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1. Introduction

The Wireless Application Protocol (WAP) is a hot topic that has been widely hyped in the mobile industry and outside of it. WAP is simply a protocol- a standardized way that a mobile phone talks to a server installed in the mobile phone network. It is amazing how in just six months, it has become imperative for all Information Technology companies in Nordic countries and beyond to have a WAP division. Many many advertising agencies and "dot.coms" have announced WAP services.

WAP is hot for several reasons:

  • It provides a standardized way of linking the Internet to mobile phones, thereby linking two of the hottest industries anywhere. 
  • Its founder members include the major wireless vendors of Nokia, Ericsson and Motorola, plus a newcomer Phone.com.
  • By April 2000, the WAP Forum had over 350 member companies.

Mobile information services, a key application for WAP, have not been as successful as many network operators expected. WAP is seen as a way to rectify this situation. 

WAP also has its detractors and controversies:

  • It is very difficult to configure WAP phones for new WAP services, with 20 or so different parameters needing to be entered to gain access to a WAP service.
  • Compared with the installed base of Short Message Service (SMS) compliant phones, the relative number of handsets supporting WAP is tiny. WAP is a protocol  that runs on top of an underlying bearer. None of the existing GSM bearers for WAP-  the Short Message Service (SMS), Unstructured Supplementary Services Data (USSD)  and Circuit Switched Data (CSD) are optimized for WAP.
  • The WAP standard is incomplete, with key elements such as Push (proactive sending of information to mobile devices) and wireless telephony (updating address reports and the like) included in the WAP 1.2, standardized in late 1999 and first expected to be implemented in the Spring of 2000.
  • There are many WAP Gateway vendors out there competing against each other with largely the same standardized product. This has led to consolidation such as the pending acquisition of APiON by Phone.com.
  • Other protocols such as SIM Application Toolkit and Mobile Station Application Execution Environment (MexE) are respectively already widely supported or designed to supercede WAP.
  • WAP services are expected to be expensive to use since the tendency is to be on-line for a long Circuit Switched Data (CSD) call as features such as interactivity and selection of more information are used by the end user. Without specific tariff initiatives, there are likely to be some surprised WAP users when they see their mobile phone bill for the first time after starting using WAP.

2. Background FORMATION

Motorola, Nokia, Ericsson and the US software company Phone.com (formerly Unwired Planet) were the initial partners that teamed up over two years ago in mid 1997 to develop and deploy the Wireless Application Protocol (WAP). WAP is an attempt to define the standard for how content from the Internet is filtered for mobile communications. Content is now readily available on the Internet and WAP was designed as the (rather than one) way of making it easily available on mobile terminals. 

The WAP Forum was formed after a US network operator Omnipoint issued a tender for the supply of mobile information services in early 1997. It received several responses from different suppliers using proprietary techniques for delivering the information such as Smart Messaging from Nokia and HDML from Phone.com (then called Unwired Planet). Omnipoint informed the tender responders that it would not accept a proprietary approach and recommended that that various vendors get together to explore defining a common standard.

After all, there was not a great deal of difference between the different approaches, which could be combined and extended to form a powerful standard. These events were the initial stimulus behind the development of the Wireless Application Protocol, with Ericsson and Motorola joining Nokia and Unwired Planet as the founder members of the WAP Forum.

PHILOSOPHY

The Wireless Application Protocol takes a client server approach. It incorporates a relatively simple microbrowser into the mobile phone, requiring only limited resources on the mobile phone. This makes WAP suitable for thin clients and early smart phones. WAP puts the intelligence in the WAP Gateways whilst adding just a microbrowser to the mobile phones themselves. Microbrowser-based services and applications reside temporarily on servers, not permanently in phones. The Wireless Application Protocol is aimed at turning a mass-market mobile phone into a "network-based smartphone". As a representative from Phone.com (formerly Unwired Planet) on the board of the WAP Forum commented "The philosophy behind Wireless Application Protocol's approach is to utilize as few resources as possible on the handheld device and compensate for the constraints of the device by enriching the functionality of the network".

The Wireless Application Protocol is envisaged as a comprehensive and scaleable protocol designed for use with: 

  •  any mobile phone from those with a  one line display to a smart phone 
  •  any existing or planned wireless service such  as the Short Message Service, Circuit Switched Data, Unstructured Supplementary Services Data (USSD)  and General Packet Radio Service (GPRS). 

Indeed, the importance of WAP can be found in the fact that it provides an evolutionary path for application developers and network operators to offer their services on different network types, bearers and terminal capabilities.

The design of the WAP standard separates the application elements from the bearer being used. This helps in the migration of some applications from SMS or Circuit Switched Data to GPRS for example. 

  • any mobile network standard such as Code Division Multiple Access (CDMA), Global System for Mobiles (GSM), or Universal Mobile Telephone System (3GSM). WAP has been designed to work with all cellular standards and is supported by major worldwide wireless leaders such as AT&T Wireless and NTT DoCoMo. 
  • multiple input terminals such as keypads, keyboards, touch-screens and styluses. 

 

3. Technical Introduction

The Wireless Application Protocol embraces and extends the previously conceived and developed wireless data protocols. Phone.com created a version of the standard HTML (HyperText Markup Language) Internet protocols designed specifically for effective and cost-effective information transfer across mobile networks. Wireless terminals incorporated a HDML (Handheld Device Markup Language) microbrowser, and Phone.com's Handheld Device Transport Protocol (HDTP) then linked the terminal to the UP.Link Server Suite which connected to the Internet or intranet where the information being requested resides. The Internet site content was tagged with HDML. 

This technology was incorporated into WAP- and renamed using some of the

many WAP-related acronyms such as WMLS, WTP and WSP. Someone with a

WAP-compliant phone uses the in-built microbrowser to: 

1. Make a request in WML (Wireless Markup Language), a language derived from HTML especially for wireless network characteristics. 

2. This request is passed to a WAP Gateway that then retrieves the information from an Internet server either in standard HTML format or preferably directly prepared for wireless terminals using WML. If the content being retrieved is in HTML format, a filter in the WAP Gateway may try to translate it into WML. A WML scripting language is available to format data such as calendar entries and electronic business cards for direct incorporation into the client device. 

3. The requested information is then sent from the WAP Gateway to the WAP client, using whatever mobile network bearer service is available and most appropriate.

4. WAP Protocol Stack

WAP has a layered architecture as shown in the diagram below:

Next we'll take a look at each layer in the WAP protocol stack:

Wireless Application Environment (WAE)

Wireless Session Protocol (WSP)

Wireless Transaction Protocol (WTP)

Wireless Transport Layer Security (WTLS)

Wireless Datagram Protocol (WDP)

Bearers e.g. Data, SMS, USSD

WIRELESS APPLICATION ENVIRONMENT

The WAE defines the user interface on the phone. The application development environment to facilitate the development of services that support multiple bearers. To achieve this, the WAE contains the Wireless Markup Language (WML), WMLScript- a scripting micro-language similar to JavaScript- and the Wireless Telephony Application (WTA). These are the tools that allow WAP-based applications to be developed.

 

WIRELESS SESSION PROTOCOL

A sandwich layer that links the WAE to two session services- one connection oriented operating above the Wireless Transaction Protocol and a connectionless service operating above the Wireless Datagram Protocol.

WIRELESS TRANSACTION PROTOCOL

Runs on top of a datagram service such as User Datagram Protocol (UDP); part of the standard suite of TCP/IP protocols, to provide a simplified protocol suitable for low bandwidth mobile stations. WTP offers three classes of transaction service: unreliable one way request, reliable one way request and reliable two way request respond. Interestingly, WTP supports Protocol Data Unit concatenation and delayed acknowledgement to help reduce the number of messages sent. This protocol therefore tries to optimize the user experience by providing the information that is needed when it is needed- it can be confusing to received confirmation of delivery messages when you are expecting the information itself. By stringing several messages together, the end user may well be able to get a better feel more quickly for what information is being communicated.

WIRELESS TRANSPORT LAYER SECURITY

WTLS incorporates security features that are based upon the established Transport Layer Security (TLS) protocol standard. Includes data integrity checks, privacy on the WAP Gateway to client leg and authentication.

WIRELESS DATAGRAM PROTOCOL

Allows WAP to be bearer independent by adapting the transport layer of the underlying bearer. WDP presents a consistent data format to the higher layers of the WAP protocol stack thereby conferring the advantage of bearer independence to application developers.

5. Optimal WAP Bearer

SHORT MESSAGE SERVICE

Given its limited length of 160 characters per short message, SMS may not be an adequate bearer for WAP because of the weight protocol of the protocol. The overhead of the WAP protocol that would be required to be transmitted in an SMS message would mean that even for the simplest of transactions several SMS messages may in fact have to be sent. This means that using SMS as a bearer can be a time consuming and expensive exercise. Only one network operator- SBC of the US- is known to be developing WAP services based on SMS.

 

CIRCUIT SWITCHED DATA

Most of the trial WAP based services use CSD as the underlying bearer. Since CSD has relatively few users currently, WAP could kick start usage of and traffic generated by this bearer. However, CSD lacks immediacy- a dial up connection taking about 10 seconds is required to connect the WAP client to the WAP Gateway, and this is the best case scenario when there is an complete end to end digital call- in the case of the need for analog modem handshaking (because the WAP phone does not support V.110 the digital protocol, or the WAP Gateway does not have a digital direct connection such as ISDN into the mobile network), the connect time is increased to about 30 seconds.

UNSTRUCTURED SUPPLEMENTARY SERVICES DATA

Unstructured Supplementary Services Data (USSD) is a means of transmitting information or instructions over a GSM network. USSD has some similarities with SMS since both use the GSM network's signaling path. Unlike SMS, USSD is not a store and forward service and is session-oriented such that when a user accesses a USSD service, a session is established and the radio connection stays open until the user, application, or time out releases it. This has more in common with Circuit Switched Data than SMS. USSD text messages can be up to 182 characters in length. 

USSD has some advantages and disadvantages as a tool for deploying services on mobile networks: 

  • Turnaround response times for interactive applications are shorter for USSD than SMS because of the session-based feature of USSD, and because it is NOT a store and forward service. According to Nokia, USSD can be up to seven times faster than SMS to carry out the same two-way transaction. 
  • Users do not need to access any particular phone menu to access services with USSD- they can enter the Unstructured Supplementary Services Data (USSD) command direct from the initial mobile phone screen. 
  • Because USSD commands are routed back to the home mobile network's

Home Location Register (HLR), services based on USSD work just as well and in exactly the same way when users are roaming. 

  • Unstructured Supplementary Services Data (USSD) works on all existing GSM mobile phones. 
  • Both SIM Application Toolkit and the Wireless Application Protocol support USSD. 
  • USSD Stage 2 has been incorporated into the GSM standard. Whereas 
  • USSD was previously a one way bearer useful for administrative purposes such as service access, Stage 2 is more advanced and interactive. By sending in a USSD2 command, the user can receive an information services menu. As such, USSD Stage 2 provides WAP-like features on EXISTING phones. 
  • USSD strings are typically complicated for the user to remember, involving the use of the "*" and "#" characters to denote the start and finish of the USSD string. However, USSD) strings for regularly used services can be stored in the phonebook, reducing the need to remember and reenter them.

As such, USSD could be am ideal bearer for WAP on GSM networks.

 

5. WAP Development Issues

WAP version 1.2 may be the first version of the protocol that is actually workable in terms of delivering easy to use and innovative non-voice mobile services.

WAP version 1.2 is expected to be finalized as a specification in late 1999 and first available in spring 2000. It will support Push services (proactive delivery of information from a WAP Gateway to a WAP terminal), User Profiles, WDP Tunneling, WMLscript, CryptoLibrary, Wireless Telephony Application, Wireless Application Environment enhancements and other features. There are several non-standardized or unresolved issues relating to WAP that application developers should be aware of:

PUSH NOT SUPPORTED

The WAP WSP specification defines the WSP push operation and a WSP push PDU (Protocol Data Unit). A push operation is not specified for the HTTP protocol, used by the WAP Gateway server to communicate with content hosts.

To support pushes, the server has to provide an application interface to allow server based applications to generate a push to a mobile client. The support of pushes on the client side depends on the capabilities of the handsets to handle pushed content. The Nokia OTA configuration proposal to the WAP Forum describes the use of a connectionless push over the SMS bearer, to transfer the configuration data to the handset.

WIRELESS TELEPHONY APPLICATION DELAYED

The so-called WirelessTelephony Application (WTA) was only defined by the WAP Forum in June 1999. The WTA gives WAP some of the features that SIM Application Toolkit incorporates such as access to phone report and call handling.

LACK OF COOKIES FOR SESSION MANAGEMENT

There are no "cookies" for session management, i.e. to hold the session together. Cookies are used on the fixed Internet to identify the web browser and thereby assist in providing customized and streamlined services. Instead, some WAP applications use indexes in the URL as an alternative. 

The cookie information is transmitted via HTTP headers. Because WAP WSP is based on HTTP headers, it should be possible to transmit cookie information to the clients. The problem may be the clients itself, which may currently not support the handling of cookie HTTP header information or to save this information to a persistent storage in the mobile phone.

PREMATURE ENCRYPTION ENDPOINT

The Wireless Transport Layer Security defines encryption between the Mobile Station and the WAP Gateway. The "endpoint" of the encrypted WTLS data is the WAP Gateway proxy server. To have a secure connection to a content host (e.g. banking server) the Gateway proxy server has to establish secure (https) connections to this hosts. In this case the proxy server has access to the decrypted data received via WTLS from the mobile station or from the content host via https.

SMALL DOWNLOADABLE UNIT SIZE

WAP incorporates no compression techniques for the textual content, although the WML markup commands are compressed. Additionally, the "deck"- the smallest unit of downloadable information in Wireless MarkUp Language- is limited to a maximum of 1400 bytes. This means that applications need to be specifically designed to be very code efficient by using templates and variables and keeping information on the server and using the cache on the phone. 

WML byte code converting defines a (maybe inefficient) compression technique by string tables. With this technique duplicate strings in the WMLC bytecode are avoided. This reduces the size of the data to transfer to the mobile client. The WSP SDU size of 1400 bytes is a default value. An increased size may be negotiated by a mobile client within the WSP capabilities. The WAP transport layer (WTP) is able to handle greater SDU sizes than 1400 too, by using SAR (Segmentation and Re-assembly).

WDP DATAGRAM PROTOCOL

The September 1999 London meeting of the WAP Forum included a decision from the SMS Experts Group that the single common standardized interface between the SMS Center and the WAP Gateway would be a subset of SMPP (Short Message Peer to Peer Protocol). A PDU (Protocol Data Unit) set has been added to SMPP version 3.4 for this purpose. There will be no SMPP specific legacy- in other words, SMS Center manufacturers that do not support SMPP can evolve their SMS Center external interface to support the new SMPP commands for connecting to WAP Gateways. 

However, programmers need to be aware of them when they commence WAP application design.

6. WAP Developer's Toolkits

There are at least four WAP toolkits available for software developers to use to assist in the speedy development of WAP-based services. These are supplied by Dynamical Systems Research (DSR), Ericsson, Nokia and Phone.com.

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