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

1 Identification/Definition of the Method

2 Brief Characteristic of the Method

3 Appraisal of the Method

The strong points of PNET are:

• The appropriateness for the behavior modeling of high complex, high distributed (sub-) systems with many but simply functioning parts-in particular when including an environment which is also event-oriented,
• easy corrections and upgrades because of the simple notation and the concept of the fusion of several networks,
• possibility for hierarchical refinement of individual components by taking into consideration certain consistency rules,
• precise specification with simple semantics,
• unique representation of the causal relations, in particular in the case of concurrent processes (conflicts, deadlocks, etc. are detected and can be solved by means of simple network changes),
• explicit representation of the dynamic by means of tokens,
• possibility to model both asynchronous and synchronous communication,
• the mathematical foundation of the PNET (network theory) makes a correct assessment of the network models possible for important operational characteristics (deadlocks, livelocks, reachability, invariants, etc.).

Peculiarities to be taken into consideration when modeling with PNET:

• In very large PNETs, it is important to decompose into subnets at an early stage in order to make the specification easier to understand.

  When modeling with PNET, the availability of a library containing network representations of frequently used function types with exactly defined characteristics is a great help (e. g. for access, synchronization, lock, conflict solution mechanisms, etc.).

Method Familiarization

• Learning the notation including the semantics is easy. Modeling complex systems with PNET requires training.

Tool Support:

• The tool support for PNET is definitely required in connection with modeling and analysis of large (sub-) systems in order to utilize the advantages of the method. A whole number of such tools is available.

4 Application of the Method in the V-Model

5 Interfaces

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

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

GDPA Online Last Updated 01.Jan.2002 Updated by Webmaster Last Revised 01.Jan.2002 Revised by Webmaster