9.6 Colored Petri nets

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9.6 Colored Petri nets

In this section we will introduce some of the basic concepts of colored Petri nets. Colored Petri nets also add another dimension to tokens as well as to selection criteria used in determining firing by the addition of different token types. Tokens now can represent different functions. For example, we can use different tokens to represent operating system calls or different classes of jobs. These different tokens can then be used to determine which transition of multiple transitions available can operate.

To represent this graphically we use colored tokens. The set of all possible colors for the tokens represents the cardinality of the token set. Using this token set we can now redefine the definition of our Petri net, specifically, to redefine the firing rules (called link algebra) for all transitions defined in our network. For example, in Figure 9.29, there are only two token types: black and white. These could represent two different types of jobs. Transitions can have priority and time associated with them as before and can also be defined to operate on only a specific token type. As also shown in Figure 9.29, transition t1 has a priority and time associated with it. Arcs also have additional details associated with them. The arc from p1 to t1 has a condition choose (n, p1), which selects n of one of the tokens to release to the transition. Other arcs are used to select only specific types of tokens. For example, the arcs leading out of transition t1 leading to places p2 and p3 have filters on them to only allow tokens of type black to traverse to place p2 and white to traverse to place p3. Conditions on arcs can be as complex as one wishes. We could use a complex condition that requires n1 of one type of token, n2 of some other type, and none of some third type before we release just one token down a specified path. Using these complex methods, we can model just about any condition that may occur in a computer system we are modeling. The reader is directed to [18-20] for details about colored Petri nets.

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Figure 9.29: Generalized Petri net.



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Computer Systems Performance Evaluation and Prediction
Computer Systems Performance Evaluation and Prediction
ISBN: 1555582605
EAN: 2147483647
Year: 2002
Pages: 136

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