0018-9162/01/$10.00 © 2001 IEEE54 Computer

C O V E R F E A T U R E

The next several years will see the rollout of 3G sys-tems in the United States, Europe, and Asia.1 3G willinitially support data rates in the tens to hundreds ofKbps range, with possible future support for data ratesas high as 2 Mbps—most likely for low-velocitymotion and short mobile-to-base transmission dis-tances. Intermediate 2.5G solutions will leverage muchof the existing network infrastructure and offer datacapabilities in excess of what is available in 2G butshort of the eventual 3G speeds.

Although the exact form in which these various high-speed wireless data services will develop is uncertain,we will clearly see substantial improvement over today’sdata rates in the very near future, thereby removing oneof the wireless Internet’s most significant hurdles.

Service technologiesAnother obstacle lies in the mobile devices them-

selves, which typically suffer from small displays, lim-ited memory, limited processing power, low batterypower, and greater vulnerability to inherent wirelessnetwork transmission problems. These usability chal-lenges make supporting common Internet standardssuch as HTML, HTTP, and TCP/IP difficult becausethey are inefficient over mobile networks.

WAP. To address these issues, a group of leading wire-less and mobile communications companies developedthe wireless application protocol for transmitting andpresenting wireless information and telephony serviceson mobile handheld devices. Whereas HTTP sends itspayload in a text format, WAP uses a compressedbinary format for greater efficiency. It offers a scalable,extensible protocol stack that handles security, sessionestablishment, and other aspects of mobile communi-cations to make systems run more efficiently overtoday’s low-bandwidth wireless networks.

Bringing theWireless Internet to Mobile Devices

Effectively mapping Internet content to mobilewireless devices requires not only new tech-nologies and standards, but also innovativesolutions that minimize cost and maximizeefficiency to the benefit of both content

providers and consumers. The wireless Internet mustdeliver information to handheld device users regard-less of where they are and how they are connected,and in a suitable format—a challenge complicated bythe dizzying array of devices, wireless standards, andapplications.

UNDERLYING TECHNOLOGIESA complex, interlocking set of technologies under-

lie wireless Internet services and devices. Equipmentproviders such as Motorola, Nokia, and Siemens pro-duce the devices and the infrastructure to support wire-less data networking. Microsoft, Palm, Symbian, andother companies provide operating systems and micro-browsers for handheld devices. Application platformsolutions from vendors including Openwave, Nokia,and Ericsson contribute the middleware infrastructuresuch as WAP (wireless application protocol) gateways.Middleware also includes a new generation of wire-less application platform infrastructure software toprovide wireless applications and device independencefor the increasing variety of handheld devices.

Network technologiesMost digital cellular networks are second-genera-

tion (2G) networks. A complicated set of overlapping,mutually incompatible 2G and 1G (analog) standardsexist in the United States, while the Global System forMobile communications (GSM) technology is the mostprevalent standard in Europe. The maximum data ratein most 2G networks is 14.4 Kbps or lower.

Transcoding and Relational Markup Language are promising middlewaresolutions to the problem of bringing Internet content to the extremelydiverse and dynamic mobile wireless devices universe.

Subhasis SahaMarkJamtgaard 2Roam Inc.

JohnVillasenorUniversity ofCalifornia,Los Angeles

June 2001 55

Instead of using HTML, WAP uses Wireless MarkupLanguage (WML), a small subset of ExtensibleMarkup Language (XML), to create and deliver con-tent. As Figure 1 shows, the WAP gateway translatesrequests from the WAP protocol stack to the TCP/IPstack so they can be submitted to Web servers. Thegateway translates WAP content into compact encodedformats that reduce the amount of data it sends overthe low-bandwidth wireless network.

WAP is currently the most widely adopted wirelessprotocol in the world among carriers and handsetmanufacturers.

Launched in 1997 by Phone.com (now OpenWave),Motorola, Nokia, and Ericsson, the WAP Forum(http://www.wapforum.org) has grown to includemore than 95 percent of the global handset market.

The forum is planning to update the WAP protocolto ensure compatibility with 2.5G and 3G wirelessstandards. On 30 January 2001, OpenWave unveiledits product architecture for wireless general packetradio service and 3G systems. This new platform,which simplifies migration from current WAP-basedmobile services deployed on 2G systems, supports notonly WML, but also XHTML, HDML (HandheldDevice Markup Language), Compact HTML (a sub-set of HTML that emphasizes text and simple graph-ics), and WAP 2.0, scheduled for release later this year.The updated protocol will support full-color graph-ics, multimedia, and, for wireless operators, subscribermanagement capabilities.

iMode. WAP is not the only protocol aimed specifi-cally at the wireless Internet. In fact, in recent months,some have questioned WAP’s long-term viability,2-4 par-ticularly in view of the explosive growth in competingtechnologies. The best-known non-WAP solution isiMode, a wireless Internet service that NTT DoCoMointroduced in Japan in February 1999. iMode relies onmodifications and extensions of existing protocols.With iMode, smart-phone users can browse the Netwith a touch of a button. iMode has a transmissionspeed of 9.6 Kbps, utilizes a packet-switched connec-tion, and has adopted CHTML as its markup language.

Other technologies Location-based services that use device location

information to modify communications content are

likely to become important for applications such ascommerce and emergency services. Clearly, location-aware systems also raise complex privacy issues.Voice-based access is also likely to be importantbecause of limited device display capabilities andbecause voice interaction has advantages in situa-tions where keypad entry is impractical—for exam-ple, while driving a car. Indoor wireless networks arealso likely to be extremely important for Internetaccess. Because wireless LANs and PANs have lesssevere bandwidth constraints than wide-area cellu-lar networks, they allow access that is closer to“wired” Internet access.

MIDDLEWARE CHALLENGESCommon to WAP, iMode, and other similar solu-

tions is the need to specifically recode Internet con-tent for wireless devices. A content provider offeringa Web site to both desktop and wireless users currentlymust maintain two parallel versions of the site cus-tomized to wired and wireless devices. Middleware,one of the most dynamic and yet least understoodtechnologies, offers an alternative to manually repli-cating content. Its basic purpose is to seamlessly andtransparently translate a Web site’s existing contentto mobile devices that support numerous operatingsystems, markup languages, microbrowsers, and pro-tocols. Creating a wireless presence using middleware,however, presents several key challenges.

Application integrationOne challenge is integrating disparate content

sources. Although some Web developers are begin-ning to store information in XML, most existing con-tent was developed for desktop-based, nonmobileHTML browsers. Typically, most Web sites have alayered structure that closely ties a presentation layerto an underlying logic layer. However, wireless devicesrequire a drastically different presentation layer, andrearchitecting the entire site to decouple these layerswould be extremely expensive.

Device independenceAnother challenge is the proliferation of devices,

browsers, and markup languages.5 For example,markup languages include such variants as HDML

Wirelessapplication

protocolmicrobrowser

running onWAP-enabled

devices

HDML/WML

WAPgateway

HTML/XML

Wireless network

Webserver

WAPprotocol

stack

TCP/IPstack

Figure 1. Accessingthe wireless Webusing a wireless-application-protocol-enabled device. AWAP gateway pro-duces a more efficientrepresentation of theWeb content that canbe more easily trans-mitted over wirelessnetworks.

56 Computer

3.0, WML 1.x for OpenWave, WML 1.x for Nokia,TinyHTML for PalmPilots, and CHTML for iMode.Different browser features—support for non-nestedtables and images, nested tables but no images,images and nested tables but only one font size, andso on—compound the problem. Display capabilitiesrange from two-line black-and-white displays to full-color displays with tens of thousands of pixels.

Optimal user interfaceCreating a compelling user interface that is appro-

priate for different device classes is another challenge.For example, a stock-trading site might want to expose

a market research function containing charts and graphsto a PalmPilot but not to a limited-display cell phone.To avoid forcing the cell phone user to scroll downnumerous lines or navigate through multiple menus toaccess desired content, the two devices’ informationarchitecture would have to be drastically different.

MIDDLEWARE SOLUTIONSAt first glance, it seems that the easiest way to

address these problems is to rewrite existing Web con-tent in a language appropriate for a particular proto-col (see the “Internet Markup Languages” sidebar).This would involve, for example, creating WML-

SGMLStandard Generalized Markup Language is a general-

ized common standard for describing an electronic docu-ment’s structure and organization. It does not specify astructure but instead allows for customized tag sets.SGML provides the primary framework for other lan-guages including HTML and XML.

HTMLDeveloped as the World Wide Web was coming to promi-

nence, Hypertext Markup Language facilitates the visualpresentation of information over the Internet. Its subsetsinclude HTML 4 and HTML Strict, which address pre-sentational and structural issues differently.

XMLAs Web developers recognized that SGML document

distribution and presentation required unique tools, Exten-sible Markup Language emerged as a powerful alternativeto HTML, which has predefined tags for specialized tasks.XML lets developers define their own markup elementsand gives content authors greater flexibility for structurallyand stylistically customizing Web documents. XML alsooffers HTML developers a framework for customizing andadding proprietary elements to HTML.

XHTMLExtensible Hypertext Markup Language supports

modular and extensible Web access based on XML. Witha few notable exceptions, it strongly resembles HTML 4.Released in January 2001 upon recommendation by theWorld Wide Web Consortium (W3C), XHTML repre-sents the most significant evolution of HTML sinceHTML 4’s introduction in 1997. XHTML essentiallyreformulates HTML as an XML application, enablingviewing by both HTML browsers and XML-based sys-tems. Consequently, most users can access Web pagesregardless of their browser device.

XSLExtensible Stylesheet Language enables transforma-

tion of XML documents into a format recognizable toa browser. For example, developers can use XSL totransform each XML element into an HTML element.XSL also provides a means for reformatting organiza-tion of the display elements: It can add completely newelements; remove, rearrange, and sort elements; and testand make decisions regarding which elements to dis-play.

RMLRelational Markup Language is an XML application

just as HTML and XML are SGML applications. RMLis tailored to meet the specific needs of wireless Internettechnologies. Developers use RML’s customized elementsto add structural context to a Web site’s content for mul-tiple presentation formats. RML retrieves and cachesclient data from requested URLs, then uses predefinedrule sets to convert the data from HTML and XML toRML. RML follows XML’s structural rules, but its spe-cific elements are unique.

WMLWireless Markup Language evolved from XML,

HTML, and Phone.com’s Handheld Device Markup Lan-guage. Designed to develop Web pages that developerscan easily render on small wireless devices, WML per-mits more flexible information displays than HTML andlets users input commands without a keyboard. Cur-rently, developers must write a Web site in WML or aserver must convert HTML to WML to translate the siteand render it properly on WAP-enabled devices. WML isscalable and extensible because, like XML but unlike ear-lier versions of HTML, it lets users add new markup tagsto meet changing needs. The WAP Forum is cooperatingwith the W3C to ensure that WML and XHTML willwork together.

Internet Markup Languages

June 2001 57

formatted content to fit various WAP devices,CHTML-formatted content for iMode devices, andso on. However, the wide range of devices, browserfunctions, gateway interfaces, and markup languagenuances make this approach intractable.

User-transparent transformationIn user-transparent transformation, a middleware

application transparently reformats content “on thefly” into a user-specific presentation, interface, andprotocol. In this context, a middleware applicationsits between the existing content server and the useragent. Because the middleware application can auto-matically detect the kind of device being used and for-mat the contents accordingly, it is not necessary tomaintain Web content in multiple formats.

User-transparent middleware approaches canprocess transcoding with or without a priori infor-mation. They can either strip down the content froman under-described source such as HTML or build upcontent from discrete pieces. However, end users stillmust scroll through hundreds of lines to get to the con-tent in which they are interested.

Relational Markup LanguageRelational Markup Language incorporates two

concepts—relational hierarchies of content andadding context to content—to resolve these prob-lems. Because it describes content through relation-ships, RML provides a write-once-deliver-anywheresolution. An interpreter can use the relational infor-mation to create an optimal information architec-ture. As Figure 2 illustrates, converting applicationcontent to a normalized RML format facilitates cre-ating a modular architecture. When a new device,browser, gateway, or markup language emerges,adding one new module supports both current andfuture users.

Atomics and groups. The RML framework uses atom-ics—discrete pieces such as a word, sentence, para-graph, image, link, or any other piece of content—tocreate a framework that supports the addition of bothrelational data and context. RML encapsulates atom-ics into groups to create a complex relational hierar-chy. As Figure 3 shows, groups contain one or more

(a)

HTML WML

XML HDML

XHTML CHTML

Page templates

(b)

HTML WML

XML HDML

XHTML CHTML

Page templates

RML

Budget Cuts Could Clip NASA’s Wings

New US astronauts eager to “kick the tires and light the fires” may find that their best chance to soar, at least in the near future, comes when they board airliners for public-speaking tours. That is one implication of the new spending plan the White House is proposing for the US space program.

World

• Pakistani City Under Security Siege to Stop Rally

• Indonesia’s Wahid Rejects Second Censure Paper

• Israeli Killed in West Bank Shooting

Business

• Vodafone to Buy Stake in Japan Telecom-FT

• Marriott to Make Strong Meridien Bid

• Reliant Resources IPO Gets $1.56 Billion

• Pier 1 Imports Cuts Forecasts

Science

• Mirror Tests Reflect Dolphins’ Intelligence

• Peptide Fights Arthritis in Mice

• Phytopharm Looks for Cures in Herbs And Spices

Figure 2. Creating a modular architecture using Relational Markup Language. (a) A tra-ditional wireless implementation can require different transcoding steps for each possi-ble combination of markup languages. (b) In contrast, an RML-based implementationprovides an intermediate format for the automatic markup of all markup languages. Theoutput can then be generated without regard to the original input markup language.

Figure 3. Breaking Web content into atomics and groups. This typical news site’s content decomposes into discretepieces—links, sentences, paragraphs, and images—to cre-ate a framework supporting the addition of both relationalinformation and context. The main group consists of all theatomics and groups that make up the site. Each top story’simage, headline, and introductory text form another group.The headline group includes three subsections: world, business, and science. Finally, each subsection and its associated story links form a group.

58 Computer

atomics, and they can themselves be part of largergroups.

Context. An interpreter can add contextual attrib-utes to the atomics and groups to optimize the infor-mation architecture for a specific mobile device.Adding context allows the RML document to handlecurrent and future application calls that differentdevices make from the browser. The context attributedefines the type of application call, and context datadefines the parameters the document needs to makethe function call. Figure 4 shows how an interpretercan use relational information to create an optimaluser experience.

RML also can differentiate between sequential andnonsequential content. For example, a page of text issequential content, and a list of links to news storiesis nonsequential content. If nonsequential contentdoes not fit on one page—for example, the headlinessubsection—RML collapses the associated groups intolinks.

Device-specific tasks may include click-to-dial, cre-ating an address book entry, or sending e-mail.Developers can use the RML framework to developalgorithms that optimize the user’s experience on avariety of mobile devices.

Bringing Internet connectivity to wireless devicesoffers a number of well-known challenges.Simply delivering bits at a high enough rate to

untethered devices and creating devices with suffi-

cient display, memory, and user interface features toallow efficient interaction with the Internet requiresinnovative technologies and new standards. The roll-out of 3G systems in the next several years willaddress many of these challenges. Equally importantbut less familiar hurdles lie in the mechanisms forbringing Internet content to an extremely diverse anddynamic universe of devices. Middleware such asRML will be critical to meeting the challenge of cre-ating a wireless presence. We anticipate that openstandards based on this or similar techniques will gainacceptance. ✸

References 1. W. Sweet, “Cell Phones Answer Internet’ s Call,” IEEE

Spectrum, Aug. 2000, pp. 42-46.2. N. Leavitt, “Will WAP Deliver the Wireless Internet?,”

Computer, May 2000, pp. 16-20.3. T. Lewis, “Why WAP May Never Get Off the Ground,”

Computer, Aug. 2000, pp. 112, 110-111.4. K.J. Bannan, “The Promise and Perils of WAP,” Scien-

tific Am., Oct. 2000, pp. 46-48.5. R. Comerford, “Handhelds Duke It Out for the Inter-

net,” IEEE Spectrum, Aug. 2000, pp. 35-41.

Subhasis Saha is a former manager of the research andtechnology group at 2Roam. His research interestsinclude multimedia compression and communication,wireless Internet and networking, and soft computing.Saha received a PhD in digital media from the Univer-sity of California, Davis. He is a member of the IEEEand the ACM. Contact him at sahas@home.com.

Mark Jamtgaard is chief technical officer of 2Roam.His research interests include wireless communicationsand adaptive systems. He received a BS in electricalengineering from Santa Clara University. Contact himat markj@2roam.com.

John Villasenor is professor and vice chair of the Elec-trical Engineering Department at the University ofCalifornia, Los Angeles. His research interests includecommunications, image and video coding, and con-figurable computing. Villasenor received a PhD inelectrical engineering from Stanford University. Con-tact him at villa@icsl.ucla.edu.

1) Top Stories2) Headlines

1) World2) Business3) Science

Top StoryBudget Cuts Could Clip NASA’s WingsNew US astronauts eager to “kick thetires and light the fires” may find thattheir best chance to soar, at least in the near future, comes when they board airliners for public-speakingtours. That is one implication of thenew spending plan the White Houseis proposing for the US space program.

WorldPakistani City Under Security Siegeto Stop Rally

(a) (b)

Figure 4. Transformed Web content on two mobile devices. (a) The first page of a four-line cell phone display consists of a link to the top story and a link to headlines, which inturn link to the world, business, and science stories, respectively. (b) A larger-displaydevice includes the top story followed by expanded headline subsections.