In the network stage a model of the system as a whole is developed and represented as a system specification diagram (SSD) (also known as a network diagram). Network diagrams show processes (rectangles) and how they communicate with each other, either via state vector connections (diamonds) or via datastream connections (circles). In this stage is the functionality of the system defined. Each entity becomes a process or program in the network diagram. External programs are later added to the network diagrams. The purpose of these programs is to process input, calculate output and to keep the entity processes up-to-date. The whole system is described with these network diagrams and are completed with descriptions about the data and connections between the processes and programs.
The initial model step specifies a simulation of the real world. The function step adds to this simulation the further executable operations and processes needed to produce output of the system. System timing step provides synchronisation among processes, introduces constraints. This stage is the combination of the former ‘Initial model’ step, the ‘function’ step and the ‘system timing’ step.