OPC provides various standards that guarantee interoperability between OPC compliant devices. More recent standards focus on Web Services technology, such as [OPCXMLDA], described in chapter 3.1, that specifies data access in fieldbus systems with SOAP Web Services.
An OPC XML-DA compliant interface is based on an underlying OPC server that communicates with the IGUANA gateway. This server may be either situated on the gateway itself or on another hardware device, such as a personal computer100, as shown in figure 45.
According to the architectural design of the IGUANA gateway, an OPC XML-DA server is simply another Internet front-end, similar to the SNMP or LDAP Internet front-end. This OPC XML-DA server will communicate with the IGUANA main module, the ``Extended Service Daemon'' (ESD) and will basically ``translate'' the ESD protocol to OPC XML-DA compliant messages. Moreover, it will have to implement OPC specific functionality, such as browsing and subscription. To accomplish this task, the OPC server has to address several issues, such as data encoding, data type conversion, addressing and data buffering/caching.
![\begin{figure}\centering
\includegraphics[scale=0.7]{graphics/iguana_opcxmlda.eps}\end{figure}](img81.png)