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COMMUNICATIONSOF THE ACM June 2000/Vol. 43, No. 6 73

Wireless and mobile networks are quickly becoming the

networks of choice, not only because of large bandwidth, but due to

the flexibility and freedom they offer.

With the increasing use of smallportable computers, wirelessnetworks, and satellites, a trendto support computing on themove has emergedthis trend isknown as mobile computing ornomadic computing [3]. Also

referred to as anytime/anywhere computing, mobilecomputing has several interesting and importantapplications for business (such as instant claim pro-cessing and e-commerce), telecommunications andpersonal communications, national defense (track-ing troop movements), emergency and disaster man-

agement, real-time control systems, remoteoperation of appliances, and in accessing the Inter-net. Since a user may not maintain a fixed positionin such environments, the mobile and wireless net-

working support allowing mobile users to communi-cate with other users (fixed or mobile) becomescrucial. A possible scenario may involve several dif-ferent networks that can support or can be modifiedto support mobile users. When dealing with differ-ent wireless networks, a universal mobile device

should be able to select the network (LAN, theInternet, PCS, or satellite) that best meets userrequirements.

Wireless and mobile networks have provided theflexibility required for an increasingly mobile work-force. As shown in Figure 1(a), the worldwide numberof cellular, GSM, and PCS subscribers increased from140 million in 1996 to over 300 million in 1999 andis expected to grow to 650 million by 2001 (see

www.gsmdata.com). In the U.S., capital investmentincreased from $6.3 billion in 1990 to $66.8 billion in1999 and service revenues were up from $4.5 billion to$38.7 billion in 1999 (see www.wow-com.com) as

shown in Figure 1(b). During the same period, theaverage local monthly bill diminished from $80 to $39as shown in Figure 1(c), indicating the technologicalmaturity and the tremendous competition among ser-vice providers.

Many general remarks can be made about wirelesssystems. First, the channel capacity typically avail-able in wireless systems is much lower than what is

Upkar Varshney and

Ron Vetter

MOBILE ANDWIRELESS NETWORKS


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quencies, which may lead to collisions. It is difficultto detect collisions in WLANs because the power

levels of signals coming to a mobile user may be dif-ferent and a station may not detect a potential com-petitor for the medium (the hidden stationproblem). The choice of frequency depends on

whether microwave, spread spectrum, or infraredcommunication will be used. Interference and secu-rity depend on the type of communications methodused in the WLAN. Because infrared cannot pene-trate walls, it encounters little interference from exter-nal sources but is limited in its coverage (typically

indoors). Spread spectrum spreads the signal over awide frequency range to reduce interference present at

certain frequencies. For security, some form ofencryption may be used. If the unlicensed ISM bandis used, some interference is likely to occur becausethe band is open to other users and agencies.

Wireless LAN standards. There are several wire-less LANs that have been proprietary in the past, suchas Motorolas Altair and AT&Ts WaveLAN. Fortu-nately, some progress has been made in standardizing

wireless LANs: two wireless standards are IEEE802.11 and HIPERLAN. The 802.11 standard sup-


Table 1. Mobile and wireless networking issues.

NetworkConfiguration

Limitations ofDevicesBandwidth andFrequency ofOperation

Handoffs

MAC Protocols

Error Control

QOSManagement

MobilityManagement

LocationTracking

Applications andMiddleware

Security

Failure

Hard handoffSoft handoffNetwork initiatedMobile assisted

User-basedApplication-basedFixed-sizeVariable

Same address in differentlocations for a mobile userDifferent addresses

Infrastructure-based configurationAd-hoc configuration

New protocols to handle device limitationsContent adaptation to device capabilitiesUse of existing frequencies(regulated/unregulated)Use of higher frequencies

Type of handoffsHandoffImplementation

Priority

Channel assignment

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)Code Division Multiple Access (CDMA)Error detection and retransmissionError correctionError correction and retransmissionAdmission control techniquesPriority to existing user's resource requestDynamic advance reservation

Adaptive error control techniquesQOS-oriented MAC protocolsChannel borrowing from underloaded regionsAddressingand Routing

Modification of existing protocols to deal withloss over wireless linksBroadcasting (paging) to locate a userLocation updating by a user after every moveCombination of paging and updatingApplications to adapt to varying QOSWireless middleware to deal with mobilityallowing no changes in existing applicationsEncryptionFrequency hoppingUse of infrared (indoors only)Deployment of back-up systems

More scalable but less flexibleLess scalable but more flexible

Interworking with existing protocolsAdditional complexity at network/serverLower bandwidth, higher interference, lower signalloss, lower costHigher bandwidth possible, lower interference,higher signal loss, higher costDevice can communicate with one access pointDevice can communicate with several access pointsNetwork to compare signal strength at several pointsMobile to compare signals from several basestations and report to the current oneKeep track of different classes of usersKeep track of different classes of applicationsEasy but not suitable to all applicationsDifficult but suitable to applicationsAnalog and amount of interference

Synchronization and slot speed match requirementsDifficult to satisfy varying bandwidth requirementsImpact on delay (not used for real-time applications)Amount of excess bandwidth requiredAdaptive to the channel conditionsAmount of processing (micro-cellular environment)Knowledge of the traffic in nearby cells/clustersDifficult to match user's future needs with networkresourcesComplexity and resource requirementsAmount of protocol processingDifficult to implementExact location information is required forcommunicationsComplex routing

Differentiation between packet loss due tocongestion and due to wireless links/user movementDelay, Amount of paging overheadAmount of updating overhead

Requires development of new applicationsCost of building middleware to deal withheterogeneous wireless networksProcessing requirements at mobile devicesComplexLimited useHigh initial cost

Issues Possible Choices Comments


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ports 1Mbps; HIPERLAN can be used to support23.5Mbps channel rates. The 802.11 also supportsseveral choices of physical medium such as spreadspectrum and infrared while HIPERLAN only allowsspread spectrum. Like HIPERLAN, 802.11 supportsprioritized access to the medium. One additional fea-ture of 802.11 is battery conservation for inactive or

idle wireless users. Many universities and companiesare encouraging the use of IEEE 802.11-based LANsfor accessing campus computing systems and theInternet. Another emerging wireless LAN standard isHIPERLAN2, which is being standardized by ETSIand expected to be ready by 2000. An exciting part ofHIPERLAN2 is the use of connections providing dif-ferent levels of quality of service for applications. It

will likely operate in a 5GHz band that is unlicensedin the U.S. and Asia and a dedicated unlicensed bandin Europe, using time-division multiplexing of uni-cast, multicast, and broadcast connections. Manymajor players in the wireless LAN area have formed

HIPERLAN2 Global Forum (www.Hiperlan2.com)to advance and complement the ETSI standardizationprocess. Not to be outdone by HIPERLAN2, IEEE802.11 is being enhanced to support 11Mbps.

Wireless Local LoopsBecause local carriers own the local loop in the U.S.,long distance companies have to pay access chargesevery time someone makes a long distance or inter-national call. The Telecom Reform Act of 1996 has

attempted to change this situation but since long dis-tance companies have not yet built their local loops,they continue to pay access charges (or excess chargesas they are often calledapproximately $20 billionlast year alone). In many developing countries, thereis little or no infrastructure in place. The situationcan be changed with the introduction of the Wireless

Local Loop (WLL). In the U.S., long-distancecompanies are looking to build their WLLs to avoidpaying access charges. Since WLLs provide fixed

wireless access (as opposed to mobile access providedby cellular and PCS), they can provide several MHzof bandwidth that can be used for high-speed Inter-net access and data transfer in addition to the basicphone service. In developing nations where layingmillions of miles of copper is impractical, WLLs canprovide phone and low-speed data transfer.

Among many choices of technologies that can beused in WLL environments, cellular and microcellu-lar systems can be used in the 900, 1800, and

1900MHz ranges with 9.6Kbps for 10100 userswithin a few kilometers or smaller area. WirelessLANs can be deployed to support WLL for users insmaller areas but with higher bandwidth require-ments. Systems that are especially designed for fixed

wireless access, such as LMDS, can provide very highbandwidth (tens of Mbps) in large areas for largenumbers of users but require a direct line of sight.

LMDS. Local Multipoint Distribution Systems(LMDS) is an emerging technology for serving point-

76 June 2000/Vol. 43, No. 6 COMMUNICATIONSOF THE ACM

Local Area

120Mbps

Data/Voice

Limited area

In use

Coverage

User

bandwidth

Application

Major issue

Status

Issues Wireless

LANs

Wireless

Loops

Cellular/

PCS

Mobile IP Wireless

ATM

Satellites

Metropolitan

19.2Kbps

Voice/Data

Bandwidth

In use

Wide Area

Network Dependent**

Data/Voice

Limited Applications

Emerging

Wide Area

120Mbps

All

Cost

Emerging

Wide Area

192Kbps to

few Mbps***

Voice/Data

Initial cost

Emerging

Local* or

Metropolitan120Mbps

Voice/Data

Interference

Emerging

*: Depending on the underlying technology such as 310 miles for LMDS.

**: Bandwidth depends on the underlying wireless network.

***: Higher limit for satellites such as Teledesic.

Table 2. A comparison of several mobile and wireless networks.

Globalstar

ICO

Teledesic

LEO

1400KmMEO

10,355Km

LEO

1400Km

48

(130,000)

10

(45,000)

288

(unspecified)

1.6, 2.4GHz users

5GHz up7GHz down

5GHz up

7GHz down

28GHz

Worldwide

(except poles)Worldwide

Worldwide

$1000 (terminal)

$.50 airtime$1000 (terminal)

$1 airtime

Unspecified

1999

2000

2003

Voice/Data

Voice/Data

Video/Voice

Data

Table 3. Some emerging satellite systems for mobile communications.

System

Name

Orbit

(height)

Satellites

(channels)

Frequency of

operation

Applications Coverage Charges Start

Date


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to-multipoint applications. It uses a 2831GHz bandand recently 1.3GHz of spectrum has been allocatedin the U.S. by the FCC to several hundred providers.LMDS is based on spread spectrum and can supportvery high bit rates for two-way data transfer. Possibleapplications are high-speed Internet data, telephony,and cable TV programming transmission at severalhundred Mbps.

SatellitesGeosynchronous satellites are in wide use providingbroadcasting services, long distance and interna-tional phone services (to stationary users), pagingservices (to mobile/stationary users), and data net-

working services. However, with advances inantenna design, signal reception, and other relatedtechnologies, it is becoming possible to providemobile services using satellites. Several such projectsare in different stages of implementation. Some ofthese are shown in Table 3.

Iridium. Iridium is a low-earth orbit (LEO) sys-tem that uses 66 satellites to provide mobile commu-nications to every point on earth and within 50 milesabove it. Since wireless users may have widely differ-ent needssome may need global roaming capabili-ties, while others may only want to supplement theircellular service in areas not served by existing cellularcarriersit is intended to provide many differentproducts and services. In Iridium, the same satellitemay not serve a user throughout the duration of a call;a call may be handed off to another approaching satel-

lite. The satellites, in concentric orbits, maintain linkswith up to four satellites in neighboring orbits. Buthandoffs are necessary among satellites in counter-rotating orbits to maintain cross-links among satel-lites. The Iridium service started in January 1999, butdue to technical problems and complaints about highprices, the number of customers that adopted the ser-vice was much smaller than needed to maintain thismultibillion-dollar networkwhich has been offi-cially declared bankrupt and has terminated commer-cial services. To avoid a similar fate, other emergingLEO-based networks must do a better job advertisingand marketing their services and products, combined

with lower initial and per-minute costs. It is difficultto derive the size of the market for such services, butthere is a large market for satellite phone service espe-cially in places where no cellular service exists or thatexperience heavy call blocking.

The most notable among the other LEO-basednetworks is the $9 billion Teledesic project funded byMicrosoft and McCaw Cellular. The project origi-nally planned to launch 840 LEO satellites but hasbeen scaled down to 288 satellites. The Boeing Co.

has recently been named as a major equity investorand a prime contractor. Worldwide licenses for fre-quency spectrums (29GHz uplink and 19GHz down-link) have been issued in 1998. Each satellite willhandle up to 155.52Mbps to and from the groundand 622.08Mbps to and from other satellites, so this

will be the first time a satellite system will providefiber-like connectivity. A typical Teledesic terminal(used by most people) will operate at 64Mbps down-link and 2Mbps uplink speeds. Teledesic is expectedto be operational in 2003 (www.teledesic.com).

Wireless ATM (Asynchronous Transfer Mode).ATM is an emerging technology for high-speed net-working, where information is transmitted in a 53-byte packet (called a cell) that can be prepared,transmitted, and switched by networks at very highspeeds [12]. The 53 bytes of an ATM cell are dividedinto 5 bytes of overhead (carrying control informa-tion) and 48 bytes of data. ATM is designed to sup-port both real-time and nonreal-time traffic withdifferent delay, loss, and throughput requirements. Inaddition, ATM has the major advantage of being scal-able; therefore ATM can be used in local-area as wellas wide-area environments at very high bit rates.

Motivation for Wireless ATM.Wireless ATM is anemerging and promising technology where ATMcells are transmitted over wireless channels andpart(s) of the ATM connection lies in the wirelessnetwork. The reasons behind the introduction of

ATM in wireless include seamless interconnectionwith backbone ATM networks, support for QOS of

wireless and mobile users, and suitability of smallpackets over wireless channels. However, the intro-duction of ATM in wireless environments createsmany interesting challenges because ATM was notoriginally designed to operate over channels of vary-ing characteristics. These challenges include how tomaintain the end-to-end ATM connection as theuser moves from one location to the other, how toimplement support for quality of service, and how todeal with wireless links to support mobile comput-ing applications. The ATM Forum, a worldwideconsortium of companies interested in ATM imple-mentation, is expected to release final standards in

the near future. Therefore early commercial deploy-ment of such systems may only be a few years away.Some of the obstacles include the present lack ofstandards, cost and complexity in implementation,and the amount of overhead. We believe increasedradio bandwidth allocation, emerging reliable andQOS-oriented protocols, along with error-controlprotocols for wireless ATM, will help in deploying

wireless ATM to support mobile applications. It isalso one of several technologies under consideration



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for third-generation wireless networks.ATM is a connection-oriented technology, so after

a mobile user moves to a new location connectionrerouting has to be performed. The connectionrerouting schemes can be based on (a) setting up anew connection, (b) providing multiple paths to amobile user, (c) forwarding ATM cells, or (d) dynam-ically rerouting the connection.

To route or reroute ATM connections, the wirelessATM network should have the information about thecurrent location of mobile hosts. Any change in loca-tion information should be reflected in the storagesystem, usually a location database. And the new loca-tion information should be available to the network

when a connection to a mobile host needs to be set up

(routed) or rerouted. Four major location manage-ment schemes are shown in Figure 2.

When ATM cells are transmitted over wirelesslinks, a high rate of cell loss may occur. Possible waysto counteract the cell loss include the use of forwarderror-correction algorithms or the use of an error-detec-tion scheme (Cyclic Redundancy Control, for exam-ple) followed by buffering and selective retransmissionof ATM cells. The retransmission and possible rese-quencing of ATM cells will require the use of sequence

number in ATM cells. It may bepossible to package sequencenumber, error-control overhead,and a 53-byte ATM cell togetherin a larger WATM cell.

Middleware andApplicationsMobile middleware can bedefined as an enabling layer ofsoftware that is used by applica-tions developers to connect theirapplications with differentmobile networks and operatingsystems without introducingmobility awareness in the appli-cations. The use of middlewaremay allow applications to run

with better response times and much greater relia-bility. Typically, middleware uses optimizationtechniques, such as header compression, delayedacknowledgements, and concatenation of severalsmaller packets into one, to reduce the amount oftraffic on the wireless networks. The middlewaredoes introduce additional complexity and signifi-cant initial cost (10300K is a range for most wire-less and mobile middleware). ExpressQ(www.nettechRF.com) is a mobile messaging mid-dleware product that enables developers to extendtheir non-IP applications to mobile users. It storesmessages when a mobile user is out of the network

range and forwards them the next time the mobileuser comes into range.

Wireless Application Protocol. Currently, manydifferent wireless access technologies exist that are notinteroperable. Designing and building network andbusiness applications for each technology would be anightmare for developers. This problem, combined

with redesigning all Web sites to support download-ing by mobile users is even more difficult. Even if allof this can be achieved, the information content stillhas to be adapted for transmission over wireless linksand is an effort to solve these problems: it allowsdevelopment of applications that are independent of

the underlying wireless access technology. WAP alsoadapts the existing Web site contents for transmissionover wireless links and display on mobile devices.

WAP specifications have been developed by the WAPForum (www.wapforum.org), a consortium of lead-ing wireless companies. The main contribution is theinteroperability of different wireless networks, devicesand applications using a common set of applicationand network protocols. The protocol architecture issimilar to that of the Web, such as the use of Wireless


Location Area 2

Location Area 1

Location AreaScheme

Always LocateScheme

Always Update

Scheme

Reporting CellScheme

Figure 2. Four major location update schemes.

MobileClient ServerEncoders

andDecoders

Response(content)

Encodedresponse

RequestEncodedrequest

Figure 3. Wireless applicationprotocol basic architecture.

Gateway

Wireless Network Wired Network


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Markup Language (WML, a cousin of HTML) opti-mized for mobile devices.

The architecture of WAP is shown in Figure 3,where a gateway acts as a proxy server to a mobileclient and translates requests from WAP protocolstacks to protocol stacks employed by the informationserver on the other side. Encoders translate the con-tent coming from the server into compact formats toreduce the size of data over the wireless network. Thisinfrastructure ensures mobile users can access a widevariety of contents and applications and also allowsapplication developers to build content services andapplications that can run on a large base of mobile ter-minals. To support this configuration, the WAP

forum defines several layers of protocols as presentedin Table 4.

Mobile OS.A general-purpose operating system isnot suitable for small handheld devices due to real-time requirements, smaller processing power, mem-ory, and screen size, and because of the types ofapplications that may be running, such as voice.Therefore, an OS with a small footprint and reducedstorage capacity is needed to support the computing-related functions of digital wireless devices. The avail-able OS for mobile devices vary in footprint size from300KB (Palm OS) to 2MB (Windows CE). Forexample, GEOS 3.0, the OS used in the Nokia 9000

Communicator, uses a footprint of 300KB. Many ofthese operating systems have attracted developers tobuild applications to run on handheld and othersmaller devices [1].

Accessing Different Mobile andWireless NetworksThe ability to roam across several different wirelessand mobile networks is necessary: in order to accessdifferent networks and services; to increase coverage

for a wireless user; to be able to use a single device;to be able to have a single bill; for providing reliable

wireless access to a user even under failure or loss ofa network or networks; and to reduce the total costof access to several networks. There are severalimportant issues in accessing different wireless net-

works as shown in Table 5; three possible architec-tures for supporting access to several differentmobile and wireless networks are discussed here (seeFigure 4).

Accessing several wireless networks using multi-mode/multifunction devices. In this configuration,the access to different services on different networks issupported using a single physical terminal with mul-

tiple interfaces. Some very early examples of thisarchitecture are the existing dual-function cell phone

AMPS/CDMA and the emerging GSM/DECT(cordless) architecture. This architecture may lead tohigher completion of calls and/or increase in the effec-tive coverage area. Since there may be overlapped cov-erage, this architecture will also provide reliable

wireless coverage in case of network, link, or switchfailure. The network design may include factors suchas the type of other networks, their pricing, regula-tions, and bandwidth. The handoff between networksmay be initiated by the user, the device, or the net-

work. Most of the additional complexity is introduced

in the device as neither wireless networks are modifiednor interworking devices are employed. Each individ-ual network can deploy a database that keeps track ofuser locations, device capabilities, network conditions,and user preferences. The location information will beneeded to complete calls to the user, for alerting ser-vices, and to implement E-911, which is required by2001 in the U.S.

Accessing several wireless networks using anoverlay network. In this architecture, a user accesses


Application

LayerSession Layer

Transaction

Layer

Security Layer

Transport Layer

Wireless Layer

Wireless Application Environment (WAE)

Wireless Session Protocol (WSP)

Wireless Transaction Protocol (WTP)

Wireless Transport Layer Security (WTLS)

Wireless Datagram Protocol (WDP)

Wireless and Mobile Networks

Provides micro browser environment and wireless

markup language (WML) and scriptHTTP functions and semantics

Facility for reliable and unreliable data push

Protocol feature negotiation

Provides several types of transaction services

Uses delayed ACKs and concatenated PDUs

Provides authentication and privacy

Provides a common interface to upper layer

protocols by adapting to specific features of the

underlying technologies

Provides a specific way to transmit information over

a wireless link

Table 4. WAP layers, protocols, and functions.

WAP layer Protocol Functions


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an overlay network consisting of several Universal

Access Points (UAPs). These access points choose awireless network for the user based on availability,QOS-specified, and user-specified choices. A UAPperforms protocol and frequency translation, andcontent adaptation. By using an overlay network, thehandoffs are not performed by the user or the devicebut by the overlay network as the user moves fromone UAP to the other. UAP stores user, network, anddevice information/capabilities and preferences. Thisarchitecture will support single billing and single

subscription for users as UAPs can keep track of var-

ious resources that have been used by a user.Accessing several wireless networks using the

Common Access Protocol. This architecture can beused if wireless networks can support one or two stan-dard access protocols, and requires interworkingbetween different networks. One possible way to sup-port this architecture is to use wireless ATM, meaningevery wireless network must allow the transmission of

ATM cells with additional headers (or WATM cells)requiring changes in the wireless networks.


Ways to access several different networks

Type of handoff

Handoff detection (how to decide when

one wireless network is not available and

start using other one)

Location coordination among networks

Adding new users (no longer depends on

one network as adding new users will

affect several networks)

Adding new services (multicasting and

other emerging services and features)

Access and bandwidth allocation

Addressing

Effect on upper layer protocols

Security

Failure and backup

Network independence (so a user may beunaware of the underlying physical network)

Regulation

Pricing issues

1. Use of multifunction devices

2. Use of an overlay network

3. Use of common access protocol

1. Allow user to access one network at a time (Hard handoff)

2. Allow user to access more than one network at a time (Soft handoff)

1. Continuous monitoring of Signal-to-Noise ratio

2. Monitoring of delay

1. Use of a centralized location database and local database for every

network

2. Location updating by broadcasting/paging when necessary

Network interaction problem (difficult to find out how much traffic will

be increased on other networks by adding a user to one network)

1. Development of minimum capability set

2. Hardware/software/implementation compatibility

3. New econometric models to divide revenue among multiple networks

1. Dynamic bandwidth division among single and multiple-network users

2. Dynamic bandwidth division among native and guest users

3. Resource allocation to high priority users

1. Network specific

2. Uniform

3. Logical (using mapping)

4. One number

Adaptation required during handoff or delayed access to a new network

1. Verification with a home location register

2. Single name, password to access different networks

1. Internal controller in a device constantly monitoring for continued

availability to networks

2. Intelligent interworking device to notify user in case of network failure

1. Common interface by using mobile middleware2. Adaptive application to adjust to change in network characteristics

1. New regulation may be required on how and what information may be

exchanged between different wireless networks

2. Wireless carriers may be required to provide FCC with data on failure

and loss of access

1. New models for dividing revenues among different wireless networks

(using total time a user was connected, number of packets/bytes

transmitted, and total overhead caused)

2. Single bill (flat pricing, usage-sensitive pricing, pricing based on QOS

delivered, pricing using guest and native networks)

Table 5. Important issues in accessing several different wireless networks.

Issue Possible Solutions


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Conclusions and Future ProspectsMobile and wireless networks represent the next

wave of networking because of their usefulness in

assisting an emerging mobile workforce in a growinginformation-oriented society. However, mobile and

wireless networks also present many challenges toapplication, hardware, software, and networkdesigners and implementers. During the past fiveyears, research has focused on systematically alleviat-ing the limitations of wireless and mobile environ-ments. For example, several optimizations have beenintroduced to improve the performance of TCP/IPto make it work in slow, failure-prone, and limited

bandwidth wireless networks. Additionally, proxyservers have been used to improve the performanceof application-specific programs (Web browsers, filesystems, database servers, and so forth) and mobileusers. Over the next five years, research on enablingarchitectures for mobile client/proxy/servers, mobileagents, and disconnected users will be carried out. Inaddition, data-centric models such as mobile andlocation-sensitive queries, mobile transactions, andmobile workflows are also recognized as importantemerging research areas.

In the near future, universal devices that can accessthe closest/best quality/cheapest wireless network outof several choices will be developed. Wireless net-

works will be able to implement a uniform addressingsystem in which a person has a consistent identifyingnumber or network address that is portable across all

wireless networks. Within two to three years, thesenetworks will compete with wired networks forapplications with low to medium bandwidth require-ments. However, with increased frequency allocations,advances in semiconductor technology, and more effi-cient coding of information over wireless channels,mobile and wireless networks will become the net-

works of choice for most users and applications, mak-ing wired networks relics of the past.

References1. Comerford, R. Pocket computers ignite OS battle. IEEE Spectrum

(May 1998).2. Foreman, G. and Zahorjan, J. The challenges of mobile computing.

IEEE Computer(Apr. 1994).

3. Kleinrock, L. Nomadic computing: An opportunity. Computer Com-munications Review(Jan. 1995).4. LaMaire, R.O., Krishna, A., and Bhagwat, P. Wireless LAN and mobile

networking: Standards and future directions. IEEE CommunicationsMagazine(Aug. 1996).

5. Miller, B. Satellites free the mobile phone. IEEE Spectrum(Mar. 1998).6. Noerpel, A.R., and Lin, Y.-B. Wireless Local Loop: Architecture, technolo-

gies, and services. IEEE Personal Communications Magazine(June 1998).7. Perkins, C.E. Mobile IP. IEEE Communications Magazine(May 1997).8. Raychaudhuri, D. and Wilson, N.D. ATM-based transport architec-

ture for multiservices wireless personal communication networks. IEEEJournal on Selected Areas in Communications(Oct. 1994).

9. Special issue on wireless computing. Commun. ACM 37, 10 (Oct.1994).

10. Tanenbaum, A. Computer Networks, 3d ed. Prentice Hall, 1996.11. Varshney, U. Supporting mobile computing using wireless ATM. IEEE

Computer(Jan. 1997).12. Vetter, R. ATM concepts, architecture, and protocols. Commun. ACM

38, 2 (Feb. 1995).

Upkar Varshney ([email protected]) is an assistant professor inthe Department of Computer Information Systems at Georgia StateUniversity; www.cis.gsu.edu/~uvarshne.Ron Vetter ([email protected]) is a professor of ComputerScience at the University of North Carolina at Wilmington;

www.uncwil.edu/people/vetterr.

2000 ACM 0002-0782/00/0600 $5.00

c


SatelliteAdapter

OverlayNetwork

LANAdapter

Cordless/FRAAdapter

Cordless/FixedRadio Access


Cellular/PCS

UAP

UAP: Universal Access Point

UAP

UAP

UAP

Wireless LAN

TunableAdapter

Interworking Unit

Terrestrial Access Protocol

Satellite Access Protocol

Interworking Unit


Wireless LAN

Wireless LAN

Figure 4. Architectures for accessing severalwireless networks: (a) Multimode device;

(b) Overlay network; (c) Common access protocol.

(a)

(b)

(c)

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Mob Wire Net