Java Reference
In-Depth Information
screens of information to a user. Lengthy documents can be divided into smaller bite-sized
pieces, which eases the display constraint. Also, links and extended logic (using the client-side
scripting language, WMLScript) enable the user to traverse through a tree of information with-
out requiring constant trips back to a server. WAP also defines a unique image format, the
Wireless Bitmap or WBMP, which is essentially a one-pixel-depth compressed bitmap for
graphic display on constrained devices.
Network Considerations
Unlike the World Wide Web, WAP was designed to support a variety of networks and trans-
ports, not just TCP/IP. As mentioned in the following section, “Bandwidth Considerations,”
WAP can be delivered using a variety of bearers, including SMS and Circuit-Switched Data.
(The
bearer
is a telecommunications service that enables transmission of user information sig-
nals between user-network interfaces.) Because nearly all wireless networks are not yet packet-
based, it was important to design a technology that transparently supported a wide variety of
underlying networking protocols. WAP does this through the Wireless Datagram Protocol, or
WDP. WDP is the interface to the wireless network's bearer service, which can be a variety of
technologies including SMS, CSD, CDMA, and UDP. WDP also provides support for error
correction, which comes in very handy given the “messy” connections often prevalent on wire-
less networks.
Bandwidth Considerations
Whereas HTTP assumes a constant TCP/IP connection, WAP's Wireless Transaction Protocol,
or WTP, does not. In fact, TCP itself was deemed to be too burdensome for use on IP-based
wireless networks. On IP-based network connections (such as connections obtained from a
packet-based network such as GPRS and CDPD), UDP (the User Datagram Protocol) is used
in conjunction with IP for information delivery. Bandwidth constraints are also addressed
through data compression. A WML file stored on a server (or WML output from server-side
technologies such as Java servlets) is compressed into a WMLC file using a WAP Gateway that
sits between the server and the communications network. A WAP gateway has two primary
responsibilities:
1.
Act as an intermediary between the “back-end” TCP/IP network and the wireless net-
work.
2.
Encode textual WML/WMLScript content obtained from an HTTP server into a com-
pressed binary format for delivery to the mobile device.
WAP gateways can also perform additional tasks. A few gateways on the market will actually
grab HTML content and attempt to intelligently convert the content to WML on-the-fly.
Several other gateways include HTTP and Java servlet scripting engines in the product directly.
