WML: The ML That Matters

The wireless industry hopes that wireless markup language holds the key for the wireless Internet.

WAP is supposed to make consumers hungry for wireless data. Will consumers line up for a browser in every phone, or will they spurn it like so many wireless-data initiatives of the past? If WAP is the new restaurant in town, then wireless markup language (WML) is its secret book of recipes. Now that WAP is open for business in Europe, and WAP-enabled phones are hot, what is going to happen now that WAP has started to open its doors for business on this side of the pond?

According to Greg Williams, WAP Forum chairman, major U.S. service providers will commercially launch WAP throughout the first half of 2000. This will allow WAP browsers to be included on cellular and PCS phones that have modestly sophisticated pixel-oriented screens (such as the Nokia 6160, which is so closely associated with AT&T's flat-rate plan). According to Williams, WAP is "air interface agnostic and bearer independent," which means that it can be used on every major digital cellular and PCS technology that provides data speeds of 8kb/s or higher. This technology independence is what has generated so much international interest. Williams joked, "Join the WAP Forum and see the world!"

WAP can be viewed best as a wireless browser for the Internet. It is not separated from the Internet, but builds upon it. Currently, the Internet, as viewed from a wireless device, is a supremely hostile environment. Most sophisticated Web sites are barely tolerable when connecting at 56kb/s, while wireless phones are generally limited in bandwidth to 8kb/s to 13kb/s. Furthermore, the tiny screens, coarse resolution and numeric keyboards provide extremely limited user-interface capabilities. Yet, in the early days of the Internet, many people surfed quite contentedly on the text-based pages using speeds that were no higher than wireless provides today.

The WAP model for delivering content is similar to the HTML/HTTP delivery model, but requires some extra processing to adapt to the limitations of wireless phones. A WAP client does not use HTML, but WML instead. A WML file looks quite similar to HTML, partly because both are based on the extensible markup language, and partly because they both provide similar capabilities. Unlike HTML, however, WML has two different forms, one based on readable ASCII characters and a compressed binary form, intended for delivery across a radio interface. WAP also provides a scripting language, which provides a subset of the capabilities of JavaScript.

WAP is based on the client-server model, where content is stored on a server, to be dished out when clients order it up. The client generally is the user's computing device that provides an interface for ordering content, and then displays it in the way that best fits the user's preferences and the characteristics of the client device. This is simply a realization that some functions must be centralized (databases, for example) and that others are best controlled by the device that is serving a single person (such as the user interface). The transition from a server-based architecture to a client-server architecture evolved when PCs started replacing dumb terminals on the desks of workers and consumers. The same transition is occurring with wireless devices now that the digital wireless revolution has transformed phones into powerful computing devices.

Methods to perform the HTML-to-WML conversion are evolving. Although manual generation of WML pages certainly is possible, there also are some partially automated approaches. Herb Williams, Spyglass director of Mobile Data Solutions, describes its Prism product that can produce WML from HTML in real-time, but first it has to work with the customer to adapt the Web site to these new requirements. Web-site managers can continue to make routine changes, but major structural changes require their consultative involvement again. Williams notes that on a well-designed site, in conjunction with sophisticated tools, content can be dished up in different forms to support text-based phones, PDAs with simple graphic capabilities and fully fledged computing platforms that just happen to have wireless connections.

WAP is independent of the underlying radio interface, which need only provide the minimum capabilities to support WML and scripting. In fact, virtually every radio interface that has data capabilities has a WAP protocol stack defined, although that does not mean that products are available yet.

According to the WAP Forum's Williams, the majority of implementations will be based on circuit-switched data (CSD), with future enhancements taking advantage of short-message service and packet data, for example. An interactive session might take advantage of the bulk data efficiencies of CSD, while a notification of a stock-price change might be delivered in a short message, even allowing delivery with a voice call in progress.

An important characteristic of markup languages such as WML and HTML is that they do not describe pages exactly, but only give guidance. This allows the display of information to be adapted to the display device being used. HTML files can be displayed on small, text-based terminals or large, high-performance graphics monitors. Similarly, it is intended that WML will be used to display information on wireless devices with screens that show only a few short lines and on those that may be the size of a PDA screen, or even larger.

Because WML only gives hints, what is seen on a variety of wireless devices could be quite different. Similarly, the input devices are not specified. When text input is required, typing on a keyboard, tracing letters on a PDA screen or speaking into a device with speech-to-text capabilities are all equivalent.

The basic syntax of WML also is similar to HTML, using the familiar angle-bracket notation (e.g. this text is to be shown in bold letters) with the insertion of a slash indicating the end of a command. Simple commands that do not have a beginning and an end can use a more compact notation, , which is equivalent to . For example, instead of

• Integrated Telephony. WML also supports telephony applications through the wireless-telephony application interface. This allows a browser to communicate not with a remote server, but with call processing on the phone. An obvious application for this would be to allow calls to be made from an address book in a PDA by clicking on an entry.

• PushmePullyu. Most HTML browser applications work in a "get"' environment. When the client wants something, it asks for it, and if it wants an update, it waits a while and asks again. This is both inconvenient and, especially in a wireless environment, a waste of bandwidth. WML supports a "push," where the client makes a request that is not fulfilled until certain criteria are met (for example, a stock reaches a certain price). At that time, a message will be sent by the server to alert an application in the WAP client. This could result in, for example, the phone beeping and displaying a flashing message on its screen.

WMLScript: Decaffeinated Java?
When HTML first was introduced, interactions with clients were restricted. HTML allowed fairly sophisticated display capabilities — tables, graphics and frames — but forms were the only method of data input. Although forms still are used heavily, they have been made somewhat more functional and a lot more glitzy through the use of Java. WAP provides a scripting language that is, according to Alastair Angwin, WAP Forum board technical committee member, "equivalent to an optimized subset with extensions for telephony." Although not a complete programming language, WMLScript does allow for more sophisticated interactions with the user, and for calculations and text processing to be performed on the client. Perl, another common scripting language in World Wide Web systems is, by comparison, executed on the server. In many applications, the WMLScript on the client will interact with scripts or other application programs on the server.

Binary WML/WMLScript: Freeze-Dried Coffee?
If WML were just a customized form of HTML, the radio interface would still be a forbidding barrier, but WAP has one more trick up its sleeve: binary encoding of the messages. HTML is not very efficient in the use of bandwidth. The common HTML code <a href="URL"> has an overhead (not counting the URL) of only 3 bytes instead of 11 bytes. WML provides short binary (numeric) equivalents for many of the text strings that are commonly used. This can reduce the transmission requirements for a WML program by several times, which is equivalent to increasing the transmission speed several times over.

The WAE Gateway
Binary WML is one of the major reasons that a WAE gateway is required. The base station or MSC is a convenient place to convert from the ASCII-formatted WML code to the binary format that will be sent over the radio interface. The WAE gateway also can choose the bearer protocol that will be optimal for transmission based on a profile of the phone and browser capabilities that is provided to it.

The following example (taken from the WAP Specification, Version 1.1) describes a simple WML Deck containing two cards. The first card is presented to the user along with a button (or other user-input device) labeled "Accept." When this button is selected, the second card is displayed, and the WML deck terminates:

<card>
<p>
<do type="accept">
<go href="#card2"/>
</do>
Hello world!
This is the first card...
</p>
</card>
<card id="card2">
<p>
This is the second card.
Good-bye.
</p>
</card>
</wml>

Readers who are interested in learning more about WML or WAP should download the WAP series of specifications from www.wapforum.org.

Playing the WAP Card
The many players behind WAP are making a major gamble in the wireless-data market, and they think that WML is their strongest card. There are no guarantees in this business, but it looks as though WAP may be the wireless data initiative with the odds in its favor. It has breathed new life into the wireless-data marketplace. Haroon Alvi, Nokia director of business development for the Americas, predicted that 95% of phones will be WAP-enabled by 2005.

WAP will not replace existing wireless data technologies because it uses them for transport. There will be a synergistic relationship between them, with WAP potentially dramatically increasing the demand for these services, allowing the data services that best support it to flourish.

Alvi anticipates that as the demand for data increases, "the market for more sophisticated phones, such as touch-screen phones, will increase dramatically." WAP may also give the coup de grace to analog cellular, as there will be a much more obvious distinction between the phone-call-only capabilities of analog compared with integrated data-voice capabilities of a WAP-enabled digital phone.

Williams of the WAP Forum notes that wireless-data proponents are no longer talking about the details of the radio interface — modulation schemes and bit rates — but focusing on building an infrastructure for applications. Manufacturers and carriers appear to have realized that only a single, global, "technology-agnostic" protocol stands a chance of success.

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Glossary

WAP is not only introducing new technology, but also new terminology. Here are some of the acronyms that you will come across in places where WAP is discussed:

CDPD — Cellular digital packet data

CSD — Circuit-switched data

GPRS — GSM/ANSI-136 general packet radio service

GUTS — Generic UDP TeleService for ANSI-136 TDMA

HTML — Hypertext markup language

JavaScript — Client-scripting language commonly used in conjunction with HTML

PD — Packet-switched data

R-Data — Message-transmission protocol within ANSI-136

SMS — Short-message service

SMSCB — Cell broadcast short-message service

USSD — GSM unstructured supplementary service data

WAE — Wireless application environment

WAP — Wireless application protocol

WML — Wireless markup language

WTA — Wireless telephony application

XML — EXtensible markup language

Crowe (crowed@cnp-wireless.com) is a wireless-standards consultant and the editor of Cellular Networking Perspectives, a wireless-standards and -technology bulletin.