hoc networks are based on wireless links, which will continue to have a
significantly lower capacity than their wired counterparts.
• Physical security is limited due to the wireless transmission.
• Mobile ad-hoc networks are affected by higher loss rates, and can present higher
delays and jitter than fixed networks due to the wireless transmission.
• Mobile ad-hoc network nodes rely on batteries or other exhaustible means for their
energy. As a consequence, energy savings are an important system design criterion.
Furthermore, nodes have to be power-aware: the set of functions offered by a node
depends on its available power (CPU, memory, etc..).
A well-designed architecture for mobile ad-hoc networks involves all networking
layers, ranging from the physical to the application layer.
Despite the fact that the management of the physical layer is of fundamental
importance, there is very little research in this area: nodes in mobile ad-hoc networks are
confronted with a number of problems, which, in existing mobile networks, are solved by
the base stations. The solution space ranges from hierarchical cell structures (a self-
organized pendant of cellular networks) to completely ad-hoc, stochastic allocations.
Power management is of paramount importance. General strategies for saving power
need to be addressed, as well as adaptation to the specifics of nodes of general channel
and source coding methods, of radio resource management and multiple access.
Mobile ad-hoc networks do not rely on one single technology; instead, they
should be able to capitalize on technology advances. One challenge is to define a set of
abstractions, which can be used by the upper layers, and still not preclude the use of new
physical layer methods as they emerge. Primitives of such an abstraction are for example
the capabilities and covering ranges of multicast and unicast channels.
Information as node distribution, network density, link failures, etc.., must be
shared among layers, and the MAC layer and the network layer need to collaborate in
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order to have a better view of the network topology and to optimise the number of
messages in the network.
In mobile ad-hoc networks, with the unique characteristic of being totally
independent from any authority and infrastructure, there is a great potential for the users.
In fact, roughly speaking, two or more users can become a mobile ad-hoc network simply
by being close enough to meet the radio constraints, without any external intervention.
Moreover, telecommunication networks are expected to grow with the advent of
new (and totally unexpected) applications. While in the past telecommunication networks
were studied and developed in separate building blocks, for mobile ad-hoc networks the
interaction between higher layers and lower layers is essential for the users.
Resilient and adaptive applications that can continue to perform effectively under
degraded conditions can significantly enhance network operations from a user's
perspective. Such applications can also ease the design pressure significantly in complex
engineering areas such as quality of service (QoS) and mobile routing at the network
layer [47].
As illustrated in Figure 2, the communication among layers is the only approach
for a demanding environment raising issues that rarely occur in other networks [35].
R apid reaction to
topology changes
Efficiency
H ighly dynam ic,
unpredictable netw ork
R elativ ely sca rce
link resources
Possible
shared link
R elativ ely lo w
bandw idth
M any updates w ith
trad itio na l ro u ting
M u ltip le alg o rith m s
at different tim escales
M u ltip le
algorithm s
Integrated algorithm s
w hich can be com plex
and m ay violate layering
Control over
tra nsm issio n p ara m e ters
Access to
overheard
inform atio n
Figure 2 Complexity of Mobile Ad-hoc Networks: the network can be highly
dynamic, implying that (1) traditional routing algorithms will either not stabilize or
will generate many routing updates, and (2) rapid response to topology change is
needed [35].
This chapter focuses on the state of the art in mobile ad-hoc networks and
highlights some of the emerging technologies, protocols, and approaches at different
layers for realizing network services for users on the move in areas with possibly no pre-
existing communication infrastructures.
The remaining of this chapter is organized as follows. In Section 2 we present the
layered architecture of mobile ad-hoc networks, and we introduce some relevant concepts
and technologies that will be discussed further. In Section 3 we cover some emerging
MAC technologies that can be used for constructing a mobile ad-hoc network: IEEE
802.11, and Bluetooth. In Section 4 we provide an overview of the standardization efforts
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within the Internet Engineering Task Force. Section 5 introduces a human based
approach to a particular class of mobile ad-hoc networks, referred to as self-organising
networks. Finally, in Section 6, we present mobile ad-hoc networking from the
users/applications point of view.
The Conclusion provides a discussion on the future evolution and applications of
mobile ad-hoc networks.
LAYERED ARCHITECTURE OF MOBILE AD-HOC
NETWORKS
Precursor of the ad-hoc networking technology was the Packet Radio Network [34],
[33]. Packet radio applies packet communications to a radio channel rather than to a wire-
based media. This technology can be used to create LANs that link devices, as well as
provide gateways to other network systems and databases [11]. The current version,
referred to as "Distributed Packet Radio", is completely distributed, permitting flexible
and rapid adaptation to changes and mobility.
Later on, research mainly focused on cellular systems that are, in principle,
single-hop wireless systems. Within the framework of the multi-hop wireless systems,
research communities worked on projects that address mainly medium access control and
routing issues.
Appropriate physical and data link protocols need to be developed for wireless
mobile networks in conjunction with the embedded MAC sub-layer and the higher-level
networking and/or transport layers.
A key aspect of wireless communications is the radio propagation channel, which
introduces co-channel and adjacent channel interference among users. Exploiting the
physical environment and controlling the location of radio users as much as possible is
one way to mitigate interference, but this is not realistic for uncoordinated wireless
systems that share the same radio spectrum. For ad-hoc networks that share the same
spectrum, new methods of cooperation are required to permit coexistence. Such methods
are difficult to research without real-world channel models and simulation
methodologies; there is still fundamental work to be done in this area [50].
There have been many successful attempts to reduce the power consumption of
digital circuits. Today, there are many different techniques known, starting from the
circuit level [19] and reaching into architecture and software. Clearly, the energy
necessary to execute a given algorithm very much depends on the implementation
technology. For current software-radio projects, large emphasis has been placed on low
power consumption, see [67] and [56].
Current research covers lower-layer issues, such as modulation and coding,
multiple access, wireless/mobile protocols, and location protocols. In the USA, most of
the research in this and in the sensors networks fields is sponsored by NSF (Advanced
Networking Infrastructure and Research Division and Computer-Communications
Research Division) and DARPA (Microelectromechanical Systems and Global Mobile
Information Systems), see [52], [3], [14], [23], [42].
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Similar projects are conducted in Europe (in the Mobility and Personal
Communications Networks Domain [2], in ETSI [25], or in some Universities e.g., [20]),
by industrial consortia (e.g., [8]), [61], and by operator (e.g., [60]).
The MAC layer specified in the IEEE 802.11 standard [29], or its variations, is
typically applied in the existing ad-hoc network projects. The standard is built on Carrier
Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme that is extended
with a capability for short channel allocation and acknowledgement control messages.
With 802.11, all the nodes must use the same channel. All nodes can communicate with
every other node that is within range. The 802.11 standard can be a good platform to
implement a one level multi-hop architecture because of its extreme simplicity. IEEE
802.11 is a digital wireless data transmission standard aimed at providing a wireless LAN
(WLAN) between portable computers and between portable computers and a fixed
network infrastructure. While it is easy to foresee that the WLANs will be the solution for
the home and office automation [5], the existing standard does not support multi-hop
systems and since only one frequency can be used, the achievable capacity is limited.
HomeRF [27] is seen as the main contender of 802.11 to be used in home network. This
is based on the Shared Wireless Access Protocol (SWAP) that defines a new common
interface that supports wireless voice and data networking in home. Wireless LAN
technology is already widely commercially available. The main aim of Bluetooth
technology [8] [10] is to guarantee the interoperability between different applications on
devices in the same area that may run over different protocol stacks, and therefore to
provide a solution for wireless Personal Area Networks. Section 2 covers in more detail
the MAC layer.
The Internet Engineering Task Force (IETF) working group on Mobile Ad-hoc
NETworks (MANET) is standardizing routing in ad-hoc networks. The group studies
routing specifications, with the goal of supporting networks scaling up to hundreds of
routers [40]. The work inside MANET relies on other existing IETF standards such as
mobile-IP and IP addressing. Most of the currently available solutions are not designed to
scale to more than a few hundred nodes. Section 3 presents some aspects of the protocols
designed to extend Internet services to mobile ad-hoc networks’ users.
Designing protocols that scale to very large wireless networks remains among the
main challenges of research in this field, and there are several factors that distinguish
protocols for realizing a wide-area mobile ad-hoc network with different peculiarities, as
explained in Section 4.
Location-management functions make it possible to access the network regardless
of the user's location. Not limited only to users, it is easily imagined that entire networks
might one day be mobile as well, e.g., networks on aircraft or other vehicles. Location
management works at several layers and is, therefore, a complex process [49].
The well-established techniques to locate mobile devices in infrastructure-based
networks, even if they contain concepts to deal with nomadic nodes, are not useful as
soon as infrastructure is no longer available.
As stated within the Zeroconf Working Group of the IETF [78], the common
TCP/IP protocols commonly used for the network configuration, e.g. DHCP, DNS,
MADCAP, and LDAP, are not appropriate for mobile ad-hoc networks because they
must be configured and maintained by an administrative staff.
![Figure 2 Complexity of Mobile Ad-hoc Networks: the network can be highly dynamic, implying that (1) traditional routing algorithms will either not stabilize or will generate many routing updates, and (2) rapid response to topology change is needed [35].](/figures/figure-2-complexity-of-mobile-ad-hoc-networks-the-network-3sjewpij.webp)
