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2.2. European Train Control System
indirectly published via the source code. This is a problem if the specification is not licensed in
the manner of OSS or FLOSS, which means generally public accessible and usable. Therefore,
it is only meaningful to develop OSS or FLOSS for train control systems which specification is
adequately licensed. This circumstance directly influenced the choice of train control system
used for the case study in Part III. Further issues related to OSS and FLOSS software for
safety-critical systems are discussed in Chapter 6.
2.2. European Train Control System
The European Train Control System (ETCS) [67, pp. 102-105] is a component of the standardised
European train traffic control system, which should replace the several different used
train safety systems in European countries. The initiative for creating a general train control
system for the European Union was already launched in 1990. Meanwhile, there exist several
revisions of the specification of ETCS, which are now published and maintained by the
European Railways Agency (ERA) [21]. The actual version of the ETCS specification can be
found in [23]. The specification itself is divided in so-called Subsets, which combine a set of
documents for a certain specification issue. The following description of ETCS mainly relates
to the Subset-026 [83], which is also called System Requirement Specification (SRS). It must
be emphasised that this work refers to the specification version 2.3.0, which was mandatory at
the beginning of this work in 2010.
The SRS 2.3.0 consists of seven documents:
Introduction general introduction to ETCS and the documents in the SRS [86]
System Description description of an ETCS [84]
Principles principles for ETCS [89]
Modes and Transition description of possible modes and transition in ETCS [88]
Procedures operation procedures [90]
ERTMS/ETCS Language structures for data transmission [85]
Messages data structures for radio communication in ETCS [87]
This SRS document often refers to another specification element outside the Subset-026, the
Functional Requirement Specification (FRS) [22]. The FRS defines the functionality that is
available in ETCS. It is more abstract because it only describes required functions, but not
how those are realised. In the development cycle of the ETCS specification, the FRS is used as
input for the SRS, in which the functional requirements are transformed to requirements of the
whole system. Therefore, the FRS is never used directly for system development.
2.2.1. General Purpose
The ETCS introduction defines some general goals and advantages [86, p. 4]:
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