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

56-6 Industrial Communication Systems
Web service
Service request
Service response
Device
related
data
Listener
Business
logic
Data
access
Data
Service control
Metainformation
FIGURE 56.5
Principle structure of a Web service server.
Model (DCOM) [RGR97] or the transport layer of the.NET specification. However, due to complexity,
management effort, and security reasons, a much simpler solution was requested. The Simple
Object Access Protocol (SOAP) [DBX00] was defined to overcome the limitations and to provide a
“firewall-friendly” way of invoking functions. SOAP is a protocol that can be bound to several underlying
transport layers, including HTTP and SMTP. The functions calls, the responses, and possible
error messages are serialized in coded format using eXtended Markup Language (XML) and are then
embedded into the transport protocol.
Depending on the implementation (see Figure 56.5), a listener function is invoked by the transport
layer, a Web server or a stand-alone application, e.g., the service request is passed to the listener,
the function called and the arguments provided by the call are passed to business logic components.
They finally provide the data for response and issue its transfer to the client. The services provided by
a Web service server are described using a service description following the definitions in Web Service
Description Language (WSDL), an XML-based format [WSD01].
Application of Web services in industrial applications is steadily growing. Because of the publicly
available transport protocols Web services can be bound to, Web services provide an easy and commonly
accepted way of handling communication between applications on different components and
perhaps different operating systems.
56.3.2 technologies for Information Description and Presentation
56.3.2.1 HyperText Markup Language
The content transferred by Internet technologies is manifold. Besides binary data representations for
different formats and coding, technologies for information descriptions have been developed. The most
important technology in this area is without any doubt the HyperText Markup Language (HTML)
[HTM00], the main description technology of the World Wide Web. A client, a Web browser, interpreting
the HTML statements of a hypertext document and thus rendering and presenting the content
described in HTML documents is available on nearly every platform. Together with presentation-related
technologies like Cascading Style Sheets (CSS) [CSS98], complex solutions for display of different types
of data have become state of the art.
HTML documents can be used for any functions in automation domain that need to display data—
especially data from different sources in different formats. In any case, the data are provided by the
server upon client request. This might be difficult for some data sources (events).
© 2011 by Taylor and Francis Group, LLC