Roshan
Pakistan Web Magazine : INFORMATION
TECHNOLOGY
The
Wireless Application Protocol
Several years ago a company called
Unwired Planet developed a micro-browser and a markup language called
Handheld Device Markup Language (HDML). After some initial success with
individual vendors in the U.S., Unwired Planet decided to push for an
open standard in order to create the critical mass needed to create the
"next big thing" in information access. They began talking to
some of the bigger players in the mobile world, Ericsson, Nokia and
Motorola, about an open, unified standard. They dubbed the standard the
Wireless Access Protocol, and formed a group called the WAP forum to
jointly develop it. While HDML hasn't disappeared entirely, the Wireless
Access Protocol has eclipsed it. WAP has gotten a lot of press the last
couple of years and appears to be on the brink of becoming the standard
for handheld, mobile devices.
Unwired Planet changed its name to Phone.com and has successfully
marketed its line of "UP" browsers and servers to most of the
WAP vendors in the United States. They have also made some inroads in
Japan. WAP's growth in Europe is driven by a company that grew up on the
banks of a river in Finland called the Nokia, and by the Swedish
telecommunications giant L.M. Ericsson.
WAP's protocols mimic the World Wide Web's to a great degree. The
Wireless Markup Language (WML) is a dialect of XML, and uses the same
type of "tag" format as the HyperText Markup Language (HTML).
World Wide Web servers communicate with WWW clients (i.e. we browsers)
using an ASCII based protocol called the HyperText Transport Protocol
(HTTP). HTTP is a very simple protocol consisting of field names and
field contents separated by carriage return and linefeed characters.
HTML and HTTP themselves are transported using the Transaction Control
Protocol/Internet Protocol (TCP/IP) whose job is to make sure that any
data sent is properly delivered to its destination. TCP/IP creates what
is known in networking parlance as a "virtual circuit". Any
data sent is either delivered properly or triggers an error condition
back to the sender.
Although the WAP protocol mirrors Internet standards to a great degree,
it obstinately manages to be almost completely incompatible with them.
Because of this incompatibility, WAP devices cannot communicate directly
with WWW servers. The WAP protocols must first be translated from their
WAP formats to the protocols used by the WWW. This is why every WAP
device needs to communicate with a WAP gateway in order to request WML
pages (or "decks" as they are referred to in WAP) on a web
server. The WAP gateway's job is to translate the WAP binary protocols
into the HTTP text protocol that the World Wide Web servers
"speak".
The WAP protocol suite is built up on hierarchical series of layers,
some of which are optional. The bottom most layer is called the WAP
Datagram Protocol (WDP) and is responsible for moving WAP data from
sender to receiver and back again. WDP is patterned after TCP/IP's
little brother, the User Datagram Protocol (UDP). Unlike TCP/IP, the
protocol used for WWW data transfers, UDP makes no guarantees about the
delivery of data. It is known as a "best effort" delivery
service. What this means is that packets may be lost or corrupted in
transit and the receiver will never know that the data is missing. UDP
is generally used on the Internet for data that is not affected if a few
pieces are missing, like audio or video streams for example. UDP was
chosen as the basis for the WAP transport mainly because of its
compactness and simplicity. Like UDP, WDP is an unreliable, "best
effort" delivery protocol.
WDP is often exactly the same as UDP, though this is not always the
case. The rule of thumb, as expressed in the WAP technical
specifications, is that WDP is synonymous with UDP (i.e. they are the
same protocol) if, and only if, the WAP devices are communicating on a
network where Internet protocols are available. Basically this means
that if UDP is present, then WDP equals UDP. But in cases where no
Internet protocols are available, the WAP specifications designate
different forms of UDP "mapping" protocols that create the
equivalent of UDP (usually through some kind of text message exchange).
The WDP to UDP "mapping" protocols specified so far are SMS,
SMS CSID, USSD, and ..95.
The next protocol up the ladder is the WAP Transaction Protocol (WTP).
WTP is designed to add back the pieces that UDP lacks compared to its
bigger brother TCP/IP. WTP's job is to make sure that packets sent via
WDP actually arrive at their destination. It does this by waiting for an
"ACK" or acknowledgement packet back from the receiver. Any
packet sent the other side must explicitly acknowledge using WTP. If an
acknowledgement packet is not received within a specific time window,
WTP will resend the packet. Unacknowledged packets will be resent a
certain number of times until an error is generated if no acknowledgment
is received. WTP is an optional protocol, however, and is not present in
all WAP communications.
Above WTP, sits the WAP Session Protocol (WSP), which is responsible for
two different types of functionality. The first is to create a
"session" between the WAP client and the WAP gateway. Each
session has a unique ID and must be explicitly started, stopped,
resumed, or disconnected. This mode is known as "connected" or
"continuous" mode WSP, and it is always used in conjunction
with WTP. A WAP client must explicitly create a session, and each packet
exchange must be sent and acknowledged via WTP (thus guaranteeing either
delivery or an error condition). But, again, WTP is an optional protocol
and so is using WSP connected mode.
The other mode of WSP exactly mirrors the functionality of HTTP. WSP's
functionality in this case is not optional and is present in every WAP
exchange. If WAP is operating with just the WSP protocol then no
sessions is created and communication becomes a simple request-response
pairing. This is very similar to how the WWW functions. When WSP is used
alone it is referred to as "connectionless" or
"temporary" WSP. Connectionless mode WSP is also called WSP/B
in some of the documentation available from the WAP forum.
