Has been used since 1978 at Purdue University. The primary emphasis has been in the use of simulation in problem solving. Building a model involves creating a network of predefined symbols. The nodes in the network create transactions, assign attributes, cause queuing, perform activities, synchronise flow, and remove transactions.
Branches route transactions from one node to another.
Transactions may require resources to process them at activities. Resources are preemptied by service requirements or may be unavailable from time to time. Examples of resources are inspectors, barbers, tugs or machines. Its dynamic features include visualisation of execution, step-through mode, tracing, saving state, analyzer interface. The networks are probabilistic and more than one links may leave a node each with a different probability.