Modellbasierte Anforderungsspezifikation sicherheitskritischer ...

1. Abstract
In common with other technical sectors, control and safety functions in the rail sector are performed
in numerous places by reactive systems. In many of these systems the actual functionality
is implemented in software. Many systems also carry responsibility for human life and material
assets.
The very important job of ensuring the correct functioning of the system begins with the creation
of the requirements specification. This must contain all necessary information for a manufacturer
to develop and produce the system and to maintain the system over its complete life-cycle. The
requirements specification therefore is a key element in the complete life-cycle of a system.
Currently mainly informal description techniques are used for the specification of requirements
in the rail sector. These techniques are, to a lesser or greater extent, open to interpretation and
subject to ambiguities and misunderstandings. In addition they do not allow automatic checks
for correctness and inconsistencies. This can lead to the introduction of functional errors already
in the requirements definition phase of the system development process.
In the last few years several new ideas and concepts have been presented in various papers as to
how the specification of systems can be improved. The research often focuses on the adaptation
or extension of existing description instruments such as the Unified Modeling Language (UML)
and mainly focuses on the system development phase of the manufacturer. Additionally the integration
of description instruments, methods and tools to a stringent overall concept, necessary
for their productive deployment, is not covered.
This research therefore develops an overall concept, which, for the first time, combines a semiformal
model-based description instrument with a process model for the creation of requirements
specifications. Test and verification methods, required for safety-critical systems, are also
an integral part of this concept.
The standardized Systems Modeling Language (SysML) from the Object Management Group
(OMG) is used as the description instrument. First a subset of the language (SysML (A)) is identified,
which appears to be particularly suitable for the description of requirements models in
the rail sector. These language elements are then applied, within a fixed model architecture, to
the description of several aspects of a future system, such as the interface with the environment,
system functions and system behavior. Grouping and organization of the requirement model -
absolutely necessary in practice - is realized through so-called “views” and a two-level recursive
hierarchy of systems and subsystems. Interfaces are also provided in order to couple information
with non-functional requirements, which cannot be sufficiently described in the modeling
language. This part of the work is completed with a listing of consistency conditions, which the
requirements model must meet in order that the integrity of the model content is guaranteed
across all grouping levels.
The second significant component of the work is a process model, which defines the model
creation procedure. As opposed to system development processes, which are carried out later
in the life-cycle and build on the requirements specification, one is confronted with a minimal
amount of mostly vague and quickly-changing information. The process model must allow for
these constraints.
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