Future Work

There are several other SOAP related issues that should be addressed to further examine the suitability of SOAP Web Services as an Internet/fieldbus interface, such as the following:

Security:

Security is more or less not addressed in this thesis. However, many fieldbus applications may need common security features, such as encryption, authentication and access control. The SOAP specification itself does not address security, instead it relies on the underlying protocol that transports the SOAP messages. Most often this will be HTTP, therefore common security measures such as HTTPS for encryption and HTTP basic authentication may be used. It will be interesting to examine whether these technologies are appropriate for fieldbus access.

Performance:

As denoted above, SOAP Web services tend to have significant hardware requirements. As fieldbus gateways will often be implemented on embedded hardware which has limited resources, the deployment of SOAP Web services on fieldbus gateways may lead to performance problems. Therefore careful selection of server modules, such as an efficient XML parser and a thorough optimization of the SOAP framework and the server are important. This issue may also be interesting for further research.

Alternative Transport Protocols:

Although HTTP is the most common transport protocol for SOAP messages, other protocols, such as SMTP, may have certain advantages for fieldbus applications and would also be an interesting topic for further examination.

The IGUANA OPC XML-DA server seems to be a good base for further research. As the design is already very modular, the implementation could be used as the base for an OPC XML-DA framework. This framework could then be used to implement other protocols beside ESD and could be enhanced to support advanced features such as security and a maintenance interface for the OPC server itself.

At the time of writing, the new SOAP 1.2 specification has been released, which should resolve various issues and will be the successor of the previous SOAP 1.1 standard. SOAP can still be seen as a new technology; SOAP frameworks seem to mature and new SOAP-based protocols and implementations are emerging fast. It will be interesting to see how this technology will further evolve.