wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Industrial Internet 56-11
FIGURE 56.9
Screenshot of a browser-based visualization using SVG.
PHP, solves this problem. However, it requires frequent updates of the content, since every change is
made at the server. Java applets seem to solve parts of the problem, since they are transferred as binary
data (class files) from prespecified servers, reside in a sandbox within the browser’s virtual machine,
but can use several underlying transport mechanisms. But again, a general solution for this problem
cannot be given.
Figure 56.9 shows an example for a browser-based application using SVG graphics with animation.
The navigation areas are created on the server with PHP, the content model uses XML, and the SVG
graphic is created from a Computer-aided design (CAD)-drawing of the machine [WOL03]. This shows
the typical combination of different Internet technologies as mentioned above.
56.4.3 Machine–Machine Communication Using Web Services
Besides the application with user interfaces, Internet technologies may also be used effectively between
distributed applications. This is often called machine–machine communication. The applications need
to transfer data or to invoke functions. Thus, Web services using SOAP as the transport protocol with
service descriptions in WSDL are the most promising technology in this scenario. Since this concept
follows the actual trend of service-oriented architectures (SOA) with loosely coupled, distributed functions,
many software development environments support the process of defining and implementing the
appropriate software components. For example, Microsoft’s Visual Studio and Eclipse provide functions
to define WSDL files to generate service components or clients. They handle serialization of the
function call and the parameters to SOAP messages and vice versa.
In the automation domain, Web services are increasingly used. The most important problem is
not the tool support, but the common definition of services. This leads to the typical standardization
topics, which are always hard to solve. However, the success of OPC enabled a definition of Web
services to access existing OPC servers for reading and writing of OPC items. This solution is known
as OPC&XML [OPCxml] and demonstrates the potential of Internet technologies. Using Web services
reduces the complex efforts of providing the appropriate DCOM configuration dramatically,
which was necessary for traditional OPC servers. The access can now easily pass firewalls; the service
analysis at the server still provides security features. Actual developments like OPC-UA (see Chapter
56) also use Web services. However, a drawback is introduced by the enormous overhead of SOAP
messages to be exchanged. So for OPC-UA, a specific binary protocol has been developed to reduce
communication effort.
© 2011 by Taylor and Francis Group, LLC