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 eciency 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 dierent 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 - 1 110 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 oering 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 eciency of mobile devices, among others. Although the commercial impacts of wireless tech- Fig. 1. Some promises of ubiquitous networking and mobile computing. 112 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 dierent 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 dierent frequencies to avoid interference. However, since transmit power and antenna height in each cell are relatively low, cells that are suciently 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 ecient 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 trac 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 dierent users can be assigned dierent 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 dierent users. 114 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 trac 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 dierent carrier frequencies are used in dierent cells. Frequencies are only reused in cells suciently 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 dierent 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 dierent 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. 116 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 trac. 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 eorts 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 ecient 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 Eorts 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 oering 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. 118 S. Pierre / Telematics and Informatics 18 (2001) 109±131 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 oce, 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 sucient 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 dierent 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 suer 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 dierent 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). 22.214.171.124. 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 dierent numbers to reach these devices. The subscriber is charged separately for each device and features may or may not work the same for the dierent devices. 126.96.36.199. 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- 120 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). 188.8.131.52. 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 dierent 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 dier 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 suers 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 ± 122 S. Pierre / Telematics and Informatics 18 (2001) 109±131 and WML in particular ± could be insuciently 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 suciently 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 dier from the 3G cell phones on steroids. 4.2. Mobile hardware and devices The issues in mobile computing are dierent 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 dierent 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 aecting the peer hosts on the other side of the proxy (Spatscheck et al., 2000). 124 S. Pierre / Telematics and Informatics 18 (2001) 109±131 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 aects 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 aected, 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 oers 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 eective 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 dicult 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 dicult 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 Die, 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. 126 S. Pierre / Telematics and Informatics 18 (2001) 109±131 Furthermore, the mobile station is allowed to roam freely. As well as making key distribution more dicult, 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 dierent service providers. Thus, the concept of trust needs to be developed to allow mobile clients to use resources of dierent servers at dierent 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 eectively, the networks and end-systems require ¯exible and ecient 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 dier 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 aect 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. 128 S. Pierre / Telematics and Informatics 18 (2001) 109±131 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 eciently (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 ecient 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 aected 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 eects 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 aect 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. 130 S. Pierre / Telematics and Informatics 18 (2001) 109±131 References Aziz, A., Die, W., 1994. 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