Feature

From EDN Europe: WAP stimulates third-generation mobile telephony

With market penetration of more than 70% in some European countries, the mobile-phone phenomenon is today's number one consumer-electronics success story. What will the service providers do to maintain and stimulate their market share, and how will future developments affect design engineers?

By David Marsh, Contributing Technical Editor -- EDN, 9/1/2000

The mobile-phone story is the consumer-electronics success story of recent times. The latest subscriber figures for the United Kingdom alone show that market penetration is now a staggering 52%, and countries such as Finland boast figures better than 70%. You might think that the service providers would be happy to sit back and enjoy the profits from tariffs that are often expensive compared with land lines, but this situation clearly isn't the case. At the recent US-style spectrum-allocation auction, five UK service providers spent some $35 billion to secure bandwidth for third-generation (3G) services. This figure is equivalent to some $1200 per subscriber. Underpinning their faith, figures from the Universal Mobile Telecommunications System group, UMTS Forum, predict that mobile-telephone users worldwide will increase from 426 million today to 1.73 billion by 2010 (Reference 1). Despite immature standards, service providers are now developing and planning deployment of technologies that will help them recoup such massive investments.

Today's European-standard mobile phones are second-generation devices that use Global System for Mobile (GSM) communications technology. These phones routinely include features such as dual- or triple-band operation and connect to the 900-, 1800-, and 1900-MHz GSM networks in more than 120 countries. You can now roam internationally and—providing that you're prepared to foot the bill—remain in direct contact pretty much wherever you go. This capability alone represents a huge leap forward from first-generation mobile telephony and effectively makes dedicated networks obsolete for normal subscribers, as Iridium famously discovered with its ill-fated satellite service. Specialist networks are now almost exclusively the preserve of the emergency and security services, with the Tetra and Tetrapol terrestrial-trunked radio technologies finding favour with agencies throughout the world (Reference 2).

Perhaps even more significant, virtually all of today's phones support short-text-message services (SMS). Figures from data-awareness group the Mobile Data Association show that text messaging accounted for 500 million messages in May 2000 in the United Kingdom alone, a tenfold increase over the same period in 1999. (May's worldwide figure was 8 billion.) Further optimism comes from Japan, where some 9.3 million of more than 40 million digital-telephony subscribers use the 9.6-kbps i-mode services to receive online sports results, download games, or even read their horoscopes. This success is tempting Japanese telecomm giant NTT to target European markets with its 3G DoCoMo "information-anywhere" technology. Meanwhile, European services that deliver mobile Internet connectivity over GSM networks are rolling out. Such services typically employ wireless-application-protocol (WAP) technology that—together with general-packet-radio-service (GPRS) data delivery—paves the way towards true 3G services.

Packets switch circuits

With a continuous connection and low end-to-end call latency, today's circuit-switched GSM technology ideally suits voice transmissions. By contrast, the Internet and the public-switched data networks employ packet-switching techniques that tolerate greater latency and variable data-delivery rates. Because virtually all mobile-telephony operators view Internet delivery as the prime target for service extensions, they urgently need a bridge between wired resources and mobile users to kick-start service deployments. On the horizon, 3G services promise to optimise mobile-data services by combining packet switching and code-division-multiple-access (CDMA) technologies (see sidebar "3G promises megabit mobiles"). But moving to 3G requires operators to commission new networks that occupy billion-dollar frequency allocations. With its emphasis on data delivery, 3G also targets a niche market in today's Europe, so you can expect this evolution to take several years to mature.

Meanwhile, "second-generation-and-a-half" (2½G) GPRS enhancements to the GSM infrastructure are beginning to extend data rates from today's 9.6-kbps baseline to a theoretical 171.2 kbps. Services that are now rolling out in countries including Germany and the United Kingdom also bridge the divide between GSM and the TCP/IP (transport-control protocol/Internet protocol) and public packet-switched X.25 wide-area networks. GPRS overlays a set of packet-data channels that concatenate transmission time slots without changing GSM's RFs, time-division-multiple-access (TDMA) frame structure, or 200-kHz carrier spacing. The basic GSM TDMA scheme supports eight 4.615-msec time slots per frame; the GPRS burst mode packs 114 data bits per time slot, with four error-protection coding levels (Table 1). Multislot transmission and reception capabilities divide into 29 classes that support one to eight transmit/receive pairs, with classes above 12 requiring frequency hopping, full-duplex communications, or both. Classes one through 12 require no architectural changes to terminal or mobile equipment, describing symmetrical and asymmetrical combinations that accommodate one to four transmit/receive pairs.

Practical estimates of the data rates that users can expect from GPRS vary but are unlikely to exceed 56 kbps. Philipp Schindera at German operator T-Mobil expects about 40 kbps when the company's service goes live on September 1, 2000, and British Telecom's fledgling roll-out delivers around 27 kbps. Lance Hiley, strategic marketing manager for Lucent Microelectronics' wireless products, observes that most operators are settling on time-slot structures that deliver 14.4 kbps with sufficient error correction for reliable operation. In partnership with phone maker Samsung, Lucent chose to start out with GPRS Class-8 mobiles that receive four time slots and transmit one: "Transmission is far more difficult than reception, with radio design being complicated by issues such as GSM's requirement for varying power levels between time slots. Our Class-8 approach provides us with a low-risk route that gets us to market early and delivers the performance that users need for typical download-heavy applications." For fast-track device development, Lucent's Berlin reference design includes the radio and baseband hardware with protocol-stack software and development tools from the company's Optimay subsidiary.

Yvan Droinet at Philips RF marketing observes, "For GPRS above 56 kbps, you need a fast synthesiser for the downlink that switches frequencies in under 250 µsec, but—given a low-noise environment—all GPRS data rates are possible without significant impact on terminal equipment or handset designs." Available now, Philips' UAA3535 transceiver IC uses a near-zero IF (N-ZIF) design to perform triple-band GSM downconversion, including 56-kbps GPRS capability. Like a zero-IF design, the N-ZIF architecture dispenses with expensive surface-acoustic-wave filters but adds a highpass-filter break that controls dc errors. Philips will also soon release a 56-kbps-capable version of its OneC-GPRS baseband controller that supports Class-12 operation (four transmit and four receive slots). Philips' architecture for 2½G phones comprises four key ICs that build a handset with an estimated $45 to $60 total bill-of-materials cost (Figure 1). Droinet concludes, "With GPRS, the idea is to get on and off the communication channel as fast as possible, which is relatively easy in hardware terms. But to manage the packet-based protocol and the new channels, the software protocol stack requires significant modification." You can license GPRS protocol stacks from independent vendors such as Condat, whose G23-GPRS package includes a user-interface development tool kit, PC simulation tools, and a compliance test suite.

But bandwidth and radio-system improvements are small parts of a much larger architectural change. Lucent's Hiley assesses, "GPRS is very much a transitory stage on the road to full IP-model mobile networks. Such networks will support "always-on" mobiles that permanently listen for their IP address to automatically download new information, such as email, or to update corporate diaries." The service provider's location register assigns temporary IP addresses as the user roams, which—if the scheme works in this native form—forces another IP-address scheme rethink due to the massive rise in the IP-capable device population. Hiley foresees that the always-on model raises issues such as power consumption and lack of choice in downloading, so users will retain ultimate control, as is the case with the GSM-based wireless location systems that companies such as Cambridge Positioning Systems and Qualcomm's subsidiary Snaptrack are developing.

GSM edges forward

Another step in GSM's evolution is the enhanced data GSM environment (EDGE) that supports data rates reaching a theoretical 384 kbps, although conservative figures from the CDMA Development Group predict practical data rates of 114 kbps. EDGE uses PSK modulation that encodes 3 bits per symbol (8-PSK) in place of GSM's normal 1-bit-per-symbol Gaussian-filtered minimum shift keying (GMSK). The technology demands a highly linear transmitter power amplifier but few changes to the receiver, so expect to see asymmetrical data exchanges with 8-PSK downstream and GMSK upstream. Industry attitudes towards EDGE vary. Philips' Droinet believes that EGDE is questionable from an application viewpoint because 56-kbps GPRS will cover most application needs until 3G arrives. Echoing this view, T-Mobil's Schindera reports that his company has made no decisions regarding EDGE. He notes, "The German authorities demand a 25% 3G coverage of our population by the end of 2003, so I expect our first 3G networks as early as the end of next year."

But in the United States, AT&T Wireless has announced a firm commitment to EDGE. Lucent's Hiley recognises that EDGE is attractive to many operators, especially if they lose out on 3G spectrum allocation or face bandwidth limitations within their existing GSM spectrum. Lucent's tests show that EDGE increases data-transfer efficiency by 40% over GPRS; that fact alone may support use of the technology in battery-powered data environments. Hiley predicts, "You can consider EDGE as a turbocharged version of GPRS that triples bandwidth, so I expect we'll see a lot more interest in EDGE when the present 3G beauty contest settles out—probably within the next six to 12 months."

WAP harmonises radio platforms

Whatever the economic realities that control the new technology roll-outs, application developers need a standard that harmonises Internet access across multiple radio platforms and on devices with diverse user interfaces. WAP aims to be a single standard for wide-area wireless-communication devices, not just phones. (From this point, the term "mobile" refers to any device that requires a mobile-telephony connection to exchange data, such as a personal digital assistant (PDA) or wireless-connected PC.) The standard's guiding body, the WAP Forum, includes founding members Ericsson, Motorola, Nokia, and Phone.com (formerly Unwired Planet), and now comprises more than 500 members. Despite much bad press—often due to early users' frustration with today's limited bandwidths—the WAP Forum's license-free, open-systems standard looks set to succeed on a global scale. You can download the full WAP specifications—currently version 1.2—from www.wapforum.org.

Crucially, WAP is independent of radio architecture, operating in 3G and legacy environments as diverse as digital-enhanced cordless telephony (DECT); GSM; Japan's PDC system; Tetra; and the main US systems, such as CDMA-based interim standard IS-95, TDMA-based IS-136, and the 1900-MHz GSM derivative, personal communications service PCS1900. WAP borrows heavily from Internet-programming conventions, making changes that suit a mobile's bandwidth and user-interface restrictions. Accordingly, WAP's thin-client model embeds minimum resources in the mobile and concentrates intelligence at the wireless network's gateway server. WAP also supports functions such as common gateway interface (CGI) and active server pages (ASPs) to provide dynamic Internet content to users on the move. Application-programming interfaces (APIs) allow WAP to work with a variety of operating systems (OSs), including dedicated mobile OSs, such as Palm's PalmOS and Symbian's Epoc.

The mobile includes a microbrowser that communicates with the gateway via wireless markup language (WML), a compact derivative of the World Wide Web Consortium's extensible markup language (XML). Because hypertext markup language (HTML) presents an entire page of information, it's inappropriate for a small-screen device with no mouse and limited keyboard facilities. WML decomposes pages into data items that present information or links to information in which each data item is a "card," and the whole page is a "deck." A scripting language, WMLScript, shares the same ECMA-262 origins (Reference 3) as JavaScript and supports procedural-logic operations, such as validating user input or accessing functions within the mobile. Script libraries provide a method for extending and reusing functions. At runtime, the gateway can binary-encode WML data to save transmission bandwidth, so WAP mobiles include "user-agent" software that processes encoded WML and compiled WMLScript.

WAP's wireless-telephony-application (WTA) environment provides the framework for creating mobile services that support the wireless-application environment (WAE). The WTA user agent is an extension to the higher level WML entity and includes an application interface (WTAI) that manages functions such as call handling, text-message processing, and phone-book management. Function libraries divide tasks into those that are common to all networks, network-specific functions, and public functions. To avoid having to continuously refresh download information during a session, the WTA environment manages the "repository," a temporary memory that holds data such as a WML deck or links to other data sources. The WTA also provides service-indication information, such as notification of new-message arrivals.

To request data from an Internet resource, the mobile issues a wireless-session-protocol (WSP) request using the normal Internet-address uniform-resource-locator (URL) mechanism (Figure 2). Functionally, WSP is a binary version of the hypertext transport protocol (HTTP) that manages conventional Internet traffic. The gateway server validates the mobile's unique client identifier and handles translation issues to request data from the Internet server and return WML-format information via a WSP response message. Gateway servers such as Geoworks' Premion Server+ translate between alternative content sources and the WML environment. The protocol stack that underpins the air-interface transactions follows the ISO-standard Open Systems Interconnection layer model, extending wired-Internet-access functions to suit the radio system (Figure 3). Each protocol-stack layer presents a defined interface to its neighbours and successively abstracts the lower levels from the highest level end-user application.

The lowest level is the physical-layer air interface that delivers messages via bearers, such as GSM's SMS or GPRS. Drivers within the mobile's OS format and exchange messages with the WAP stack's transport layer via API calls, using the wireless-datagram-protocol (WDP) or user-datagram-protocol (UDP) message formats. The wireless-control-message protocol (WCMP) monitors non-IP datagrams and reports errors, such as fragmented messages. Wireless-transport-layer security (WTLS) derives from Internet secure-sockets-layer (SSL) practices and optionally establishes secure connections based on network port addresses. WTLS also handles other security issues, such as message encryption; in the original concept, the WTA user agent communicates exclusively with the service provider's infrastructure to access operator-approved content, but commercial pressures demand direct-to-Internet connections.

Above the security function lies the wireless transaction protocol (WTP), which controls message exchanges. WTP can optionally send a message once ("send-and-forget"), resend if there's no acknowledgment from the recipient, or initiate a send-receive-acknowledge sequence for maximum reliability. The supervisory WSP layer manages WDP, WTLS, and WTP services to support four message-delivery options. The connectionless mode provides a simple datagram service that requires no acknowledgments, but connection mode implies a longer session that requires reception acknowledgments and can thus retransmit lost data. Either mode can also employ WTLS functions. The highest application-layer level is the WAE, which executes WML and WMLScript code, providing each WAP device with a man-machine interface (MMI) that has a sufficient look and feel to provide product differentiation.

Develop WAP apps for free

To promote their core products, such as server software, Ericsson, Nokia, and Phone.com provide free WAP software-development kits (SDKs) on their Web sites, where you can also find online developer forums. Alternatively, you can purchase a third-party package from an independent vendor, such as Dynamical Systems Research, for approximately $900 (for the first copy, $350 per copy thereafter), which includes development support. You can also extend the functionality of the major vendors' SDKs with Motorola's free MobileADK application-development kit. MobileADK includes features such as VoxML, Motorola's text-to-speech version of XML. Today's SDKs comply with WAP Forum 1.1 specifications that reflect available devices, such as the first phone to win WAP Forum approval, Ericsson's R320. But tool kits are on the way that will meet WAP Forum "June 2000" specification versions (formerly called version 1.2), including support for push technology. Push technology allows a mobile to automatically download information from the service provider and is currently the greatest development issue for WAP Forum's standards committee. Expect to see a lot of work in this area.

Currently at release 4, Phone.com's UP.SDK typifies what you can expect from a WAP development environment. The core of a Phone.com system is the proprietary UP.Link server software that provides proxy access to any Internet site, HTML-to-WML translation, operator-management services, and optional value-added services from the UP.Apps suite. The free MS Windows- and Sun Solaris-compatible SDK includes a generic UP.Phone model with a minimum three-line-by-12-character display and three function keys that complement a standard keypad. The latest version (3.2) of the UP.Browser includes support for colour displays, and the SDK includes a Windows-only UP.Simulator for testing (Figure 4). The simulator's HTTP-direct mode allows your PC to load WML directly from any capable Web server, such as Phone.com's developer area; alternatively, you can use a mode that connects with a real UP.Link server. The SDK also includes a library of C, C++, and Perl routines and scripts to help you generate WML and process HTTP requests.

WAP's relative immaturity and continuous development complicate interoperability issues. To help application developers, the WAP Forum aims to ensure interoperability by layering compliance tests into device- and content-level tests. Low-level device tests subdivide into application and protocol-stack tests using a test pool of at least three devices to provide reasonable confidence that the device under test is compatible. Higher level content tests evaluate WML and WMLScript for syntactical correctness, and a supplementary authoring component provides hints for application developers. A separate but related problem is the potential requirement for device-specific decks. But, according to David Corfan, technical manger at Phone.com, most decks now work on virtually all devices. Says Corfan, "When you've got such limited display resources, every pixel counts, so decks that work simply may not be optimised for the specific device." To help you navigate the burgeoning new-product maze, vendors maintain up-to-date lists of compatible devices and decks in the developers' areas of their Web sites.