`
` Upkar Varshney and
`Ron Vetter
`
`MOBILE AND
`WIRELESS NETWORKS
`
`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 small
`
`portable computers, wireless
`networks, and satellites, a trend
`to support computing on the
`move has emerged—this trend is
`known as mobile computing or
`nomadic computing [3]. Also
`referred to as anytime/anywhere computing, mobile
`computing has several interesting and important
`applications for business (such as instant claim pro-
`cessing and e-commerce), telecommunications and
`personal communications, national defense (track-
`ing troop movements), emergency and disaster man-
`agement,
`real-time control
`systems,
`remote
`operation of appliances, and in accessing the Inter-
`net. Since a user may not maintain a fixed position
`in such environments, the mobile and wireless net-
`working support allowing mobile users to communi-
`cate with other users (fixed or mobile) becomes
`crucial. A possible scenario may involve several dif-
`ferent networks that can support or can be modified
`to support mobile users. When dealing with differ-
`ent wireless networks, a universal mobile device
`
`should be able to select the network (LAN, the
`Internet, PCS, or satellite) that best meets user
`requirements.
`Wireless and mobile networks have provided the
`flexibility required for an increasingly mobile work-
`force. As shown in Figure 1(a), the worldwide number
`of cellular, GSM, and PCS subscribers increased from
`140 million in 1996 to over 300 million in 1999 and
`is expected to grow to 650 million by 2001 (see
`www.gsmdata.com). In the U.S., capital investment
`increased from $6.3 billion in 1990 to $66.8 billion in
`1999 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, the
`average local monthly bill diminished from $80 to $39
`as shown in Figure 1(c), indicating the technological
`maturity and the tremendous competition among ser-
`vice providers.
`Many general remarks can be made about wireless
`systems. First, the channel capacity typically avail-
`able in wireless systems is much lower than what is
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`

`

`Figure 1(c). Decreasing monthly charges for
`wireless phone service in the U.S.
`100
`90
`80
`70
`60
`50
`40
`30
`20
`10
`
`Average Local Monthly Bill
`
`90 91 92 93 94 95 96 97 98
`Calendar Year
`Source: CTIA
`
`Figure 1. Recent changes in usage.
`
`Figure 1(a). Increasing number
`of wireless subscribers.
`
`Figure 1(b). Increasing capital investment
`and service revenues in the U.S.
`
`Capital Investment
`Revenues
`
`70
`60
`50
`40
`30
`20
`10
`
`Revenues (in billions)
`
`Investment and
`
`World
`Europe
`U.S.
`
`700
`600
`500
`400
`300
`200
`100
`
`Subscribers (in millions)
`
`98
`97
`96
`Calendar Year
`Source: CTIA, GSMDATA
`
`99
`
`00
`
`01
`
`90 91 92 93 94 95 96 97 98
`Calendar Year
`Source: CTIA
`
`
`
`available in wired networks due to the limited spec-
`trum available, power restrictions, and noise levels.
`Even with advances in coding schemes, the capacity
`remains limited due to these reasons and also
`because users share the available capacity in one way
`or another. Second, noise and interference have
`more impact on systems design for wireless systems
`than on wired systems. Third, before building a
`wireless system some sort of frequency allocation (by
`the Federal Communications Commission in the
`U.S.) is required. Fourth, security is a greater con-
`cern in wireless systems than in wired systems since
`information may be traveling in free space (with the
`exception of infrared LANs). More details on these
`issues can be found in [2, 9, 10].
`Mobile users do not necessarily need to use wireless
`interfaces and wireless interfaces do not necessarily
`support mobility. A mobile user can simply connect
`to fixed networks using wired interfaces as he or she
`moves. Likewise, a fixed-location user might use a
`wireless interface (via a LAN) while sitting in an
`office. Therefore, mobile and wireless systems are not
`the same even though there is considerable overlap.
`Mobile networks provide support for routing (how to
`maintain communication with mobility) and location
`management (keeping track of the location) func-
`tions. Wireless networks provide wireless interfaces to
`users (both mobile and stationary) by supporting
`bandwidth allocation and error-control functions.
`When combined, there are several interesting issues
`that arise, including optimal use of low bandwidth
`channels due to limited frequency allocation, man-
`agement of large bit-error rates due to high noise lev-
`els, application-level quality of service support,
`increased security concerns, and failure or malfunc-
`tioning of equipment (Table 1).
`The choice of media access control (MAC) can
`affect both performance and use of wireless networks.
`The MAC protocols used in cellular and PCS systems
`in the U.S. and Europe differ considerably. For exam-
`
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`
`June 2000/Vol. 43, No. 6 COMMUNICATIONS OF THE ACM
`
`ple, the U.S. standards use FDMA (in AMPS),
`TDMA (in PCS), and CDMA (IS-95), while GSM
`uses TDMA/FDMA over different frequencies. This
`directly affects the interoperability and global roam-
`ing of mobile users. These differences are also affect-
`ing the standardization of the next (third)
`generation—3G—of wireless/mobile systems where
`North American companies are pushing for CDMA
`(or Wideband CDMA) to allow for backward com-
`patibility with CDMA-based IS-95 while Europe is
`supporting TDMA for GSM compatibility. Some
`agreements on 3G systems have been reached, allow-
`ing all of the previously mentioned networks to inter-
`operate with or evolve into 3G wireless networks
`(www.itu.org).
`
`Emerging Mobile and Wireless
`Networks
`Mobile and wireless networks are also experiencing
`significant progress in the form of wireless local area
`networks (WLANs) [4], satellite-based networks
`[5], Wireless Local Loops (WLL) [6], mobile Inter-
`net Protocol (IP) [7], and wireless Asynchronous
`Transfer Mode (ATM) networks [8, 11]. A compar-
`ison is shown in Table 2. One emerging wireless
`technology
`is Bluetooth (www.bluetooth.net),
`which provides low-cost and short-range radio links
`for wireless connectivity among computers, printers,
`and scanners. Since the range is small, it can use the
`unlicensed ISM band in 2.4GHz.
`
`Wireless LANs
`Wireless local area networks are designed to provide
`coverage in a small area, such as a building, hallway,
`park, or office complex by extending or replacing
`wired LANs (such as Ethernet). The main attraction
`is the flexibility and mobility supported by a wireless
`LAN; bandwidth considerations are secondary.
`Unlike cellular networks where a frequency (chan-
`nel) is allocated, users in WLANs have to share fre-
`
`

`

`
`
`Table 1. Mobile and wireless networking issues.
`
`Issues
`Network
`Configuration
`Limitations of
`Devices
`Bandwidth and
`Frequency of
`Operation
`
`Handoffs
`
`Possible Choices
`Infrastructure-based configuration
`Ad-hoc configuration
`New protocols to handle device limitations
`Content adaptation to device capabilities
`Use of existing frequencies
`(regulated/unregulated)
`Use of higher frequencies
`
`Type of handoffs
`Handoff
`Implementation
`
`Hard handoff
`Soft handoff
`Network initiated
`Mobile assisted
`
`Priority
`
`Channel assignment
`
`User-based
`Application-based
`Fixed-size
`Variable
`Frequency Division Multiple Access (FDMA)
`Time Division Multiple Access (TDMA)
`Code Division Multiple Access (CDMA)
`Error detection and retransmission
`Error correction
`Error correction and retransmission
`Admission control techniques
`Priority to existing user's resource request
`Dynamic advance reservation
`
`MAC Protocols
`
`Error Control
`
`QOS
`Management
`
`Adaptive error control techniques
`QOS-oriented MAC protocols
`Channel borrowing from underloaded regions
`Same address in different
`Addressing
`locations for a mobile user
`and Routing
`Different addresses
`Modification of existing protocols to deal with
`loss over wireless links
`Broadcasting (paging) to locate a user
`Location updating by a user after every move
`Combination of paging and updating
`Applications to adapt to varying QOS
`Wireless middleware to deal with mobility
`allowing no changes in existing applications
`Encryption
`Frequency hopping
`Use of infrared (indoors only)
`Deployment of back-up systems
`
`Mobility
`Management
`
`Location
`Tracking
`
`Applications and
`Middleware
`
`Security
`
`Failure
`
`Comments
`More scalable but less flexible
`Less scalable but more flexible
`Interworking with existing protocols
`Additional complexity at network/server
`Lower bandwidth, higher interference, lower signal
`loss, lower cost
`Higher bandwidth possible, lower interference,
`higher signal loss, higher cost
`Device can communicate with one access point
`Device can communicate with several access points
`Network to compare signal strength at several points
`Mobile to compare signals from several base
`stations and report to the current one
`Keep track of different classes of users
`Keep track of different classes of applications
`Easy but not suitable to all applications
`Difficult but suitable to applications
`Analog and amount of interference
`Synchronization and slot speed match requirements
`Difficult to satisfy varying bandwidth requirements
`Impact on delay (not used for real-time applications)
`Amount of excess bandwidth required
`Adaptive to the channel conditions
`Amount of processing (micro-cellular environment)
`Knowledge of the traffic in nearby cells/clusters
`Difficult to match user's future needs with network
`resources
`Complexity and resource requirements
`Amount of protocol processing
`Difficult to implement
`Exact location information is required for
`communications
`Complex routing
`Differentiation between packet loss due to
`congestion and due to wireless links/user movement
`Delay, Amount of paging overhead
`Amount of updating overhead
`
`Requires development of new applications
`Cost of building middleware to deal with
`heterogeneous wireless networks
`Processing requirements at mobile devices
`Complex
`Limited use
`High initial cost
`
`quencies, which may lead to collisions. It is difficult
`to 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 station
`problem). The choice of frequency depends on
`whether microwave, spread spectrum, or infrared
`communication will be used. Interference and secu-
`rity depend on the type of communications method
`used 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 a
`wide frequency range to reduce interference present at
`certain frequencies. For security, some form of
`encryption may be used. If the unlicensed ISM band
`is used, some interference is likely to occur because
`the band is open to other users and agencies.
`Wireless LAN standards. There are several wire-
`less LANs that have been proprietary in the past, such
`as Motorola’s Altair and AT&T’s WaveLAN. Fortu-
`nately, some progress has been made in standardizing
`wireless LANs: two wireless standards are IEEE
`802.11 and HIPERLAN. The 802.11 standard sup-
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`
`
`Issues
`
`Coverage
`
`User
`bandwidth
`Application
`Major issue
`Status
`
`Table 2. A comparison of several mobile and wireless networks.
`Wireless
` Wireless
`Cellular/
`Mobile IP
`Wireless
`LANs
`Loops
`PCS
`ATM
`Local* or
`Wide Area
`Metropolitan
`Local Area
`Metropolitan
`1–20Mbps
`
`1–20Mbps
`
`19.2Kbps
`
`Network Dependent**
`
`1–20Mbps
`
`Wide Area
`
`Data/Voice
`Limited area
`In use
`
`Voice/Data
`Interference
`Emerging
`
`Voice/Data
`Bandwidth
`In use
`
`Data/Voice
`Limited Applications
`Emerging
`
`All
`Cost
`Emerging
`
`Satellites
`
`Wide Area
`
`19–2Kbps to
`few Mbps***
`Voice/Data
`Initial cost
`Emerging
`
`*: Depending on the underlying technology such as 3–10 miles for LMDS.
`**: Bandwidth depends on the underlying wireless network.
`***: Higher limit for satellites such as Teledesic.
`
`Table 3. Some emerging satellite systems for mobile communications.
`Orbit
`Satellites
`Frequency of
`Applications
`Coverage
`Charges
`(height)
`(channels)
`operation
`LEO
`48
`1.6, 2.4GHz users
`1400Km
`(130,000)
`5GHz up
`MEO
`7GHz down
`10,355Km
`5GHz up
`LEO
`7GHz down
`1400Km
`28GHz
`
`Voice/Data
`
`Voice/Data
`
`Video/Voice
`Data
`
`Worldwide
`(except poles)
`Worldwide
`
`Worldwide
`
`$1000 (terminal)
`$.50 airtime
`$1000 (terminal)
`$1 airtime
`Unspecified
`
`System
`Name
`Globalstar
`
`ICO
`
`Teledesic
`
`10
`(45,000)
`288
`(unspecified)
`
`Start
`Date
`1999
`
`2000
`
`2003
`
`ports 1Mbps; HIPERLAN can be used to support
`23.5Mbps channel rates. The 802.11 also supports
`several choices of physical medium such as spread
`spectrum and infrared while HIPERLAN only allows
`spread spectrum. Like HIPERLAN, 802.11 supports
`prioritized access to the medium. One additional fea-
`ture of 802.11 is battery conservation for inactive or
`idle wireless users. Many universities and companies
`are encouraging the use of IEEE 802.11-based LANs
`for accessing campus computing systems and the
`Internet. Another emerging wireless LAN standard is
`HIPERLAN2, which is being standardized by ETSI
`and expected to be ready by 2000. An exciting part of
`HIPERLAN2 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 unlicensed
`in the U.S. and Asia and a dedicated unlicensed band
`in Europe, using time-division multiplexing of uni-
`cast, multicast, and broadcast connections. Many
`major players in the wireless LAN area have formed
`HIPERLAN2 Global Forum (www.Hiperlan2.com)
`to advance and complement the ETSI standardization
`process. Not to be outdone by HIPERLAN2, IEEE
`802.11 is being enhanced to support 11Mbps.
`
`Wireless Local Loops
`Because local carriers own the local loop in the U.S.,
`long distance companies have to pay access charges
`every time someone makes a long distance or inter-
`national call. The Telecom Reform Act of 1996 has
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`
`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 charges
`as they are often called—approximately $20 billion
`last year alone). In many developing countries, there
`is little or no infrastructure in place. The situation
`can be changed with the introduction of the Wireless
`Local Loop (WLL). In the U.S., long-distance
`companies are looking to build their WLLs to avoid
`paying access charges. Since WLLs provide fixed
`wireless access (as opposed to mobile access provided
`by cellular and PCS), they can provide several MHz
`of bandwidth that can be used for high-speed Inter-
`net access and data transfer in addition to the basic
`phone service. In developing nations where laying
`millions of miles of copper is impractical, WLLs can
`provide phone and low-speed data transfer.
`Among many choices of technologies that can be
`used in WLL environments, cellular and microcellu-
`lar systems can be used in the 900, 1800, and
`1900MHz ranges with 9.6Kbps for 10–100 users
`within a few kilometers or smaller area. Wireless
`LANs can be deployed to support WLL for users in
`smaller areas but with higher bandwidth require-
`ments. Systems that are especially designed for fixed
`wireless access, such as LMDS, can provide very high
`bandwidth (tens of Mbps) in large areas for large
`numbers of users but require a direct line of sight.
`LMDS. Local Multipoint Distribution Systems
`(LMDS) is an emerging technology for serving point-
`
`

`

`
`
`to-multipoint applications. It uses a 28–31GHz band
`and recently 1.3GHz of spectrum has been allocated
`in the U.S. by the FCC to several hundred providers.
`LMDS is based on spread spectrum and can support
`very high bit rates for two-way data transfer. Possible
`applications are high-speed Internet data, telephony,
`and cable TV programming transmission at several
`hundred Mbps.
`
`Satellites
`Geosynchronous satellites are in wide use providing
`broadcasting services, long distance and interna-
`tional phone services (to stationary users), paging
`services (to mobile/stationary users), and data net-
`working services. However, with advances in
`antenna design, signal reception, and other related
`technologies, it is becoming possible to provide
`mobile services using satellites. Several such projects
`are in different stages of implementation. Some of
`these 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 miles
`above it. Since wireless users may have widely differ-
`ent needs—some may need global roaming capabili-
`ties, while others may only want to supplement their
`cellular service in areas not served by existing cellular
`carriers—it is intended to provide many different
`products and services. In Iridium, the same satellite
`may 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 links
`with up to four satellites in neighboring orbits. But
`handoffs are necessary among satellites in counter-
`rotating orbits to maintain cross-links among satel-
`lites. The Iridium service started in January 1999, but
`due to technical problems and complaints about high
`prices, the number of customers that adopted the ser-
`vice was much smaller than needed to maintain this
`multibillion-dollar network—which has been offi-
`cially declared bankrupt and has terminated commer-
`cial services. To avoid a similar fate, other emerging
`LEO-based networks must do a better job advertising
`and marketing their services and products, combined
`with lower initial and per-minute costs. It is difficult
`to derive the size of the market for such services, but
`there is a large market for satellite phone service espe-
`cially in places where no cellular service exists or that
`experience heavy call blocking.
`The most notable among the other LEO-based
`networks is the $9 billion Teledesic project funded by
`Microsoft and McCaw Cellular. The project origi-
`nally planned to launch 840 LEO satellites but has
`been scaled down to 288 satellites. The Boeing Co.
`
`has recently been named as a major equity investor
`and a prime contractor. Worldwide licenses for fre-
`quency spectrums (29GHz uplink and 19GHz down-
`link) have been issued in 1998. Each satellite will
`handle up to 155.52Mbps to and from the ground
`and 622.08Mbps to and from other satellites, so this
`will be the first time a satellite system will provide
`“fiber-like” connectivity. A typical Teledesic terminal
`(used by most people) will operate at 64Mbps down-
`link and 2Mbps uplink speeds. Teledesic is expected
`to 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 high
`speeds [12]. The 53 bytes of an ATM cell are divided
`into 5 bytes of overhead (carrying control informa-
`tion) and 48 bytes of data. ATM is designed to sup-
`port both real-time and non–real-time traffic with
`different delay, loss, and throughput requirements. In
`addition, ATM has the major advantage of being scal-
`able; therefore ATM can be used in local-area as well
`as wide-area environments at very high bit rates.
`Motivation for Wireless ATM. Wireless ATM is an
`emerging and promising technology where ATM
`cells are transmitted over wireless channels and
`part(s) of the ATM connection lies in the wireless
`network. The reasons behind the introduction of
`ATM in wireless include seamless interconnection
`with backbone ATM networks, support for QOS of
`wireless and mobile users, and suitability of small
`packets over wireless channels. However, the intro-
`duction of ATM in wireless environments creates
`many interesting challenges because ATM was not
`originally designed to operate over channels of vary-
`ing characteristics. These challenges include how to
`maintain the end-to-end ATM connection as the
`user moves from one location to the other, how to
`implement support for quality of service, and how to
`deal with wireless links to support mobile comput-
`ing applications. The ATM Forum, a worldwide
`consortium 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 of
`standards, cost and complexity in implementation,
`and the amount of overhead. We believe increased
`radio bandwidth allocation, emerging reliable and
`QOS-oriented protocols, along with error-control
`protocols for wireless ATM, will help in deploying
`wireless ATM to support mobile applications. It is
`also one of several technologies under consideration
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`
`
`Figure 2. Four major location update schemes.
`
`Always Locate
`Scheme
`
`number in ATM cells. It may be
`possible to package sequence
`number, error-control overhead,
`and a 53-byte ATM cell together
`in a larger WATM cell.
`
`Location Area 2
`
`Always Update
`Scheme
`
`Reporting Cell
`Scheme
`
`Figure 3. Wireless application
`protocol basic architecture.
`
`Mobile
`Client
`
`Encoded
`request
`
`Encoded
`response
`
`Gateway
`
`Encoders
`and
`Decoders
`
`Request
`
`Response
`(content)
`
`Server
`
`Wireless Network
`
`Wired Network
`
`for third-generation wireless networks.
`ATM is a connection-oriented technology, so after
`a mobile user moves to a new location connection
`rerouting has to be performed. The connection
`rerouting schemes can be based on (a) setting up a
`new connection, (b) providing multiple paths to a
`mobile user, (c) forwarding ATM cells, or (d) dynam-
`ically rerouting the connection.
`To route or reroute ATM connections, the wireless
`ATM network should have the information about the
`current location of mobile hosts. Any change in loca-
`tion information should be reflected in the storage
`system, 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 wireless
`links, a high rate of cell loss may occur. Possible ways
`to counteract the cell loss include the use of forward
`error-correction algorithms or the use of an error-detec-
`tion scheme (Cyclic Redundancy Control, for exam-
`ple) followed by buffering and selective retransmission
`of ATM cells. The retransmission and possible rese-
`quencing of ATM cells will require the use of sequence
`
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`
`Location Area 1
`
`Location Area
`Scheme
`
`Middleware and
`Applications
`Mobile middleware can be
`defined as an enabling layer of
`software that is used by applica-
`tions developers to connect their
`applications with different
`mobile networks and operating
`systems without introducing
`mobility awareness in the appli-
`cations. The use of middleware
`may allow applications to run
`with better response times and much greater relia-
`bility. Typically, middleware uses optimization
`techniques, such as header compression, delayed
`acknowledgements, and concatenation of several
`smaller packets into one, to reduce the amount of
`traffic on the wireless networks. The middleware
`does introduce additional complexity and signifi-
`cant initial cost (10–300K 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 extend
`their non-IP applications to mobile users. It stores
`messages when a mobile user is out of the network
`range and forwards them the next time the mobile
`user comes into range.
`Wireless Application Protocol. Currently, many
`different wireless access technologies exist that are not
`interoperable. Designing and building network and
`business applications for each technology would be a
`nightmare for developers. This problem, combined
`with redesigning all Web sites to support download-
`ing by mobile users is even more difficult. Even if all
`of this can be achieved, the information content still
`has to be adapted for transmission over wireless links
`and is an effort to solve these problems: it allows
`development of applications that are independent of
`the underlying wireless access technology. WAP also
`adapts the existing Web site contents for transmission
`over wireless links and display on mobile devices.
`WAP specifications have been developed by the WAP
`Forum (www.wapforum.org), a consortium of lead-
`ing wireless companies. The main contribution is the
`interoperability of different wireless networks, devices
`and applications using a common set of application
`and network protocols. The protocol architecture is
`similar to that of the Web, such as the use of Wireless
`
`

`

`Table 4. WAP layers, protocols, and functions.
`
`Protocol
`
`Functions
`
`Wireless Application Environment (WAE)
`
`Wireless Session Protocol (WSP)
`
`Provides micro browser environment and wireless
`markup language (WML) and script
`HTTP 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
`
`
`
`WAP layer
`
`Application
`Layer
`Session Layer
`
`Transaction
`Layer
`Security Layer
`Transport Layer
`
`Wireless Transaction Protocol (WTP)
`
`Wireless Transport Layer Security (WTLS)
`Wireless Datagram Protocol (WDP)
`
`Wireless Layer
`
`Wireless and Mobile Networks
`
`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 mobile
`client and translates requests from WAP protocol
`stacks to protocol stacks employed by the information
`server on the other side. Encoders translate the con-
`tent coming from the server into compact formats to
`reduce the size of data over the wireless network. This
`infrastructure ensures mobile users can access a wide
`variety of contents and applications and also allows
`application developers to build content services and
`applications that can run on a large base of mobile ter-
`minals. To support this configuration, the WAP
`forum defines several layers of protocols as presented
`in Table 4.
`Mobile OS. A general-purpose operating system is
`not suitable for small handheld devices due to real-
`time requirements, smaller processing power, mem-
`ory, and screen size, and because of the types of
`applications that may be running, such as voice.
`Therefore, an OS with a small footprint and reduced
`storage capacity is needed to support the computing-
`related functions of digital wireless devices. The avail-
`able OS for mobile devices vary in footprint size from
`300KB (Palm OS) to 2MB (Windows CE). For
`example, GEOS 3.0, the OS used in the Nokia 9000
`Communicator, uses a footprint of 300KB. Many of
`these operating systems have attracted developers to
`build applications to run on handheld and other
`smaller devices [1].
`
`Accessing Different Mobile and
`Wireless Networks
`The ability to roam across several different wireless
`and mobile networks is necessary: in order to access
`different 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 of
`a network or networks; and to reduce the total cost
`of access to several networks. There are several
`important issues in accessing different wireless net-
`works as shown in Table 5; three possible architec-
`tures for supporting access to several different
`mobile and wireless networks are discussed here (see
`Figure 4).
`Accessing several wireless networks using multi-
`mode/multifunction devices. In this configuration,
`the access to different services on different networks is
`supported using a single physical terminal with mul-
`tiple interfaces. Some very early examples of this
`architecture are the existing dual-function cell phone
`AMPS/CDMA and the emerging GSM/DECT
`(cordless) architecture. This architecture may lead to
`higher 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 switch
`failure. The network design may include factors such
`as the type of other networks, their pricing, regula-
`tions, and bandwidth. The handoff between networks
`may 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 modified
`nor interworking devices are employed. Each individ-
`ual network can deploy a database that keeps track of
`user locations, device capabilities, network conditions,
`and user preferences. The location information will be
`needed to complete calls to the user, for alerting ser-
`vices, and to implement E-911, which is required by
`2001 in the U.S.
`Accessing several wireless networks using an
`overlay network. In this architecture, a user accesses
`
`COMMUNICATIONS OF THE ACM June 2000/Vol. 43, No. 6
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`79
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`Table 5. Important issues in accessing several different wireless networks.
`
`Issue
`
`Possible Solutions
`
`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 be
`unaware 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 middleware
`2. 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)
`
`an overlay network consisting of several Universal
`Access Points (UAPs). These access points choose a
`wireless network for the user based on availability,
`QOS-specified, and user-specified choices. A UAP
`performs protocol and frequency translation, and
`content adaptation. By using an overlay network, the
`handoffs are not performed by the user or the device
`but by the overlay network as the user moves from
`one UAP to the other. UAP