Protocols for wireless applications
5.5 WIRELESS APPLICATION PROTOCOL (WAP)
The WAP architecture provides a scalable and extensible environment for application development for mobile communication devices. The WAP protocol stack has a layered design, and each layer is accessible by the layers above and by other services and appli- cations. The WAP layered architecture enables other services and applications to use the features of the WAP stack through a set of well-defined interfaces. External applications can access the session, transaction, security, and transport layers directly. The protocol stack of the WAP architecture is shown in Figure 5.1.
The Wireless Application Environment (WAE) is a general-purpose application envi- ronment based on the combination of WWW and Mobile Telephony technologies. The WAE allows operators and service providers to build applications and services that can reach wireless platforms in an efficient and useful manner. WAE contains a microbrowser environment containing the following functionality:
Application layer (WAE) Session layer (WSP) Transaction layer (WTP) Security layer (WTLS) Transport layer (WDP) Bearers: GSM CDMA etc.
Other services and applications
Figure 5.1 WAP architecture.
• Wireless Markup Language (WML): a lightweight markup language, similar to HTML, and optimized for use in handheld mobile devices;
• WMLScript: a lightweight scripting language, similar to JavaScript;
• Wireless Telephony Application (WTA): telephony services and programming inter- faces; and
• Content formats: a set of well-defined data formats, including images, phone book records, and calendar information.
Wireless Session Protocol (WSP) provides the application layer of WAP with an inter- face for two session services. The connection-oriented service operates above the Wireless Transaction Protocol (WTP). The connectionless service operates above a secure or non- secure Wireless Datagram Protocol (WDP). The WSP consists of services for browsing applications providing the following functionality:
• HTTP/1.1 functionality and semantics in a compact over-the-air encoding;
• Long-lived session state;
• Session suspend and resume with session migration;
• A common facility for reliable and unreliable data push; and
• Protocol feature negotiation.
The WTP runs on top of a datagram service and is suitable for implementation of Mobile Stations (MSs) as a lightweight transaction oriented protocol. WTP operates efficiently over secure or nonsecure wireless datagram networks and provides the fol- lowing features:
• Three classes of transaction service: unreliable one-way requests, reliable one-way
WIRELESS APPLICATION PROTOCOL (WAP) 87
• Optional user-to-user reliability – WTP user triggers the confirmation of each
received message;
• Optional out-of-band data on acknowledgments;
• Protocol Description Unit (PDU) concatenation and delayed Acknowledgement to
reduce the number of messages sent; and
• Asynchronous transactions.
Wireless Transport Layer Security (WTLS) is a security protocol based upon the indus- try standard Transport Layer Security (TLS) protocol, formerly known as Secure Sockets Layer (SSL). WTLS is used with the WAP transport protocols and is optimized for use over narrow-band communication channels. WTLS provides the following features:
• Data integrity: WTLS ensures that data sent between the terminal and an application server is unchanged and uncorrupted;
• Privacy: WTLS ensures that data transmitted between the terminal and an application server is private and cannot be understood by any intermediate parties that may have intercepted the data stream;
• Authentication: WTLS contains facilities to establish the authenticity of the terminal and application server;
• Denial of service protection: WTLS detects and rejects data that is replayed or not successfully verified. WTLS protects the upper protocol layers.
WTLS can also be used for secure communication between terminals, for authentication of electronic business card exchange. The applications can selectively enable or disable WTLS features depending on their security requirements and the characteristics of the underlying network.
WDP is the transport layer protocol in the WAP architecture. The WDP layer operates above the data capable bearer services supported by the various network types. WDP offers a consistent service to the upper layer protocols of WAP and communicates transparently over one of the available bearer services.
The WDP provides a common interface to the upper layer protocols, and the security, session, and application layers can function independently of the underlying wireless network. The transport layer is adapted to specific features of the underlying bearer. The global interoperability can be achieved by using mediating gateways.
The WAP protocols operate over different bearer services, including short message, circuit switched data, and packet data. The bearers offer different quality of service with respect to throughput, error rate, and delays. The WAP protocols are designed to com- pensate for or tolerate these varying levels of service.
The WAP-layered architecture enables other services and applications to use the fea- tures of the WAP stack through a set of well-defined interfaces. External applications can access the session, transaction, security, and transport layers directly. This allows the WAP stack to be used for applications and services not currently specified by WAP, but valuable for the wireless market.
Figure 5.2 shows possible protocol stacks using WAP technology. The first stack repre- sents a WAP application, a WAE user agent, running over WAP technology. The next stack
WAE WSP
WTP
WDP WDP
WTP
No layer WTLS No layer WTLS No layer WTLS
UDP UDP UDP
IP Non-IP IP Non-IP IP Non-IP
WDP WAP technology Outside of WAP Applications over datagram transport Applications over transactions WAE user agents
Figure 5.2 WAP stacks.
shows applications and services that require transaction services with or without security. The last stack shows applications and services that only require datagram transport with or without security.
5.6 SUMMARY
Mobile networks are growing in complexity and the cost of providing new value-added services to wireless users is increasing. To meet the requirements of mobile network operators, solutions must be: interoperable, scalable, efficient, reliable, and secure.
The WAP Forum is an industry group dedicated to the goal of enabling sophisticated telephony and information services on handheld wireless devices such as mobile tele- phones, pagers, PDAs and other WTs. Recognizing the value and utility of the World Wide Web architecture, the WAP Forum has chosen to align certain components of its technology very tightly with the Internet and the WWW. The WAP specifications extend and leverage mobile networking technologies (such as digital data networking standards) and Internet technologies, such as IP, HTTP, XML, URLs, scripting, and other content formats.
Structured data such as spreadsheets, address books, configuration parameters, financial transactions, technical drawings, and so on are often stored on a disk, for which they can use either a binary format or a text format. The latter allows the user, if necessary, to look at the data without the program that produced it. XML is a set of rules, guidelines, and conventions for designing text formats for such data, in a way that produces files that are easy to generate and read by a computer, that are unambiguous and that avoid common pitfalls such as lack of extensibility, lack of support for internationalization/localization, and platform-dependency.
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The XP specification must define the concept of an envelope or outermost syntactical construct or structure within which all other syntactical elements of the message must be enclosed. The envelope must be described with XML Schema.
The WAP protocols operate over different bearer services, including short message, circuit switched data, and packet data. The bearers offer different quality of service with respect to throughput, error rate, and delays. The WAP protocols are designed to com- pensate for or tolerate these varying levels of service.
The WAP-layered architecture enables other services and applications to use the fea- tures of the WAP stack through a set of well-defined interfaces. External applications can access the session, transaction, security, and transport layers directly. This allows the WAP stack to be used for applications and services not currently specified by WAP, but valuable for the wireless market.
PROBLEMS TO CHAPTER 5
Protocols for wireless applications
Learning objectives
After completing this chapter, you are able to
• demonstrate an understanding of mobile and wireless devices;
• explain the features of mobile and wireless devices;
• explain how to access the Web;
• explain the protocols, and applications;
• demonstrate an understanding of WAP.
• explain the role of WAE;
• explain WSP, WTP, and WDP;
• explain WTLS.
Practice problems
5.1: What are the features of mobile and wireless devices?
5.2: What are the special considerations for mobile devices when it comes to using Web information?
5.3: Why are XML files text files? 5.4: What is WAP architecture?
5.5: What is the functionality of the WAE microbrowser environment? 5.6: What is the role of WSP?
5.7: What is the role of WTP? 5.8: What is the role of WTLS? 5.9: What is the role of WDP? Practice problem solutions
5.1: Mobile and wireless devices are usually handheld devices, and accessing the World Wide Web presents a more constrained computing environment compared to desktop
computers because of fundamental limitations of power and form factor. Mass-market handheld wireless devices tend to have: less powerful CPUs, less memory (both ROM and RAM), restricted power consumption, smaller displays, and different input devices (e.g., a phone keypad, voice input, etc.).
5.2: Mobile devices need special consideration when it comes to using Web information. Their displays are generally much smaller than a conventional computer screen and are capable of showing only a small amount of text. On a cellular phone, for example, there may be only enough space for three or four rows of text. Palmtop pocket-sized computers have screens smaller than a PC or laptop, but large enough to read e-mail (electronic mail) and documents with a small amount of text. Mobile devices have limited memory and processing speeds, and these considerations also need to be taken into account.
5.3: XML files are text files because that allows experts, such as programmers, to more easily debug applications, and in emergencies they can use a simple text editor to fix a broken XML file. But the rules for XML files are much stricter than for HTML. A forgotten tag, or an attribute without quotes makes the file unusable, while in HTML such practice is often explicitly allowed, or at least tolerated. In the official XML specification, the applications are not allowed to try to second-guess the creator of a broken XML file; if the file is broken, an application has to stop right there and issue an error message.
5.4: The WAP architecture provides a scalable and extensible environment for applica- tion development for mobile communication devices. The WAP protocol stack has a layered design, and each layer is accessible by the layers above, and by other services and applications. The WAP-layered architecture enables other services and applications to use the features of the WAP stack through a set of well-defined inter- faces. External applications can access the session, transaction, security, and transport layers directly.
5.5: The WAE is a general-purpose application environment based on the combination of WWW and Mobile Telephony technologies. The WAE allows operators and service providers to build applications and services that can reach wireless platforms in an efficient and useful manner. WAE contains a microbrowser environment containing the following functionality:
• Wireless Markup Language (WML): a lightweight markup language, similar to HTML, and optimized for use in handheld mobile devices;
• WMLScript: a lightweight scripting language, similar to JavaScript;
• Wireless Telephony Application (WTA): telephony services and programming inter- faces; and
• Content formats: a set of well-defined data formats, including images, phone book records, and calendar information.
5.6: WSP provides the application layer of WAP with an interface for two session services. The connection oriented service operates above the WTP. The connectionless service operates above a secure or nonsecure WDP.
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5.7: The WTP runs on top of a datagram service and is suitable for implementation of MSs as a lightweight transaction oriented protocol. WTP operates efficiently over secure or nonsecure wireless datagram networks.
5.8: WTLS is a security protocol based upon the industry standard TLS protocol, formerly known as SSL. WTLS is used with the WAP transport protocols and is optimized for use over narrow-band communication channels.
WTLS can also be used for secure communication between terminals, for authenti- cation of electronic business card exchange. The applications can selectively enable or disable WTLS features depending on their security requirements and the charac- teristics of the underlying network.
5.9: WDP is the transport layer protocol in the WAP architecture. The WDP layer operates above the data capable bearer services supported by the various network types. WDP offers a consistent service to the upper layer protocols of WAP and communicates transparently over one of the available bearer services.