Ad hoc networking in future wireless communications

David Remondoa, Ignas G. Niemegeersb

aCatalonia University of Technology (UPC), Barcelona, SpainbDelft University of Technology, Delft, The Netherlands

Received 26 February 2002; accepted 26 February 2002

Abstract

This paper contains an overview of the discussions on future research directions within the subject of ad hoc networking, held at the

Wireless World Research Forum meetings during 2001. Ad hoc networking is an emerging research field where ad hoc networks are no

longer viewed as stand-alone groups of wireless terminals. On the contrary, ad hoc networks are expected to become fundamental in the

future development of infrastructure networks and they will be the basis for enabling ubiquitous communications. Ad hoc networking

involves new research issues at all layers.

q 2002 Elsevier Science B.V. All rights reserved.

Keywords: Wireless networks; Ubiquitous communications; Pervasive computing

1. Introduction

In early 2001, the Wireless World Research Forum was

founded with the objective of formulating visions on strategic

future research directions in wireless communications. The

objective was to involve industry and academia in generating,

identifying and promoting research areas and technical trends

for mobile and wireless system technologies. The time frame

of reflection is in the range of 10–15 years from now. This

paper gives an overview of the discussions that have been held

within the Wireless World Research Forum (WWRF) on the

subject of ad hoc networking.

2. Rationale

Ad hoc networks are formed by wireless1 devices that

communicate without necessarily using a pre-existing

network infrastructure. Ad hoc networks are self-configur-

ing, i.e. there is no (central) management system with

configuration responsibilities. Some, if not all, nodes in an

ad hoc network are capable of assuming router functionality

when needed. This enables terminals to communicate with

each other when they are out of range, provided they can

reach each other via intermediate terminals acting as routers

that relay the packets from source to destination. The structure

of the network can change constantly because of the

movement of the nodes. Therefore, we include the possibility

of having mobile routers in the network. In contrast with

cellular networks, there is no need to build up a network

infrastructure with base stations. Ad hoc networks can be

viewed as stand-alone groups of mobile terminals, but they

may also be connected to a pre-existing network infrastructure

(such as fixed access networks or cellular systems) and use it to

access hosts which are not part of the ad hoc network. In

conclusion, ad hoc networking is about supporting spon-

taneous communication among mobile and fixed terminals

whenever there is the physical possibility to do so.

3. The added value of ad hoc networking

What is the motivation behind research on ad hoc

networking? There are two main aspects: to enable

ubiquitous communications and to complement and

enhance wireless access networks. It can be expected that

in the near future there will be a proliferation of wireless

devices. Ad hoc network functionality such as self-

configurability and independence of existing infrastructures

are key issues in this context. Examples of future networks

are Personal Area Networks (PANs) [1], Body Area

Networks (BANs) [2], home networks, networks of sensors

Computer Communications 26 (2003) 36–40

www.elsevier.com/locate/comcom

0140-3664/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.

PII: S0 1 40 -3 66 4 (0 2) 00 1 17 -2

E-mail address: remondo@entel.upc.es (D. Remondo).1 Strictly speaking, ad hoc networks can make use of other technologies.

However, wireless is the natural choice for spontaneous networking.

and actuators (e.g. at home, in cars, or those for ambient

intelligence), or vehicle-to-vehicle networks. All these new

forms of ad hoc networks will make many new applications

and services possible.

Ad hoc networking and in particular the multi-hop

communication capabilities can also be used to extend the

coverage of existing wireless access technologies. In

general, direct communication between neighbouring term-

inals without intervention of relatively remote base stations

results in a decreased interference level, which implies that

the general capacity of the system will be increased. This

can represent a reduction in the cost of wireless access

infrastructures, at the expense of some additional network-

ing complexity. Not only is this an interesting approach for

cellular networks, but also it is particularly interesting in the

case of high-frequency Wireless Local Area Networks

(WLANs) due to opacity problems [3]. The use of ad hoc

networking to enhance wireless access was proposed as one

of the access methods the Universal Mobile Telecommu-

nications System (UMTS), the Opportunity Driven Multiple

Access (ODMA) [25].

Intrinsic characteristics of ad hoc networks such as self-

configurability [12] and neighbour discovery imply that

these networks will be a key element for enhancing the

interoperability among different wireless technologies,

which is considered to be the fundamental underlying idea

of future wireless networks.

There is a wide range of applications that could

potentially be supported by ad hoc networks. Some

examples are the following:

† disaster-relief networks;

† networks at construction sites;

† inter-vehicle networks;

† interaction with an organisations’ intranet and extranet in

the office;

† interaction of PANs or BANs with a Vehicle Area

Network;

† home environment networks;

† supporting doctors and nurses in hospitals;

† networks of visitors at airports, conference centres, trade

fairs, sport centres and shopping malls;

† PANs and BANs;

† linking up with the Internet, intranets or extranets via

cellular networks and WLANs.

4. State of the art

Experimental ad hoc networks are already being built

these days. These experiments mainly focus on the

implementation and the operation of already proposed

routing protocols for ad hoc networks. We find an overview

of research activities within this field in [4]. Some have led

to working demonstrations. A nice example can be found in

the Mobile Communication System for the Construction

and Mining Industry project [26].

Because of the unique characteristics of ad hoc networks,

other issues such as QoS, security and multicasting are

being reconsidered too.

Some ongoing research projects are concerned with the

specific functionality of interconnecting sensors, actuators

and processors. The networks in this context are character-

ised by a large number of nodes, mainly static, with highly

constrained energy resources. For example, protocols for

self-organisation in combination with energy-aware routing

and co-operative information processing have been pro-

posed [13]. The Sensit project [15] is investigating the

technology for building self-organising sensor networks

using reconfigurable smart sensor nodes. Networked

embedded sensor packs for critical applications are being

studied in the ESP project [16].

One fundamental point on ad hoc networking is self-

configurability. Some projects, such as the WIND project

[14], are developing middleware and protocols to enable

applications on networks of devices, sensors, and computers

to communicate with each other with minimal manual or a

priori configuration. It is also interesting to mention the

concept of Smart Spaces, where the environment adapts to

the needs of the information consumer while moving [17].

Auto-configurability can be used for providing network

interoperability. The use of software-radio aware architec-

tures for this purpose is studied [19].

An example of the use of ad hoc networking as a

complement instead of an alternative to cellular networks

has been studied for example [18]. In Fig. 1, we find an

illustration of how a mobile phone belonging to a cellular

system could communicate with neighbouring PDAs,

allowing the PDAs to use very low power while accessing

the cellular infrastructure.

The intrinsic differences between ad hoc networks and

fixed networks have been investigated in many contri-

butions. An example of the implication of having multi-hop

ad hoc communication is found [20].

An essential part of research in ad hoc networking is

performance evaluation. One important aspect therein is

mobility modelling. In cellular networks, for instance a

user’s mobility behaviour directly affects the signalling

Fig. 1. Ad hoc networking as a complement of cellular systems.

D. Remondo, I.G. Niemegeers / Computer Communications 26 (2003) 36–40 37

traffic needed for handover and location management, the

channel holding time and the call blocking and dropping

probability. The choice of the mobility model has a

significant effect on the obtained results. If the model is

unrealistic, invalid conclusions may be drawn. A categor-

isation and survey of mobility models used in current

wireless research can be found in [25].

5. Research issues

Before discussing specific research issues, it is important

to reflect on specific characteristics of ad hoc networks. We

can expect that most ad hoc networks will be formed by

different types of terminals, e.g. PDA-like devices, mobile

phones, sensors or desktop computers, with different

capabilities in terms of maximum transmission power,

energy availability, mobility patterns and QoS require-

ments. Therefore, ad hoc networks will, in general, be

heterogeneous in terms of terminals and offered services.

On the other hand, ad hoc networks will be connected to

external networks such as fixed infrastructures (e.g. the

Internet, a company’s Intranet or a home network) or

cellular systems. The interworking should occur when the

opportunity and the need arises. This is closely related to the

self-organising and self-managing properties of ad hoc

networks, that make these networks blend into the

environment of the person they are associated with. In

terms of energy and power, we have to consider not only

node heterogeneity in terms of transmission power and

energy availability, but also varying (possibly adaptive)

communication ranges, such as sleeping/active modes and

the existence of energy supplies. A point of attention is the

fact that information on the geographical location of (some

of) the terminals may be available. This can be a powerful

tool for improving network architecture, routing schemes

and location-aware services. Finally, we must be aware that

ad hoc networks raise new issues concerning security and

privacy.

Let us now briefly introduce some specific research areas

and issues that need to be addressed, without claiming to be

exhaustive.

5.1. Network layer issues

As mentioned above, ad hoc networks involve self-

configurability, interoperability and new networking con-

cepts such as mobile routers. This creates new challenges in

the networking layer.

There exists a consensus on implementing the network

layer of ad hoc networks with the Internet Protocol (IP) due

to its relative simplicity, flexibility and because of its

widespread use. Under this assumption, terminals will have

IP addresses. Since future ad hoc networks are likely to be

used in a pervasive computing context, large numbers of IP

devices will be involved. This will make the use of IPv6

mandatory due to its larger address space. In addition, the

address assignment mechanisms of IPv6 offer advantages

over IPv4, and the optional fields of IPv6 allow for the

implementation of novel network layer algorithms.

There has been quite a lot of work in routing. Most

contributions focus on networks with terminals that have

identical capabilities (homogeneous networks) and look at

the trade-off between rapidly adapting to the changing

network topology (because of mobility) and maintaining

signalling overhead low. However, there is a lot to be done

in this area, since ad hoc networks will be heterogeneous in

general and routing should be related to a variety of

parameters across protocol layers, such as energy avail-

ability, terminal mobility, terminal processing power, or

buffer sizes. Policy may also be a factor in routing in ad hoc

networks, e.g. some devices may not be trusted to fulfil

particular tasks such as routing. Other important parameters

are delay, bandwidth consumption, other QoS parameters

and fairness in the use of resources. Some researchers

propose active networking mechanisms in this context [31].

The basic idea of micro-mobility, which is to confine

mobility-associated signalling to a small geographical area,

can be useful in the context of relatively large ad hoc

networks with one or more access points to the Internet. A

specific problem in some ad hoc networks, e.g. PANs, is that

with respect to other networks and fixed infrastructures one

does not deal with mobile hosts but with mobile routers and

mobile networks. Some work in this direction is going on in

the IETF Working Group on Mobile IP.

The support of mobility encompasses horizontal and

vertical handovers between different domains and network

technologies. In relation to infrastructure networks, the

issue of paging, tracking and location updating may also

have to be reconsidered regarding some ad hoc networks,

e.g. PANs.

An important field of research is the support of QoS and

real-time traffic for some services across layers and during

handovers and reconfigurations.

Obviously, research on using ad hoc techniques for

enhancing fixed network infrastructures is still going on. An

example is described in [24], although this case does not

necessarily involve spontaneous auto-configurable

networking.

5.2. Medium access control (MAC) issues

Research within this area includes the classical aspects of

efficiency and fairness in the context of the heterogeneity of

devices and applications. In addition to this, it is important

to consider whether to have a centralised or distributed

MAC, in particular in the light of the heterogeneity of

devices. The possibility of having terminals with different

ranges and even adaptive ranges has an impact on the MAC

as well. MAC schemes should also look at the possibility of

allowing terminals to enter dormant states with the objective

of saving energy.

D. Remondo, I.G. Niemegeers / Computer Communications 26 (2003) 36–4038

5.3. Physical layer issues

We should explore enhancements of radio transmission

techniques to use the scarce spectrum more efficiently.

Examples are diversity techniques, smart antennas, beam

forming, adaptive modulation and multi-user reception.

This may include adaptive reconfiguration of radio inter-

faces, to allow devices to be always connected or select the

best radio interface for the environment and the application.

New radio technologies such as ultra wideband (UWB)

radio [23] may provide new physical layer functionality

such as geographical positioning. Another point of attention

is the coexistence of multiple radio interfaces, e.g. W-LANs

and Bluetooth.

5.4. Issues related to the need for and potential of self-

organisation

Self-organisation issues comprise service discovery,

including service availability beyond the first hop, the use

of addressing hierarchies and its relation with topology and

mobility, address assignment methods and mechanisms and

methods for the exploitation of location and context

awareness.

5.5. Administration, authentication and accounting (AAA)

aspects

Authentication and authorization become a very big issue

due to the wireless nature of the connections, which makes

eavesdropping easier.

The potentially automatic incorporation of foreign

devices (in particular without the user being aware of it)

in ad hoc networks introduces a high security risk.

Accounting becomes an issue because ad hoc networking

creates the opportunity to make use of resources belonging

to others, e.g. multi-hop access from a PAN to a fixed

network via somebody else’s node acting as a relay.

5.6. Applications issues

This is a very wide area of research. It is being addressed

in many projects at the middleware and application level in

the context of pervasive computing or ubiquitous comput-

ing. For example, how applications using ad hoc networks

can cope with the dynamics of link quality, disconnections

and topology changes.

5.7. Power and energy issues

Energy availability in wireless terminals is expected to

be an important bottleneck due to the relatively slow

progress in battery technology. Therefore, many aspects of

ad hoc networks, throughout the protocol stack, should

consider this. For instance, routing mechanisms could be

designed to balance power consumption. In addition, it must

be realized that battery life depends not only on the total

energy consumption, but also on the characteristics of the

power surges.

6. Conclusions

In this paper, we have given an overview of the state of

the WWRF discussions on the subject of ad hoc networking.

The ad hoc networking field is just opening up and offers

many perspectives beyond the work that has been done on

homogeneous ad hoc networks for communication among

people, ad hoc networks for areas where fixed networks are

not possible and for military applications.

New technical challenges have to be faced when ad hoc

networks become a communication substrate for pervasive

computing and for the enhancement of infrastructure

networks. Some of the tough challenges are how to deal

with the heterogeneity found at all levels, from the radio

level to the applications, the need for self-organization of

complex ad hoc networks, and the interaction between ad

hoc networks and infrastructure networks. The related

research issues have been briefly introduced in this paper.

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