Research and analysis in this thesis show that SOAP as a fieldbus protocol has its strengths and its weaknesses. These facts help to decide whether SOAP should be used for certain applications or whether alternative technologies should be favoured. Table 1 tries to sum up the most important advantages and disadvantages of SOAP.
OPC XML-DA seems to be a thoroughly designed and mature specification of a SOAP-based fieldbus interface. However, some parts of the standard tend to be complex to implement, such as the subscription architecture, moreover, some elements seem to exist in order to maintain backwards compatibility with previous OPC standards which may be obstructive to new implementations that are not related to older OPC specifications. Nevertheless the openness and high level of interoperability of the OPC XML-DA specification seem to lead to a widespread usage of this fieldbus interface and may prove a long-term replacement of older, DCOM-based OPC standards.
Currently a new standard is being developed by the OPC consortium, namely the ``OPC Unified Architecture'' (OPC-UA). As shown in chapter 3, OPC today has very different specifications, covering issues such as data access, alarms and events and historical access. The new OPC-UA standard tries to bring all different OPC specifications into one, all-embracing standard163, which is structured in different sections, covering the following topics:
It can be observed that all access types, such as data access and alarms, are based on one information model. This approach may lead to a higher level of compatibility between different OPC applications and may also reduce implementation efforts. It is also very interesting that OPC has decided to also cover security, an aspect that current OPC standards lack.
OPC-UA compliant clients and servers utilize SOAP Web services for their communication, therefore it seems that SOAP has a bright future for Internet/fieldbus interfaces. Further information about this emerging standard can be found in [OPCXMLUA], the first publicly released specification of the OPC-UA standard.