WAP -> Gateways

The simplest way to think about a WAP gateway is that it is the system that WAP devices communicate directly with, and any content delivered to WAP devices via IP networks (like the Internet) must travel through these gateways on their way to those little, tiny screens.

The WAP gateway performs a host of duties, which we'll cover here in detail, but the most important is the protocol translations that are necessary to connect the TCP/IP-based world of the wired Internet with the WAP-based world of wireless mobile devices. Without something to perform this essential task, WAP simply wouldn't work.

A WAP gateway is a piece of software that usually resides on a dedicated Unix-type of server. Often the box itself will also be referred to as the gateway, but for the purposes of this article we will only be considering the software functionality of WAP gateways. There are also WAP gateways that can run on Windows-based systems, as well as combination content-server/gateway products, but the norm is to have a stand-alone Unix server running the gateway software

What does a Gateway Do?

• implementation of the WAP stack
• converting protocols
• converting markup languages
• compiling WMLScript programs
• encoding WML into a binary bitstream
• providing access control
• caching
• domain name resolution services (DNS)
• security features

Clearly, these gateways can pack a lot of punch. There is a wide range of products available, with differing feature sets and price ranges (from free to big bucks) so we'll deal with the most commonly implemented and important features here.

Providing the software to maintain the WAP stack layers is a critical function of any WAP gateway, though they will not all implement the same layers. Gateways that support connectionless services will not need to implement the WTP layer, just as products that don't support secure services won't need to include the WTLS layer. A gateway that will be used for secure, connection-oriented services will need to implement WSP, WTP, WTLS, and WDP.

Converting between WAP's WSP and the Internet's HTTP is another central function of any WAP gateway. One of the main differences between WSP and HTTP is that WSP's headers are encoded in binary form, or "tokenized." For example, the token "Accept" from the HTTP/1.1 header becomes "0x80". This is a way for WAP to save on bandwidth by transmitting the bare minimum bits possible to represent a specific group of characters.

Some gateways also offer HTML to WML conversion, though this feature rarely results in functional WAP services. It is generally agreed that mobile applications should serve WML directly from the origin server, and that the code needs to be designed and tested separately from HTML implementations.

Another important function of the WAP gateway is the compilation of WMLScript programs. WMLScript is the only scripting language that works with WAP devices, and although there are many similarities to Javascript, there are some key differences in how it is called and compiled. Where Javascript code is enclosed inside of HTML files and interpreted by the client browser, WMLScript code is put in a separate file, and compiled at the gateway, in the constant quest to reduce processing requirements at the device end. The compiled script is then sent to the device as a binary encoded stream, also reducing bandwidth.

Encoding the WML content is another mandatory function of a gateway, and again it is done to save on the precious bandwidth of the wireless networks. By converting the WML to a binary, tokenized format, the amount of data sent over the wireless network is greatly diminished. The gateway also checks the WML for errors as part of this process, which is rigorous as WML is derived from XML and has strict syntax requirements.

List of WAP Gateways

http://www.wapdrive.com/DOCS/wap_gateway/index.html

Some WAP gateways are free to use, and some of the gateways require registration with the service provider.

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