Wireless application protocol stimulates 3G mobile telephony

Mobile phones are the consumer electronics success story of recent times. The latest subscriber figures for the UK alone show that market penetration is now a staggering 52%; in contrast, countries such as Finland boast figures better than 70%. You might think that service providers would be happy to sit back and enjoy the profits from relatively expensive tariffs, but this clearly isn't the case. 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 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 GSM (Global System for Mobile Communications) technology. These phones routinely include features, such as dual- or triple-band operation, connecting 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 wherever you go.

Perhaps more significantly, virtually all today's phones support SMS (short message services). Figures from data awareness group the Mobile Data Association show that text messaging accounted for 500 million messages in May 2000 in the UK alone. Further optimism comes from Japanese experience, where 9.3 million of more than 40 million digital telephony subscribers use the 9.6 kbps i-mode services to receive on-line sports results, download games, or read their horoscopes. This success is tempting Japanese telecom's giant, NTT, to target European markets with its 3G (third-generation) DoCoMo "information anywhere" technology. Meanwhile, European service providers are beginning to deliver mobile Internet connectivity over GSM networks. These services typically employ WAP (wireless application protocol) technology, which combined with GPRS (General Packet Radio Services) data delivery, are paving 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 optimize mobile data services by combining packet switching and CDMA technologies.

Meanwhile, 2½G (second-generation-and-a-half) 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. GPRS overlays a set of packet data channels that concatenate transmission timeslots without changing GSM's RF frequencies, TDMA (time-division multiple-access) frame structure, or 200- kHz carrier spacing. The basic GSM TDMA scheme supports eight 4.615-msec timeslots per frame; the GPRS burst mode packs 114 data bits per timeslot, with four error-protection coding levels (Table 1). Multislot transmission and reception capabilities divide into 29 classes that support 1 to 8 transmit/receive pairs, with more than 12 classes requiring frequency hopping and/or full-duplex communications. Classes 1 to 12 require no architectural changes to terminal or mobile equipment, describing symmetrical and asymmetrical combinations that accommodate 1 to 4 transmit/receive pairs.

Practical estimates of the data rates that users can expect from GPRS vary but are unlikely to exceed 56 kbps. Lance Hiley, strategic marketing manager for Lucent Microelectronics' wireless products, observes that most operators are settling on timeslot 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 timeslots and transmit one. Hiley notes that Lucent's approach provides a low-risk route that delivers the performance that users need for heavy downloading.

Yvan Droinet at Philips RF marketing observes that, for GPRS as fast as 56 kbps, users need a synthesizer capable of switching frequencies in less than 250 µsec. Available now, Philips' UAA3535 transceiver IC uses a N-ZIF (near-zero intermediate frequency) 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 high-pass filter break that controls dc errors. 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 points out that to manage the packet-based protocol and the new channels, you need to significantly modify software protocol stacks. You can license GPRS protocol stacks from independent vendors such as Condat, whose G23-GPRS package includes a user interface development toolkit, PC simulation tools, and a compliance test suite.

But bandwidth and radio-system improvements are a small part of a much larger architectural change. Hiley assesses that GPRS is a transitory stage to full IP-model (Internet protocol) mobile networks, which support "always-on" mobiles that listen for their IP address to download new information. The service provider's location register will assign temporary IP addresses as the user roams, which will force 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, which is the case with the GSM-based wireless location systems that companies such as Cambridge Positioning Systems and Qualcomm subsidiary, Snaptrack, are now developing.

GSM edges forward

Another step in GSM's evolution is the EDGE (enhanced data GSM environment) that supports data rates as fast as a theoretical 384 kbps, although conservative figures from the CDMA Development Group predict practical data rates of 114 kbps. EDGE uses PSK (phase-shift keying) modulation that encodes 3 bits per symbol (8-PSK) in place of GSM's normal 1-bit/symbol GMSK (Gaussian-filtered minimum shift keying). The technology demands a highly linear transmitter power amplifier but few changes to the receiver, so you should expect to see asymmetrical data exchanges with 8-PSK downstream and GMSK upstream. Industry attitudes towards EDGE vary. Droinet believes that EDGE is questionable from an application viewpoint because 56-kbps GPRS will cover most application needs until 3G arrives.

But in the US, AT&T Wireless has announced a firm commitment to EDGE. Lucent's tests show that EDGE increases data-transfer efficiency by 40% compared with GPRS; this fact alone may support use of the technology in battery-powered data environments. Hiley predicts an increased interest in EDGE within the next six to twelve months due to EDGE's tripled bandwidth.

WAP harmonizes radio platforms

Application developers need a standard that harmonizes Internet access across multiple radio platforms and on devices with diverse user interfaces. WAP aims to be a standard for wide-area wireless communication devices, not just phones. From this point, it makes sense to use the term "mobile" to refer to any device that requires a mobile telephony connection to exchange data, such as a PDA (personal digital assistant) or wireless-connected PC. The standard's guiding body, the WAP Forum, includes founder members Ericsson, Motorola, Nokia, and Phone.com (formerly Unwired Planet) and now comprises more than 500 members. Despite bad press because of users' frustration with limited bandwidth, WAP Forum's license-free, open-systems standard seems set to succeed on a global scale. Currently at version 1.2, you can download the full WAP specifications from www.wapforum.org.

Crucially, WAP is independent of radio architecture, operating in 3G and legacy environments as diverse as DECT (digital enhanced cordless telephony), GSM, Japan's PDC (personal digital cellular) system, TETRA, the main US systems, CDMA-based interim standard IS-95, TDMA-based IS-136, and the 1900-MHz GSM derivative, PCS 1900. WAP borrows heavily from Internet programming conventions and makes 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 CGI (common gateway interface) and ASP (active server pages) to provide dynamic Internet content to users on the move. APIs (application programming interfaces) allow WAP to work with a variety of operating systems, including dedicated mobile OSs, such as Palm's PalmOS and Symbian's Epoc.

The mobile includes a microbrowser that communicates with the gateway via WML (wireless markup language), a compact derivative of the Worldwide Web Consortium's XML (extensible markup language). Because HTML presents an entire page of information, the language is inappropriate for a small-screen device without a mouse and with limited keyboard facilities. WML decomposes pages into data items that present information or links to information, where each data item is a "card" and the whole page is a "deck." A scripting language, WMLScript, shares the same ECMA-262 origins (Reference 2) 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 re-using 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 WTA (wireless telephony application) environment provides the framework for creating mobile services that support the WAE (wireless application environment). The WTA user-agent is an extension to the higher-level WML entity and includes a WTA interface that manages functions, such as call handling, text-message processing, and phonebook management. Function libraries divide tasks into those that are common to all networks, network-specific functions, and public functions. To avoid the need 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 is also responsible for providing service indication information, such as notifying new message arrivals.

To request data from an Internet resource, the mobile issues a WSP (wireless session protocol) request using the normal Internet URL mechanism (Figure 2). Functionally, WSP is a binary version of the 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, which extends wired Internet access functions to suit the radio system (Figure 3). Each protocol stack layer presents a defined interface to its neighbors 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 communications with the WAP protocol stack's transport layer via API calls using the WDP (wireless datagram protocol) or UDP (user datagram protocol) message formats. The WCMP (wireless control message protocol) monitors non-Internet protocol datagrams and reports errors, such as fragmented messages.

Above the security function lies the WTP (wireless transaction protocol) that controls message exchanges. WTP can optionally send a message once ("send-and-forget"), resend the message if there's no acknowledgement from the recipient, or initiate a send-receive-acknowledge sequence for maximum reliability. The supervisory WSP (wireless session protocol) layer manages WDP, WTLS, and WTP services to support four message delivery options. The connectionless mode provides a simple datagram service that requires no acknowledgements; in contrast, connection-mode implies a longer session that requires reception acknowledgements and can retransmit lost data. Either mode can also employ WTLS security functions.

Develop free WAP applications

To promote their core products, such as server software, Ericsson, Nokia, and Phone.com provide free WAP SDKs (software development kits) on their Web sites, where you can also find on-line 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 and $350 per copy thereafter. The package 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 the XML markup language. 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 toolkits are on the way that will meet WAP Forum "June 2000" specification versions (formerly version 1.2), including support for push technology. Push technology allows a mobile to automatically download information from the service provider.

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 color displays, and the SDK includes a Windows-only UP.Simulator for test purposes. 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.

WAP's relative immaturity and continuous development complicates interoperability issues. To help application developers, the WAP Forum aims to ensure interoperability by layering compliance tests into device-level tests and content-level tests. Low-level device tests subdivide into application tests and protocol stack tests and use 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. To help you navigate the burgeoning new product maze, vendors maintain up-to-date lists of compatible devices and decks on their Web sites.

Author info

You can reach Contributing Editor David Marsh at forncett@compuserve.com.

Reference

"The future mobile market," UMTS Forum, March 1999, www.umts-forum.org.

European Computer Manufacturer Association's standard ECMA-262, www.ecma.ch.