The Internet Encyclopedia (Volume 3)

P1: IML

Cassel WL040/Bidgoli-Vol III-Ch-65 September 15, 2003 8:52 Char Count= 0

WIRELESSMARKUPLANGUAGE 807

vCARDandvCalendarIndustry standards for sharing

address and calendar information.

Wireless Session Protocol

WSP offers the application layer consistent interfaces for

session services. Two modes are offered. A connection

mode runs over the wireless transaction protocol and

a connectionless service runs directly over a datagram

transport service. Initial versions of WSP offer browsing

capabilities including the equivalent of HTTP with im-

plementation more suitable for wireless devices, plus

facilities for long-lived sessions, session suspend/resume,

data push, and capability negotiation (Wireless Appli-

cation Protocol Wireless Session Protocol Specification,

1999; Wireless Transport Layer Security V, 2001).

Wireless Transaction Protocol

WTP provides services to allow browsing; specifically,

WTP provides request response interaction between a

client and a server. Unlike regular Web surfing with HTTP,

WTP runs over a datagram service at the network layer.

WTP enhances the service provided by unreliable data-

grams in order to provide appropriate service levels to the

higher layers, relieving the wireless device of the need to

support all the services of TCP but providing a reliable

service to the application. By using a datagram transport

service, WTP eliminates the need for connection estab-

lishment and connection release activities. The protocol

transmits a message as its basic unit, rather than a stream

of bytes. WTP defines three classes of service:

Class 0Unreliable invoke message with no result mes-

sage,

Class 1Reliable invoke message with no result message,

and

Class 2Reliable invoke message with exactly one reliable

result message.

Class 2 is the basic invoke/response service. Class 0 is

available for an invoke message that will not be retrans-

mitted if delivery fails and for which no response is ex-

pected. Class 1 is used for an invoke message that must

be delivered or re-sent. The variations allow an applica-

tion to function with a minimum of data transmission

required.

Wireless Transport Layer Security

WTLS exists to provide privacy and security between com-

municating entities in a wireless environment. Security

includes both data integrity and authentication services.

The services of WTLS are optional and their use depends

on the nature of the application and the vulnerability of

the data it transmits.

Datagrams

Datagrams are transport protocol mechanisms that of-

fer best-effort delivery service without the high overhead

required to provide highly dependable transport. Data-

gram transport protocols are lightweight, fast, and suit-

able for situations where the underlying network services

are highly reliable or where the application will provide

Client

Server

Protocol

Server

CGI

processes

Content

Requests

Responses

Figure 2: Direct interaction between the wireless client and

the wireless application server.

reliability and so services at the transport layer are redun-

dant.

Access to wireless applications can be provided by di-

rect connection to a WAP server that is also a content

provider, as illustrated in Figure 2. However, because most

of the Internet content is present in Web sites designed to

respond to HTTP requests and formatted in HTML that

is often not suitable for wireless devices, standard access

includes a gateway. The gateway receives requests using

the WAP protocols and passes those requests to conven-

tional Web servers using HTTP. The gateway reformats re-

sponses for display on the limited device and encodes the

response for transmission using the WAP protocol stack.

Figure 3 shows wireless client interaction with a conven-

tional Web server.

WIRELESS MARKUP LANGUAGE

WML Basics

As we have seen, normal HTML-specified Web pages are

seldom suitable for display on handheld wireless devices.

The WAP protocol suite includes an XML language de-

signed to specify items for display on these devices. The

wireless markup language (WML) borrows heavily from

HTML, but eliminates features not suitable for these de-

vices and adds functions that address the screen size and

user input options (Evans & Ashworth, 2001; Wireless

Markup Language Version 2.0, 2001).

The first difference to notice is the paradigm of pre-

sentation. Where the Web metaphor is a page, the WML

metaphor is a card. A single application display is a deck

Client

Gateway

interface

Encoders and

decoders

Server

Protocol

Server

CGI

processes

Content

Requests

Responses

Requests

Figure 3: Wireless application client interaction with a con-

ventional Web server.