|
|
Wireless Application Protocol - WAP WAP Communication Protocol & its ComponentsThe
WAP Protocols cover both the application (WAE), and the underlying transport
layers (WSP and WTP, WTLS, and WDP). WML and WMLScript are collectively known as
WAE, the Wireless Application Environment. As described earlier the 'bearer'
level of WAP depends on the type of mobile network. It could be CSD, SMS, CDMA,
or any of a large number of possible data carriers. Whichever bearer your target
client is using, the development above remains the same. WAE WML For
those who know HTML, WML should be quite easy to learn since many tags and
attributes in WML are almost the same as in HTML, but there are fewer tags.
Unlike HTML, you can play around with variables in WML, making it more dynamic.
In WML it is also possible to have many sub-pages (cards) in one WML page
(deck). Each WML card works like a web page and it's content is displayed to the
user. WML
introduces the concept of decks and cards. A WML deck consists of one or several
cards, and each card functions like an independent page, with it’s own title
and preferences. This approach helps conserve valuable bandwidth because so the
HTTP header information is not occupying bandwidth for each single little page. WML is an application of XML. So WML is more XML than HTML, but the syntax of WML is a lot like HTML. Where as in HTML one can take some liberties of skipping some end tags and omit quotation marks, the rules in WML are much stricter. In WML documents, the DOCTYPE definition must always be written in the beginning of each deck, after the XML declaration, while this is optional in HTML documents. The head tag in WML is not used very often. Instead, http header information is sent using some server-side script language. In WML the <card> tag replaces the <head> tag in HTML. There is no body tag in WML. In WML, it is possible to use variables, which opens for slight dynamic functionality. My
aim is not to teach you how to code in WML and WML script, I’ll briefly talk
about the WML syntax to give a better feel of WML. All decks must start with the following XML header: The first line gives the version number of the XML version used. The second line shows the URL to the Document Type Definition provided by W3C The body of a WML file starts with <wml>. The 'id' must be unique inside each deck. Whereas the id is used to navigate between different cards in a deck the title is not that significant. It actually specifies the text that is presented on top of the window in WAP browsers but not all browsers display it. In order to display text inside a card, you must write a
paragraph start and end tag and apply the text inside them: align= ”center” or “left” or “right” and line break <br/> tags do the normal formatting. (tags that do not delimit content must have a '/' at the end) The image tag is similar as in HTML. <img src="image.wbmp" alt="description"/> Note that there is no closing tag for the <img> tag, but as you can see, the tag ends with a / (slash) unlike the tags that need a closing tag Links can either be written like this: In the example below, the
second card automatically loads after 4 seconds. <card
id="card1" title="" ontimer="#card2"> <card id="card2" title="Card two">
Note that
there is a limitation on the maximum compiled deck code size. The size
limitation varies from phone to phone. Maximum size for a compiled deck or image
on Nokia 7110 is 1397 bytes. Maximum deck size on Ericsson R320 is 3000 bytes
(excluding images). Therefore it
makes sense to limit the size of your deck to less than 1300 bytes. WML SCRIPT Server based
computation means that several round trips to and from mobile to server have to
made in case of any interaction or computation. This is not very desirable in
case of low bandwidth systems. WMLScript allows code to be built into files
transferred to mobile client so that
many of these round-trips can be eliminated. WML script also allows developer to
provide interactivity in WAP pages without taxing the very valuable air
interface. You can get
lots of examples on WML script at www.wmlscript.com/library/ A big
difference with javascript though, is that WMLScript must be kept in a separate
file, and can not be used with inline tags like JavaScript. WMLScript can
perform mathematical calculations, manipulate strings of text, make redirections
etc. A listing the entire syntax of WMLScript can be obtained from specification at WAPForum. A good WMLScript tutorial available at Wireless Developer Network WBMPWBMP stands for Wireless BitMaP. It is the default picture format for WAP. The current version of WBMP is called type 0. WBMPs are uncompressed, monochrome black/white bitmaps intended for use in devices with small screens and narrow bandwidth connection. The constraints when using WBMP are the small screen size, limited graphics capabilities and the limited bandwidths available. As a thumb rule, a WBMP should not be wider than 96 pixels and higher than 48 pixels (at 72 dots per inch). Nokia’s toolkit have a nice WBMP editor where you can draw and edit. Other WBMP tools create the WBMPs from BMPs, GIFs or JPGs. There are also plug-ins available for Paintshop, Photoshop and Gimp, which let you save WBMP files with these programs.
You
can also
use online services like to convert
your existing pictures to WBMP A look at the following pictures will give you a clear idea about graphics and graphics support in WAP. Remember that WAP is designed to enable Internet access in low bandwidth systems. Therefore the idea is providing content fit for low bandwidth and low resolution devices rather than providing high resolution content. If you are confused as to which picture is in WDMP format, feel free to mail me!
WSP WTA The Security layer protocol in the WAP architecture is called the Wireless Transport Layer Security, WTLS. The WTLS layer operates above the transport protocol layer. The WTLS layer is modular and it depends on the required security level of the given application whether it is used or not. WTLS provides the upper-level layer of WAP with a secure transport service interface that preserves the transport service interface below it. In addition, WTLS provides an interface for managing (e.g., creating and terminating) secure connections. WTLS is fast becoming the de facto standard for providing privacy, data integrity, and authentication for applications in cellular phones and other small wireless terminals. WTLS bears a close resemblance to the SSL and TLS protocols. Already WTLS has found place in a large number of mobile devices. WTLS,
though similar to SSL and TLS has some differences basically because of the
nature of mobile data communications. The differences arise due to specific
requirements of the WTLS protocol because of the constraints presented by the
mobile data systems.
In
other words, design of WTLS is based on the requirements to provide WTLS
is designed to function on connection-oriented and/or datagram transport
protocols. Security is assumed to be an optional layer above the transport
layer. The security layer preserves the transport service interfaces. The
session or application management entities are assumed to provide additional
support required to manage (e.g., initiate and terminate) secure connections.
Using WTLS the client and server agree on protocol options to use. The
negotiation may include the security parameters (e.g., cryptographic algorithms
and key lengths), key exchange and authentication. Since this exchange of
parameters involve several steps it is possible for either client or server to
terminate the negotiation process at will (if e.g. some parameters are not
supported. For
more details you can refer to the WTLS specification at WAP forum WDP WTP The
protocols in the WAP family are designed for use over narrow band bearers in
wireless telecommunications networks. Since the WDP protocols provide a common
interface to the upper layer protocols (Security, Transaction and Session
layers), they are able to function independently of the underlying wireless
network. This is accomplished by adapting the transport layer to specific
features of the underlying bearer. WDP services include application addressing
by port numbers, segmentation and reassembly of messages and error detection
(optional)
Source: WAP- WDP Specifications As I mentioned above
WDP can be mapped onto different bearers, with different characteristics. In
order to optimize the protocol with respect to memory usage and radio
transmission efficiency, the protocol performance over each bearer may vary.
However, the WDP service and service primitives will remain the same, providing
a consistent interface to the higher layers. This is visible in the figure
above. WDP supports several simultaneous communication instances from a higher
layer over a single underlying WDP bearer service. The port number identifies
the higher layer entity above WDP. This may be another protocol layer such as
the Wireless Transaction Protocol (WTP) or the Wireless Session Protocol (WSP)
or an application such as electronic mail. By reusing the elements of the
underlying bearers, WDP can be implemented to support multiple bearers and yet
be optimized for efficient operation within the limited resources of a mobile
device. The figure below describes the WDP architecture.
(Dedicated to providing comprehensive information to mobile computing community of IT professionals, user organizations, and vendors) |
|||||||||||||||||||||||||||||||||
