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Telematics and Informatics 18 (2001) 109±131
www.elsevier.com/locate/tele
Mobile computing and ubiquitous networking:
concepts, technologies and challenges
Samuel Pierre *
Mobile Computing and Networking Research Laboratory (LARIM), Department of Electrical and
Computer Engineering, Ecole
Polytechnique de Montr
eal, C.P. 6079, Succ. Centre-Ville, Montr
eal, Qu
e,
Canada H3C 3A7
Abstract
With the rapidly increasing penetration of laptop computers, which are primarily used by
mobile users to access Internet services, support of Internet services in a mobile environment
become an increasing need. The opportunities emerging from these technologies give rise to
new paradigms such as mobile computing and ubiquitous networking. However, there are numerous challenges such as reliability and quality of service, infrastructure costs, energy eciency of mobile devices, among others. This paper analyzes concepts, technologies and
challenges related to these paradigms. The major challenges include mobility, disconnection
and scale, new information medium and new resource limitations. As a fundamental characteristics of mobile environments, the user can see the same computing environment regardless of his or her current location. The most exciting promises of mobile computing and
ubiquitous networking stay an entire new class of applications and potential new massive
markets combining personal computing and consumer electronics. Ó 2001 Elsevier Science
Ltd. All rights reserved.
Keywords: Mobile computing; Ubiquitous networking; Wireless Internet; Cellular system
1. Introduction
Mobile communication networks are made possible by the convergence of several
di€erent technologies, speci®cally computer networking protocols, wireless/mobile
communication systems, distributed computing and Internet (Danneels, 1998;
*
Tel.: +1-514-340-4711 ext. 4685; fax: +1-514-340-3240.
E-mail address: [email protected] (S. Pierre).
0736-5853/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 7 3 6 - 5 8 5 3 ( 0 0 ) 0 0 0 2 4 - 1
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S. Pierre / Telematics and Informatics 18 (2001) 109±131
Acronyms
ATM
BS
BSC
CDMA
CPU
DCS-1800
DECT
EDGE
ETSI
FDMA
GSA
GSM
HLR
IMT-2000
IP
IS
ITU
ITU-T
Kbps
kHz
LAN
MAN
Mbps
MHz
MSC
PC
PCS
PDA
PDC
PN
PSTN
RF
SS7
TCP
TDMA
TIA
UMTS
UPT
The singular and plural of an acronym are always spelled the same
Asynchronous Transfer Mode
Base Station
Base Station Controller
Code Division Multiple Access
Central Processing Unit
Digital Cellular System, 1800 Hz
Digital Enhanced Cordless Telecommunications
Enhanced Data rates for GSM Evolution
European Telecommunications Standards Institute
Frequency Division Multiple Access
Geographical Service Area
Global System for Mobile communication
Home Location Register
International Mobile Telecommunications 2000; 2000 indicates the
target availability date (year 2000) as well as the operational radio
frequency band (2000 MHz range) for the standard.
Internet Protocol
Interim Standard
International Telecommunication Union
ITU-Telecommunication standardization sector
Kilobit per second
kilohertz
Local Area Network
Metropolitan Area Network
Megabit per second
Megahertz
Mobile Switching Center
Personal Computer
Personal Communications System
Personal Digital Assistant
Personal Digital Cellular
Pseudo-random Noise
Public Switched Telephone Network
Radio Frequency
Signaling System 7
Transmission Control Protocol
Time Division Multiple Access
Telecommunications Industry Association
Universal Mobile Telecommunications System
Universal Personal Telecommunications
S. Pierre / Telematics and Informatics 18 (2001) 109±131
VLR
WAN
WAP
WML
WWW
3G
111
Visiting Location Register
Wide Area Network
Wireless Application Protocol
Wireless Markup Language
World Wide Web
Third-generation
Goodman, 2000). With the rapidly increasing penetration of laptop computers,
which are primarily used by mobile users to access Internet services like e-mail and
WWW, support of Internet services in a mobile environment become a growing
requirement. Mobile IP attempts to solve the key problem of developing a mechanism that allows IP nodes to change physical location without having to change IP
address, thereby o€ering the so-called nomadicity to Internet users.
As shown in Fig. 1, next-generation wireless technologies promise ubiquitous
networking and mobile computing on a large scale, with high-bandwidth data services and a wireless Internet (Fasbender and Reichert, 1999; Gibson, 1999; Negus et
al., 2000; Ojanpera and Prasad, 1998). However, there are still numerous challenges
such as reliability and quality of service, infrastructure costs, energy eciency of
mobile devices, among others. Although the commercial impacts of wireless tech-
Fig. 1. Some promises of ubiquitous networking and mobile computing.
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S. Pierre / Telematics and Informatics 18 (2001) 109±131
nologies has far been limited to cellular telephones, the business and technical
communities anticipate rapid growth in wireless data services. Almost daily, some
prominent company announces plans for a ``wireless e-commerce'' enhancement to
its business (Goodman, 2000).
Technology advances have made it conceivable to build and deploy dense wireless
networks of heterogeneous nodes collecting and disseminating wide ranges of environmental data. Applications of such sensor and monitoring networks include
smart homes equipped with security, identi®cation, and personalization systems;
intelligent assembly systems; warehouse inventory control; interactive learning toys;
and disaster mitigation. The opportunities emerging from this technology give rise to
new paradigms such as mobile computing and ubiquitous networking.
This paper analyzes concepts, technologies and challenges related to mobile
computing and networking. It is organized as follows. Section 2 de®nes some basic
concepts of cellular systems. Section 3 describes the evolution of wireless technologies which constitute the foundations of mobile computing and ubiquitous networking. Section 4 presents characterization and issues of mobile computing. Section
5 analyzes the economical and social impacts of these new paradigms. Section 6 gives
some concluding remarks.
2. Basic concepts of cellular systems
A cellular system is generally characterized as a high-capacity land mobile system
in which available frequency spectrum is partitioned into discrete channels assigned
in groups to geographic cells covering a cellular GSA. The discrete channels are
capable of being reused in di€erent cells within the service area (Pandya, 1999).
The principle of cellular systems is to divide a large geographic service area into
cells with diameters from 2 to 50 km, each of which is allocated a number of RF
channels (Rappaport, 1996). Transmitters in each adjacent cell operate on di€erent
frequencies to avoid interference. However, since transmit power and antenna height
in each cell are relatively low, cells that are suciently far apart can reuse the same
set of frequencies without causing cochannel interference. The theoretical coverage
range and capacity of a cellular system are therefore unlimited.
Generally, a ®xed amount of frequency spectrum is allocated to a cellular system
by the national regulator. Multiple access techniques are then deployed so that many
users can share the available spectrum in an ecient manner. The three basic multiple access methods currently in use in cellular systems are: FDMA, TDMA, and
CDMA. Figs. 2±4 illustrate these multiple access methods, where Fx refers to frequency slot x, Tx to time slot x, Cx to PN code x, and Ux to user number x (Pandya,
1999).
In the case of FDMA, users share the available spectrum in the frequency domain,
and a user is allocated a part of the frequency band called the trac channel. The
user's signal power is, therefore, concentrated in this relatively narrow band in the
S. Pierre / Telematics and Informatics 18 (2001) 109±131
113
Fig. 2. Basic principle of FDMA.
Fig. 3. Basic principle of TDMA.
Fig. 4. Basic principle of CDMA.
frequency domain, such that di€erent users can be assigned di€erent frequency
channels on a demand basis (Gibson, 1999). As shown in Fig. 2, interference from
adjacent channels is limited by the use of guard bands and bandpass ®lters that
maintain separation of signals associated with di€erent users.
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S. Pierre / Telematics and Informatics 18 (2001) 109±131
In TDMA techniques widely used in digital cellular systems, the available spectrum is partitioned into narrow frequency bands or frequency channels (as in
FDMA), which in turn are divided into a number of time slots (Pandya, 1999). An
individual user is assigned a time slot that permits access to the frequency channel for
the duration of the time slot, as shown in Fig. 3. In this context, the trac channel
consists of a time slot in a periodic train of time slots that make up a frame. In case
of the North American digital cellular standard IS-136, each frequency channel (30
kHz) is divided into three time slots, whereas for the European digital standard
GSM, each frequency channel (200 kHz) is divided into eight time slots (full rate)
(Sollenberger et al., 1999). In the case of TDMA systems, guard bands are needed
both between frequency channels and between time slots.
TDMA is the multiple access technique of choice for several digital cellular and
PCS systems. It is usually combined with FDMA, because di€erent carrier frequencies are used in di€erent cells. Frequencies are only reused in cells suciently
distant in order to minimize interference (Falconer et al., 1995).
In the case of CDMA, the spread spectrum technique is used. In fact, a spreading
code, also called a PN code, is used to allow multiple users to share a block of
frequency spectrum. In CDMA cellular systems (e.g., IS-95 in the United States) that
use direct sequence spread spectrum techniques, the digital information from an
individual user is modulated by means of a unique PN code assigned to each user.
All the PN code modulated signals from di€erent users are then transmitted over the
entire CDMA frequency channel (e.g., 1.23 MHz in case of IS-95). At the receiving
end, the desired signal is recovered by despreading the signal with a copy of the
spreading sequence or PN code for the individual user in the receiving correlator. All
the other signals (belonging to other users), whose PN codes do not match with that
of the desired signals, are not despread and, as a result, they are considered as a noise
by the correlator. As shown in Fig. 4, since the signals in the case of CDMA utilize
the entire allocated block of spectrum, no guard bands of any kind are necessary
within the allocated block (Pandya, 1999).
De®nition of channels for assignment to individual cells within a cellular system is
straightforward in the case of systems based on FDMA and TDMA methods. By
contrast, systems based on the spread spectrum CDMA technique require a di€erent
view of what constitutes a radio channel in this context.
3. Evolution of wireless technologies for mobility support
The analog cellular mobile systems fall in the category of ®rst-generation mobile
systems, the digital cellular, low-power wireless, and PCS are rather perceived as
second-generation systems. The original ``®rst-generation'' cellular systems used
analog frequency modulation to transmit voice signals. Most of today's cellular
phones use ``second-generation'' technology that conveys speech in digital format at
bit rates that are around 10 kbps.
S. Pierre / Telematics and Informatics 18 (2001) 109±131
115
3.1. General architecture
The general architecture of next-generation wireless networks will include and
extend existing infrastructures such as cellular architecture, wireless LAN, ®xed
networks (LAN, MAN, WAN, Internet, etc.), as well as specialized service oriented
architectures including radio and satellite services.
As shown in Fig. 5, a typical cellular wireless network infrastructure consists of a
number of components such as:
· A BS which serves hundred of mobile users in a given area (cell) by allocating resources that allow users to make new calls or continue their calls if they move to
the cell.
· A BSC which provides switching support for several neighboring BS, serving
thousands of users; links between BSC and BS usually have been wireline or ®berline, but they also can be wireless microwave links.
· An MSC which is a larger switch that is capable of serving more than 100,000 users; links between MSC and BSC are also increasingly wireless.
Fig. 5. A typical cellular wireless network infrastructure.
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S. Pierre / Telematics and Informatics 18 (2001) 109±131
· HLR and VLR which keep track of users who are permanently registered or who
are just visiting the area, respectively.
· SS7 which performs the call setup between MSC and the PSTN.
· High-capacity trunks (T1 or T3) which carry calls between MSC and the PSTN.
3.2. Digital cellular systems
The ®rst digital cellular system speci®cation was released in 1990 by ETSI for the
GSM system. The GSM, DCS 1800 and DECT systems developed by ETSI form the
basis for mobile and personal communication services, not only in Europe but in
many other parts of the world including North America. The number of GSM
subscribers worldwide exceeds 100 million and is growing rapidly. In the United
States, the implementation of digital cellular standards developed by TIA is progressing at a rapid rate. These standards are based on time and code division multiple access technologies. The intent of the emerging PCS standards in the United
States is to provide a combination of terminal mobility, personal mobility, and
service portability to the end users utilizing a range of wireless technologies and
network capabilities (Kim and Litman, 1999).
A digital cellular system called PDC was developed in Japan and is in full commercial operation in that country. To a large extent, the speci®cations for these
second-generation cellular systems are being developed to meet business and regulatory requirements in speci®c countries and/or regions, leading to incompatible
systems that are unable to provide global mobility (Pandya, 1999).
All the second-generation digital cellular system designs were optimized for telephone trac. The initial support of data services on cellular networks was essentially
restricted to dial-up modem-based services. The user data rates for these services
were further constrained by the interference-prone nature of the radio environment.
If second-generation cellular mobile systems are expected to enhance their datahandling capabilities in order to support the emerging high-speed data and multimedia market, the radio access part must be modi®ed. Initial e€orts in this direction
have been under way for some time in Europe, and the resulting EDGE proposal has
been developed. Support of EDGE in GSM is considered to be a key point in its
evolution to the third-generation mobile system (UMTS/IMT-2000).
The EDGE concept proposes to use the 200 kHz bandwidth in GSM with more
high-level modulation schemes and a range of ecient encoding methods that can be
combined to provide higher bit rates across the radio interface. The development of
the EDGE speci®cation is under way in ETSI SMG 2, and various high-level
modulation schemes and encoding methods are under study and analysis. The
EDGE concept, which applies to both circuit mode and packet mode data, is suf®ciently generic for application to other digital cellular systems. For example, it has
been adopted by the TIA in the United States for evolving the data capabilities of the
IS-136 TDMA system toward third-generation capabilities.
S. Pierre / Telematics and Informatics 18 (2001) 109±131
117
3.3. Toward the third-generation systems
E€orts are deployed at the international as well as the regional/national levels to
design and implement third-generation mobile telecommunication systems. In fact,
international or global standards are needed; they aim not only at ensuring seamless
global mobility and service delivery but also integrating the wireline and wireless
network to provide telecommunication services transparently to the users. These
global standards must be ¯exible enough to meet local needs and allow current regional/national systems to evolve smoothly toward the third-generation system.
The international standard proposed by ITU-T for wireless access to worldwide
telecommunication infrastructure is known as IMT-2000. IMT-2000 is intended to
form the basis for 3G wireless systems, which will consolidate today's diverse and
incompatible mobile environments into a seamless radio and network infrastructure
capable of o€ering a wide range of telecommunication services on a global scale. It
will provide capabilities constituting signi®cant improvements over the current
mobile systems, especially in terms of global mobility for the users and support of
services like high-speed data, multimedia and Internet. Since these IMT-2000 capabilities will to a large extent be achieved by evolving existing wireline and wireless
networks, IMT-2000 will be a family of systems rather than a single, monolithic
network.
3.4. Choice of an access standard
The three technologies that seem to be gaining momentum in digital cellular and
PCS industry are: CDMA, TDMA, and GSM (Kim and Litman, 1999). Picking
the ultimate winner technology can yield advantages in lower cost from economies
of scale and make it easier to enter into roaming agreements. On the other hand,
picking the wrong technology can leave the provider and customer stranded. Because such a decision is critical and strategic, cellular or PCS providers tend to
enter into nationwide strategic alliances, and the strategic alliance may select a
technological standard (Westerhold, 1996). In this context, strategic groups can be
de®ned in terms of choice of a technological standard in the wireless telephone
industry.
In the United States, there has been a raging debate among CDMA, TDMA and
GSM proponents to dominate the next generation of wireless services, but it is likely
that multiple standards will co-exist. This is because the US government supports
multiple standards for third-generation wireless services and the ITU has recently
given up the long-sought goal of a single, global standard that would apply around
the world for delivering ``mobile multimedia'' services. As computing becomes increasingly mobile, the limitations of third-generation cellular telephony and the
wireless applications protocol become increasingly apparent.
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4. Characterization and issues of mobile computing
Advances in wireless networking technologies and portable information appliances have engendered a new paradigm of computing, called mobile computing.
According to this concept, users who carry portable devices have access to information services through a shared infrastructure, regardless of their physical location
or movement behavior. Such a new environment introduces new technical challenges
in the area of information access. Traditional techniques for information access are
based on the assumptions that the location of hosts in distributed systems does not
change during the computation. In a mobile environment, these assumptions are
rarely valid or appropriate.
4.1. Mobility characterization
The need for mobility is a fundamental market factor that is driving the evolution
of telecommunications networks. In telecommunications terms, mobility can be
de®ned as the ability to access all of the services that one would normally have in a
®xed wireline environment such as a home or oce, from anywhere (Jing et al., 1999;
Fasbender and Reichert, 1999). Examples include the ability to have a telephone
conversation from your car, while on the move, or at the beach. More complex
examples would be the possibility to be reached via your traditional telephone
number or IP address anywhere in the world. Other examples are cellular roaming or
the ability to receive all of your voice, email and fax messages while travelling in a
foreign country.
4.1.1. In¯uence of mobility on network infrastructure
In third-generation systems, mainly due to the huge number of mobile users in
conjunction with the small cell size, the in¯uence of mobility on the network performance is strengthened. In particular, the accuracy of mobility models becomes
essential for the evaluation of system design alternatives and network implementation cost issues (Markoulidakis et al., 1997).
In a mobile environment, the location of devices is not know a priori and call
routing in general implies mobility management procedures. The problems which
arise from subscriber mobility are solved in such a way, that both a certain degree of
mobility and a sucient quality of the aspired services are achieved.
Mobile computing is distinguished from classical, ®xed-connection computing due
to the following elements: (1) the mobility of nomadic users and their computers; (2)
the mobile resource constraints such as limited wireless bandwidth and limited
battery life. The mobility of nomadic users implies that the users might connect from
di€erent access points through wireless links and might want to stay connected while
on the move, despite possible intermittent disconnections. Wireless links are relatively unreliable and currently are 2±3 orders of magnitude slower than wireline
networks. Moreover, mobile hosts powered by batteries su€er from limited battery
S. Pierre / Telematics and Informatics 18 (2001) 109±131
119
life constraints. These limitations and constraints leave much work to be done before
mobile computing is fully operational This remains true despite the recent advances
in wireless data communication networks and hand-held device technologies (Jing et
al., 1999).
4.1.2. Universal personal communication concepts
A mobile computing infrastructure should support di€erent wireless and wireline
communications devices optimized for their speci®c environment. As a result, a
person is able to initiate or receive information anywhere, at any time. The concepts enabling the provision of universal personal communications include terminal
mobility provided by wireless access, personal mobility based on personal numbers,
and service portability through the use of Intelligent Network capabilities (Pandya,
1999).
4.1.2.1. Terminal mobility. Terminal mobility systems are characterized by their
ability to locate and identify a mobile terminal as it moves, and to allow the mobile
terminal to access telecommunication services from any location. It is associated
with wireless access and requires that the user carry a wireless terminal while being
within a radio coverage area.
Terminal mobility places some important requirements on the network. The ®rst
one is giving subscribers the ability to roam from one radio coverage area to another.
This implies that the network should either register continuously the movements of
the terminal or be able to ®nd the terminal as soon as an incoming call has to be
delivered. Another feature is the ability to continue an existing call from one radio
coverage area to another, i.e., perform handover. Managing the radio spectrum,
providing uniform coverage, reducing interference problems and accessing user
pro®les over the network are important network considerations associated with
terminal mobility (Jabbari et al., 1995).
Based on a common radio access, terminal mobility has been supported in conventional mobile communications networks by allowing users to carry their own
portable terminals. Ordinary terminal mobility still has a limitation in the sense that
the directory number is associated with a particular terminal, rather than a subscriber. The network routes the call to that terminal without regard to whether or
not it is most convenient to the subscriber. Another consideration is that if a subscriber with terminal mobility owns more than one device, these devices have no
relation to one another, i.e., the caller must dial di€erent numbers to reach these
devices. The subscriber is charged separately for each device and features may or
may not work the same for the di€erent devices.
4.1.2.2. Personal mobility. Personal mobility, on the other hand, is centered around a
user carrying a personal subscription identity (personal telecommunication number)
rather than a terminal. With an identity card containing a personal telecommuni-
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S. Pierre / Telematics and Informatics 18 (2001) 109±131
cation number, a user can access services from any terminal, whether it is in ®xed or
mobile communications network. When a caller dials this number, it is the network's
responsibility to route the call to the terminal of the subscriber's choice. The subscriber could make this choice known to the network by the use of a personal
identity module, based on time-of-day/day-of-week, or the network could make
attempts to deliver the call at more than one terminal.
Personal mobility has impacts on both mobile communications networks and
®xed networks. In the mobile network, some personal mobility-related services such
as universal mobile terminal rental (several users share a mobile terminal rental, each
of the users as his own) and multiple mobile terminals (a subscriber uses several
mobile terminal as his own) are foreseen in the future.
Associated with this type of reachability is the need to give subscribers the ability
to screen incoming calls. Static screening is based on prede®ned lists, whereas dynamic screeningis based on caller identi®cation or subscriber's convenience. Another
identi®ed requirement for personal mobility is a degree of subscriber's control over
the service pro®le. For this purpose, subscribers need a user-friendly interface to
interact with the service pro®le database and modify or customize their own service
pro®les. The ability to change static-call screening criteria, time-of-day routing, add
or delete new terminals, change calling privileges, and activate or disable features fall
in this category (Jabbari et al., 1995).
4.1.2.3. Service portability. Personal mobility systems are characterized by their
ability to identify end users as they move, and to allow them to originate and receive
calls, and to access subscribed services on any terminal, in any location. The
emerging implementations and integration of intelligent network capabilities within
®xed and mobile networks provide a dynamic relationship between a terminal and a
user. Service portability refers to the capability of a network to provide subscribed
services at the terminal or location designated by the user. The exact services that a
user can evoke depend on the capability of both the terminal and the network in
service.
The term Intelligent Network describes an architectural concept that can be applied to all communications networks. It is aimed at facilitating the introduction of
new services by decoupling the functions required to support call and connection
control from those required to support service control, allowing both sets of functions to be placed on di€erent physical platforms.
Personal communications require addressing the problem of mobility and service
portability for subscribers and terminals in order to provide reachability and access
through a single directory number. Terminal mobility is accommodated using a
portable terminal through a wireless access to a ®xed base station. Personal mobility
can be accommodated either through a wired access or through a wireless access
using a portable identity card. Personal mobility services fall under what is referred
to as UPT by the ITU-T recommendations (Jabbari et al., 1995).
S. Pierre / Telematics and Informatics 18 (2001) 109±131
121
4.1.3. Implementation approaches
To implement mobile computing applications, two complementary approaches or
technologies have been developed: a third-generation cellular radio transmission
technology (3G), and a wireless application protocol (WAP). 3G focuses on highdata-rate communications with portable computers. Data rates cited in technical
standards are 384 kbps for devices moving outdoors at high speed ± in cars or trains,
for example, ± and 2 Mbps for slowly moving devices in or near suitably equipped
buildings. At these rates, the proponents of 3G expect people to use portable
computers for many of the exotic information services they enjoy at home and work
(Goodman, 2000).
Unfortunately, a closer look at 3G technology reveals ¯aws in these glamorous
pictures. These ¯aws arise because 3G addresses only one of the barriers to the
adoption of wireless data, say transmission speed. The problems of cost and power
consumption remain. 3G's basic problem is that it adds only incrementally to the
radio transmission technology that has supported the remarkable commercial success of cellular telephones. Further, the rate adaptation built into 3G, compromises
the ``anytime, anywhere'' theme that has motivated advances in cellular telephony.
Furthermore, in those places and times when high-data-rates can be attained, thirdgeneration communications will be power hungry and costly.
WAP's underlying assumptions di€er fundamentally from 3G's. Rather than
transmitting a Web content and other Internet applications through the air, WAP
recognizes that cellular phones are not PC, and that many information services
developed for PC are of little use to people moving about with small devices.
Therefore, WAP focuses on applications tailored to the capabilities of cell phones
and their users' needs. By taking into account the constraints of mobile radio
channels, WAP uses various compression techniques to reduce the number of bits
transmitted through air.
With respect to information delivered to the phones, WAP uses WML, to display
text and icons on a telephone screen. Instead of point-and-click navigation through
hypertext, people use the phone's small keypad to send information upstream. Thus,
WAP creates an information web for cellular phones, distinct from the PC-centric
Web. WAP functions well in a low-data-rate, low-power environment of present
cellular systems.
On the other hand, the wireless web created for cellular phones su€ers from several
de®ciencies relative to the Internet. In contrast to the Web's organic evolution,
cellular operating companies manage the wireless Web's development and will
control the material customers can access through it. The kind of spontaneous
creativity seen during the Web's expansion will likely be excluded from the WAP's
world (Goodman, 2000).
On the technical side, WML is primitive relative to other languages that incorporate modern software engineering features designed to promote reliability.
Moreover, its capabilities are too closely matched to today's cellular telephone
technology. Consequently, we have reason to be concerned that WAP in general ±
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and WML in particular ± could be insuciently powerful to deliver a wide range of
services to future-generation terminals. WAP critics assert that the technology will
serve as a stopgap for a few years, until 3G radios, proxies, and advanced terminals
bring the entire Web to small portable devices. WAP proponents claim that the
technology is suciently scalable and extensible to do the job inde®nitely.
WAP and 3G products reaching the market in the next few years could provide
only partial solutions to the present shortcomings of wireless data. Their de®ciencies
will become increasingly acute relative to the requirements of advanced information
services. In addition to wireless communications that link computers, telephones,
and PDA, the computer industry anticipates a future with billions of small microprocessors that contain wireless modems for exchanging information with each other
and with backbone networks. The radio communications techniques used by these
tiny, inexpensive devices must radically di€er from the 3G cell phones on steroids.
4.2. Mobile hardware and devices
The issues in mobile computing are di€erent from conventional distributed systems due to the unique characteristics of mobile hardware and devices. One of the
important characteristics of mobile devices is the low-bandwidth and high-latency,
communications with the rest of the system. Other drawbacks of mobile devices
include limited battery capacity, small screen sizes and the unreliability of the environment in which they operate. As mobile computers get more powerful in terms
of CPU speed and memory size, there is a need to exploit that power to minimize the
response time to users.
One class of mobile devices is PDA. These devices are small enough to be carried
around at all times. They have a screen large enough to be carried around at all times
and allow user interaction for extended periods of time. Among other things, PDA
are used for browsing and updating databases. Some examples of this type of use are
banking transactions, schedule coordination and airline information.
When two entities indirectly communicate with each other through two separate
TCP connections, an entity called a proxy mediates the communications between the
two connections, and controls the ¯ow of data between the communicating parties.
The proxy decides whether the parties can communicate, and if so, what is communicated. A proxy can both restrict and enhance communications, according to
two operation modes: control mode and forwarding mode. In control mode, the proxy
processes either out-of-band or in-band control information. Once the control
functions have been completed, the proxy switches into forwarding mode to transfer
data between the connections.
Processes commonly communicate with each other indirectly through a proxy.
This happens; for example, in a ®rewall where proxy mediates the ¯ow of information between a TCP connection to a not-trusted external entity and a TCP
connection to a trusted local entity. The term TCP forwarding is used to denote the
S. Pierre / Telematics and Informatics 18 (2001) 109±131
123
general pattern of indirect communication over a pair of TCP connections via a
proxy (Spatscheck et al., 2000).
Proxies can be broadly classi®ed into four categories, depending on the degree of
control processing they perform. The ®rst class of proxies performs a minimum of
control processing; they typically perform level-4 routing based on IP addresses and
port numbers. They are in control mode only during connection setup, after which,
they switch to forwarding mode for the duration of the connection. An FTP proxy is
an example: it processes an FTP request in control mode on the control connection,
sets up a data connection between the two computers, and switches to forwarding
mode on the data connection until it is closed. The control connection remains in
control mode to process subsequent FTP requests.
The second class of proxies performs more control processing because they authenticate the user or request and base routing decisions on either the result of the
authentication or control information passed in the TCP connection. A Telnet proxy
is a member of this class.
The third class of proxies remains in control mode for all data transferred in one
direction, but switch to forwarding mode for data transferred in the other. An example is an HTTP proxy that processes the HTTP requests (control information)
sent by clients, but simply forwards the data returned by the HTTP server.
The fourth class remains in control mode and continuously monitors data passed
in both directions. This might be the case for a proxy that allows users on a protected
network to access HTTP servers on the Internet. The proxy could ®lter outgoing
accesses to restrict the servers that can be reached, and ®lter incoming access responses to remove untrusted Java code (Spatscheck et al., 2000).
TCP forwarding has many uses, including functions such as a network ®rewall, an
HTTP proxy, and a mobile computing system. In the area of mobile computing,
proxies are used to improve the performance of mobile hosts operating across
wireless links by separating TCP connections into two connections: one covering the
wireless link and another covering the wired network. The performance enhancement can either be simply an improvement caused by the separation of ¯ow control
on two di€erent types of network, or it can rely on transformation or ®ltering of
data. In the last case, the proxy can reduce the resolution on graphics sent to the
mobile host over a low-capacity link and remove all video clips from e-mail. When
the mobile host is connected to a wired network, the proxy merely relays data in the
forwarding mode, but cannot be removed from the communication path due to the
presence of bipartite TCP connections.
Another use of proxies is to allow a mobile host to change its point of attachment to
the network without jeopardizing any open connections. In this case, the proxy would
operate in forwarding mode when the mobile host is connected, but would switch to
control mode both when the mobile host connects and disconnects. This would allow
the mobile host to terminate its TCP connections, move to a new location with a new
IP address, and establish a new set of TCP connections to the proxy, without a€ecting
the peer hosts on the other side of the proxy (Spatscheck et al., 2000).
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4.3. Data management issues
In general, techniques used for distributed data management have been based on
the assumption that the location of hosts in the distributed system as well as the
connections among hosts do not change. In mobile computing, such assumptions are
no longer valid, since distributed data management is fundamentally changed due to
mobility.
In managing data for mobile systems, there exist two broad classes of issues: those
that arise because the mobile devices move, and those that arise because of the
unique way mobile devices operate, that is, over slow wireless networks and in
disconnected mode. Other relevant questions are how to organize and query location
dependent data, and how to handle disconnection. One of the big impacts of mobility on data management is that the algorithms, as far as possible, have to shift the
costs of computation and communication to the static (strongly connected) portion
of the network. Then the mobility of users a€ects data placement and may require
the dynamic creation of servers during the execution of a transaction.
Maintaining data on mobile hosts implies additional search cost to access the data,
consumption of scarce energy resources, and even unavailability of data due to
frequent disconnection. To overcome these drawbacks, the approach to designing
algorithms, to managing distributed mobile data should be, to the extent possible, to
shift the computation and communication costs of an algorithm to the static portion
of the network. Then the number of operations performed at the mobile hosts and
thereby the power consumption is minimized
4.4. Reliability, survivability and security issues
Most mobile applications involve connections through ®xed networks. A connection usually consists of the concatenation of ®xed and mobile network infrastructure circuits. As a result, any consideration of reliability must consider the
reliability of the entire end-to-end connection.
A network's ability to avoid or cope with failure is measured in three ways:
· Reliability is a network's ability to perform a designated set of functions under
certain conditions for speci®ed operational times.
· Availability is a network's ability to perform its functions at any given instant under certain conditions; average availability is a function of how often something
fails and what it takes to make it recover from a failure.
· Survivability is a network's ability to perform its designated set of functions given
network infrastructure component failures, resulting in a service outage, which
can be described by the number of services a€ected, the number of subscribers affected, and the duration of the outage.
Reliability, availability, and survivability have long been important areas of research
for wireline networks. Unfortunately, similar attention has not been directed toward
S. Pierre / Telematics and Informatics 18 (2001) 109±131
125
wireless and mobile networks, even though they are more prone to failure and loose
access (Boyd and Mathuria, 2000).
Several con®gurations and architectures can improve survivability. Adding redundancy is one way to enhance reliability through the end-to-end connection.
Another way to improve survivability is to use multifunction/multimode devices in
which a single terminal o€ers multiple interfaces (LAN adapter, satellite adapter,
cordless/®xed-radio-access adapter). This architecture provides overlapped services
to ensure wireless coverage in the case of network, link, or switch failure. It may also
increase the e€ective coverage area.
Yet another way to improve survivability and hide network failure is to deploy an
overlay network. Based on availability, a user accesses an overlay network consisting
of several universal access points, which choose a wireless network for the user,
speci®ed quality of service, and user-speci®ed choices. A universal access point
performs protocol and frequency translation, as well as content adaptation.
All of these techniques involve capital investment. It is up to each carrier to
evaluate the trade-o€ between the increased expenditures and customer satisfaction.
Such a dicult decision will increasingly become necessary in the future as dependence on wireless grows.
In addition to directing some attention to designing survivable wireless and mobile
networks, developers must also keep in mind that increasingly pervasive and demanding services will further escalate the importance of reliability and survivability
(Snow et al., 2000).
Security is a critical issue in mobile radio applications, both for the users and
providers of such systems. Mobile applications have special requirements and vulnerabilities, and are therefore of special concern. The problem is to use appropriate
cryptographic algorithms to provide the required security services. In other words,
we need to design protocols for authentication and key management.
The problem of designing correct protocols for authentication and key management is dicult to solve in any environment (Boyd and Mathuria, 2000). In mobile
environments, the extra constraints and requirements make this problem all more
hard. In fact, a variety of protocols speci®cally designed for use in mobile applications has been proposed in recent years (Aziz and Die, 1994; Beller et al., 1991,
1992, 1993; Carlsen, 1994; Mitchell, 1995). The wide diversity of these proposals,
together with the subtlety required for analyzing security protocols, make it hard for
designers of mobile applications to compare the various protocols and choose the
one best suited for their needs.
Typically, authentication protocols must include verifying that the identity of
some party involved is the same as that claimed, and establishing a session key for
use, in conjunction with chosen cryptographic algorithms to secure the subsequent
session. Due to the speci®c nature of mobile environments, additional factors such as
heterogeneous communications path, user roaming and computational constraints
must be taken into account. In fact, the communications channel is split into a
number of parts, one of which (the radio link) is particularly vulnerable to attack.
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Furthermore, the mobile station is allowed to roam freely. As well as making key
distribution more dicult, this means that information about the mobile stations'
location may be valuable to an adversary. Finally, the mobile station is computationally limited in comparison with typical communication devices. In particular,
there is an asymmetry between the computational power of the mobile and the base
station (Boyd and Mathuria, 2000).
On the other hand, mobile users employ resources, including software, at various
locations. These resources may be provided by di€erent service providers. Thus, the
concept of trust needs to be developed to allow mobile clients to use resources of
di€erent servers at di€erent locations.
4.5. Mobile applications and services
The explosive growth of mobile/wireless communication systems, such as PCS, as
well as ultra-portable intelligent devices, such as palmtop computers and PDA, is
leading to a variety of new mobile applications and services. The goal is to make
services such as access to web, corporate Intranet or databases, electronic mails and
teleconferencing universal, accessible from anywhere, anytime. To support this
universal service availability e€ectively, the networks and end-systems require ¯exible and ecient service-enabling platform. These networks should include the
functions of accessing and sharing various types of resources and provide generic
support for service creation, service delivery and service management (Mukherjee et
al., 2000).
One can distinguish two types of mobile applications: horizontal applications
which are domain independent, and vertical applications which are written for a
speci®c application domain. Horizontal mobile applications includes information
services such as local yellow pages possibly extended with online information such as
movies currently playing at local theaters or merchandises on sale at the local supermarket. Location will play a signi®cant role in selecting relevant information.
Such services will require access to databases from anywhere and at any time.
A great number of vertical mobile applications already exists, they are built for
niche markets that respond to the very speci®c needs of a mobile work force. Field
technicians who require access to manuals while on a repair assignment, ¯eet
management applications (localization and dispatch of trucks) and tracking packages for express mail providers, are some of these vertical markets.
Another existing application of mobile computing is the so-called active badge
technology, where infrared communication is used for locating employees and redirecting voice mail and data. In this way, needed information can follow the moves
of the recipients. All these applications di€er from traditional numerical and computationally intensive applications which are typically not run on small palmtops but
rather on the static hosts or, quite possibly, on the more powerful laptop computers.
S. Pierre / Telematics and Informatics 18 (2001) 109±131
127
5. Economical and social considerations
The world is becoming more dependent on wireless and mobile services, but the
ability of wireless network infrastructures to handle the growing demand is questionable. As wireless and mobile services grow, weaknesses in network infrastructures become clearer. Failures not only a€ect current voice and data use but could
also limit emerging wireless applications such as e-commerce and high-bandwidth
Internet access. As wireless and mobile systems play greater roles in emergency response, including 911 and enhanced 911 services, network failures take on life-ordeath signi®cance (Snow et al., 2000).
New wireless and ®ber optic technologies are greatly improving the means of
access to modern communications in both the developed and developing worlds. The
convergence of wireless communications and the Internet provides limitless access to
information in all forms and extraordinary opportunity. Motorola (Tooker, 2000)
estimates that there could be a billion wireless phone users by 2002, up from 470
million wireless and 260 million Internet users in 1999, and that the industry will ship
a billion devices with wireless Internet access capability by 2003.
The business opportunities that spring from the wireless Internet revolution includes the delivery of public services, such as education and health care to a much
broader population. The delivery of information is the most powerful tool in
building a knowledge-based economy. As the basis of the educational system, it
creates the trained workforce that is essential for alleviating poverty and succeeding
in a global knowledge economy. The wireless Internet promises access to information without restrictions of time or location. For the ®rst time, it sets the stage for
universal access and closing the gap between ``information haves and information
have-nots'' (Tooker, 2000).
In the early 1980s, only one in ®ve people in the entire world had access to a
telephone. Since then, the convergence of telecommunications and computing has
had a huge impact in the developed world, but the gap between information poor
and information rich has actually increased, as shown in Table 1. In 1982, in Asia,
for example, over half the people have never made a phone call, despite the growth in
Table 1
Technology input indicators 1997±1999a
a
Mobile phones per 1000 persons
1997
1998
1999
1997±1999
OECD countries
Middle East & North Africa
Sub-Saharan Africa
Latin America & Caribbean
Eastern Europe & Central Asia
Asia Paci®c
195
20
10
25
13
12
268
28
15
41
27
20
332
40
19
66
45
31
+70%
+100%
+90%
+164%
+246%
+158%
Source: Pyramid Research.
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the number of telecom users in the developed economies of the region. China's record of installing 20 million telephone lines a year is impressive, but it only keeps up
with China's population growth. Investment has been concentrated in densely
populated urban areas where an early revenue stream can be achieved and the existing network operates most eciently (Tooker, 2000).
In much of the developing world, the existing telecommunications network is
inadequate to support the deployment of high-speed data Internet applications. In
some cases, the appropriate regulatory environment does not exist to allow for the
introduction of wireless Internet access in an expeditious and cost ecient manner.
Since the wireless Internet is inherently borderless, nations will need to work together and achieve a new level of cooperation among private companies, governments and ®nancing institutions. This is needed to assist developing countries in
achieving universal access by helping them develop strategies and long-term visions,
providing technical assistance to build strong competent regulatory frameworks, and
building partnerships between the public and private sectors to invest where needed.
The convergence of the Internet and wireless technologies presents an opportunity to
transform the nature of communications, as we know it today. According to Tooker
(2000), properly managed, this convergence can enhance commerce, education, and
health care and bring poor, rural economies into the global marketplace for the ®rst
time.
6. Conclusions
With the convergence of computing and communications, today, access is no
longer de®ned by a dial tone, but by a much broader vision of the digital future.
Universal access and Internet access have become synonymous. The Internet's unprecedented popularity has profoundly a€ected society, commerce, politics and the
media, and is widely recognized as the fundamental building block for bringing basic
telephony and data services to the peoples of the world.
As mobile and personal communication services and networks evolve toward
providing seamless global roaming and improved quality of service to their users, the
role of network aspects such as numbering, identities and quality of service will
become increasingly important. Well-de®ned standards in these areas, as well as
network performance for present and future mobile and personal communications
networks, will need to be addressed.
To support mobility management functions in mobile communication networks,
and to provide national and international roaming, well-de®ned, standardized subscriber/terminal numbers and identities are required. Some form of station (walkstation) equipment identities is also needed to ensure that service can be denied to
non-type-approved or fraudulent terminals. ITU-T plays a key role in providing
these standards and is currently revising existing standards and developing new ones
to support a range of terminal and personal mobility services as wireless networks
S. Pierre / Telematics and Informatics 18 (2001) 109±131
129
evolve into the new century. An appreciation of the role of numbering and identities
in mobility management, international roaming, call delivery, billing and charging is
important in understanding the operation of mobile and personal communication
networks.
Mobile computing brings about a new style of computing. Due to battery power
restrictions, the mobile units are frequently disconnected (powered o€). For instance,
reading and sending e-mail, or querying local databases are separated by substantial
periods of disconnection.
As a result, the major challenges which arise in mobile computing can be grouped
into the two categories: mobility, disconnection and scale; new information medium
and new resource limitations. Mobility is a behavior that has e€ects on both ®xed
and wireless networks. The main distinction between disconnection and failure is its
elective nature, in that a disconnection can be treated as a planned failure which can
be anticipated and prepared for. Scale refers to the massive size of the potential set of
users, i.e., massive distribution of services and their organization. On the other hand,
the wireless medium provides a powerful new method of disseminating information
to a large number of users. New access methods and new data organization paradigms have to be developed both for providers of broadcast information and recipients. Also, limited bandwidth wireless connection and battery power limitations
of the mobile hosts are new resource limitations which will substantially a€ect data
management.
In general, mobile computers are equipped with a wireless connection to the ®xed
part of the network and, possibly, to other mobile computers. The resulting mobile
or nomadic computing environment no longer requires a user to maintain a ®xed
position in the network and enables almost unrestricted user mobility (Imielinski and
Korth, 1996). Thus, the user sees the same computing environment regardless of his
or her current location. According to Imielinski and Korth (1996), the most exciting
promises of mobile computing and ubiquitous networking stay an entire new class of
applications and potential new massive markets combining personal computing and
consumer electronics.
On the other hand, by allowing users to remain connected regardless of their location, such mobile environments can also stimulate more collaborative forms of
computing. All these advances and promises should be shared by all social classes in
each country as well as by all countries. In other words, we need a mechanism that
enables the rich and poor alike to partake in the bene®ts and promises of ubiquitous
networking and mobile computing.
Acknowledgements
This work was supported in part by the Natural Science and Engineering Research
Council (NSERC) of Canada under grants 140264-98 and 115877.
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S. Pierre / Telematics and Informatics 18 (2001) 109±131
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