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

Industrial Internet 56-7
56.3.2.2 eXtensible Markup Language
HTML has limitations concerning its mixture of presentation-related and content-related markup. In
order to separate the content from its presentation, the XML has been specified [TBR98]. XML was
developed and established as a markup language for generic purpose. XML is often treated as a Meta
language suitable for development of customized, specific languages.
XML expands the description language HTML with user-defined tags, data types, and structures.
Furthermore, it addresses one of the limitations in HTML—it introduces a clear separation between the
data descriptions, the data themselves, and their representation (Figure 56.6). For declaring syntactical
information, a separate definition file XML Schema Definition Language (XSD) [XSD01] or Document
Type Definition (DTD) is used. This allows reusing the description structure in different contexts. This
provides a number of benefits when using the same XML description file for different tasks. In combination
with Namespace definitions semantics can be defined. Namespaces allow establishing a clear
relation between tags defined in a schema and their usage context.
The availability of a wide range of software tools allows creating effective XML-based solutions.
Publicly available solutions for parsers and development frameworks supporting different platforms and
different operating systems contribute to the actual hype of using XML as a general-purpose description
language. The XML data can be filtered and associated to software. The selection of the necessary
information and the definition of their presentation details can be performed by means of scripts and
style sheets. The style sheets [FBO98] are part of the development of XML. In most cases, they are
implemented using the eXtensible Style Language for Transformation (XSLT). An XML document can
be related to different style sheets for different presentation or conversion functions, so that only the
XSLT references have to be changed. Since style sheets in XSLT can contain script elements, a Document
Object Model (DOM) [DOM98] has been developed to support scripting. This DOM is used to access
the XML file and its elements from a script. The DOM programming objects provide interfaces to the
original XML file, representing its structure as a tree of objects. The output data of the DOM functions
can be used to generate any textual or binary files, including new XML files with a different structure.
The major benefit of this solution is a unique, reusable description with an excellent consistency and
reduced efforts of the description process.
Applications of XML in the industrial domain have developed with increasing speed. They cover
static descriptions for multiple purposes, for example, device descriptions, data exchange descriptions,
and protocol descriptions using SOAP and WSDL (e.g., OPC and XML specification [OPC04]).
A special importance for visualization tasks is assigned to Scalable Vector Graphics (SVG) [SVG01],
an XML format for presentation of two-dimensional vector-based graphics including functions for
Value
XML-file
(Description
content)
Document type
definition—DTD
Schema
(Syntax, Semantics)
Output files
Style-sheet
(Formatting
instructions)
FIGURE 56.6
Structure of an XML file (left) and the XML environment.
© 2011 by Taylor and Francis Group, LLC