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Annex 1  
5.8.2.1 Petri Networks (PNET)  

  Petri-Netze (PNET)

Contents  
  • 1 Identification/Definition of the Method
  • 2 Brief Characteristic of the Method
  • 3 Appraisal of the Method
  • 4 Application of the Method in the V-Model
  • 5 Interfaces
  • 6 Further Information
  • 7 Literature
  • 8 See Also
  • 1 Identification/Definition of the Method

    /Reisig, 1986/

    2 Brief Characteristic of the Method

    Petri Nets (PNET) are a modeling method based on a special network theory (developed by C.A. Petri). By means of PNET, event-driven (also not technical) systems with discrete parts and discrete local state transitions are with regard to its causal structure and dynamic (e. g. manufacturing system including control software, service system, communication system, real-time computer system). Particularly the concurrent, parallel processes and its corresponding synchronizations can be exactly specified by means of a unique representation of its causal relations. A special characteristic of the PNET is its dynamic notation of the timewise actual filling of places (marking by tokens).

    3 Appraisal of the Method

    The strong points of PNET are:

    Peculiarities to be taken into consideration when modeling with PNET:

    Method Familiarization

    Tool Support:

    4 Application of the Method in the V-Model

    The method is applied in SD1.5 - User-Level System Structure and SD3.3 - Definition of Requirements for the Functionality in order to model the courses of functions.

    5 Interfaces

    Support by a tool, data flow diagrams can be transformed into to Petri Nets that can be analyzed. The user changes processes into transitions by means of graphical identification, and via an additional labeling of the arrows in the data flow diagram; he can insert places between the transitions. The data flow diagram thus prepared is transferred to a PNET-based tool for further processing (analysis, simulation). This way, a dynamic consistency assessment is possible for data flow diagrams.

    According to /Graubmann, 85/ and /Grabowski, 90/, automata represented by Specification and Description Language (SDL) can be transformed into Petri Nets and may then be analyzed with regard to deadlocks and other dynamic characteristics. Petri Nets resulting from an automatic transformation with Specification and Description Language (SDL) can also be embedded into existing subnets within the scope of a total design.

    6 Further Information

    Further Developments/Versions

    Apart from the most simple PNET form-the condition/transition nets-there are several other versions. A simple upgrade are the place/transition nets where each place may have several similar tokens which can be transported even in various numbers while a transition is switched. Even very complex systems can be easily specified with this PNET. Time-related PNETs allow the definition of time delays for the switching and thus a detailed specification of a system. The most powerful form of PNET are the predicate/transition nets. In this connection, there are various so-called individual tokens that may fill a place in a mixed way. When switching, they are transported by the arrows in different combinations. This and additional switching conditions are described by predicates. In case of real-time systems this refers to the local storage and the controlled exchange of data that are specified on a high abstraction level. The causal process model of the PNET is effectively completed by these possibilities.

    7 Literature

    /Baumgarten, 1990/ generally oriented introduction into Petri Nets with several examples
    /Grabowski, 1990/ representation of the principles required for the transformation of SDL diagrams into Petri Nets
    /Graubmann, 1985/ tool-oriented representation of the validation of SDL diagrams via transformation into Petri Nets
    /Reisig, 1985/ introduction into the design with Petri Nets
    /Reisig, 1986/ mathematically oriented introduction into the basics of Petri Nets with proofs; standard and reference documentation

    8 See Also

    Other publications on Petri-Nets

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