The WTP provides a request/reply based transport mechanism. It
includes many of the facilities that are typical of reliable
transport protocols, such as retransmission and selective
retransmission of lost or corrupted messages, message
concatenation, and so on. For the purpose of efficiency, it
excludes the explicit session setup and tear down semantics typical
of reliable transports. It also provides an abort capability, where
a message can be sent to the server to indicate that the results of
any processing underway are no longer required.
To match the different requirements of different types of
applications, three classes of transport are defined:
Ø Class 0 is much like
UDP in the fixed-wire world, in that it provides an unreliable,
stateless connection, in which no result is communicated and the
abort function is not supported.
Ø Class 1 defines a
reliable stateful transport, which supports retransmission, but
does not transmit a result. The abort function is supported.
Ø Class 2 is also
reliable and stateful, but returns a result for every message sent.
This class also supports a 'hold on' reply as a result, which
indicates that the server is busy processing the request.
Wireless Transport Layer Security
The wireless equivalent of TLS is WTLS. It provides secure
transmission of data across the air network, and supports privacy,
message integrity and authentication functions. It is based on its
Internet equivalent, but can operate over unreliable transports, in
particular WDP and UDP.
Several key exchange suites are included, as well as a number of
symmetric and asymmetric encryption ciphers and signature
functions. Because much of its authentication is certificate based
it can be integrated with Public Key Infrastructure
(PKI).
Transport Layer
The preferred transport layer is UDP over IP. However, many of
the mobile networks cannot support IP at this point in time, so
there is an additional protocol that allows transmission of packets
over circuit-switched networks. This is the Wireless Datagram
Protocol. It provides a connectionless non-reliable transport
protocol with support for message segmentation and reassembly, and
UDP-based port numbers. It also provides a consistent interface to
all of the underlying over-the-air bearers.
Because WDP is an unreliable transport protocol, it needs a
message protocol to signal error conditions, which is the Wireless
Control Message Protocol. WCMP is similar to ICMP from the
fixed-wire world, and performs similar functions.
Bearers
Underlying the entire protocol stack is a number of over-the-air
bearers that can be used to communicate with the mobile device.
Some of these, such as GSM, IS-136, CDMA and iDEN, you may be
familiar with. All of the most popular standards are supported,
including some that you possibly wouldn't expect, like Short
Message Service (SMS).
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