100 
From the numbering of the data flow diagrams is obtained the allocation to the particular overriding pro 
cess and thus also the respective classification level. The “top-level diagram” is given the number 0. 
The processes 8 represented there are numbered from 1 to n (see Figure 10). The data flow diagrams 
of the level underneath are identified with these numbers. The processes represented in these dia 
grams are again numbered from 1 to n, the respective number of the diagram being placed before the 
process number. This number code is continued recursively until the lowest classification level Is 
reached. Figure 11 shows this method in schematic form. 
Figure 11 : Model hierarchy 
Data memories are represented by two parallel lines. Memories are needed if access to certain data is 
asynchronous, for example if data are to be used at a later time. The data types contained in the par 
ticular memory are specified in the data dictionary. 
Data flows are channels for information flows with a known composition. Data packages may be 
transmitted via these channels and trigger the receiving process, i.e. the data package serves as an 
“incident” which “triggers” the receiving process. Input information may be converted into output infor 
mation by the particular process. The composition of the data packages transmitted at any given time 
characterises the respective channel. The description of the data packages is contained in the data 
dictionary. 
External units, i.e. objects of the environment with which the system concerned communicates, are 
identified in the context diagram and in data flow diagrams by “terminators”. These are not described in 
greater detail 9 and appear in the model only as transmitters and receivers of data. The data flows be 
tween terminators and the system represent the interfaces of the system with the environment. Data 
flows between terminators are not represented. 
* In this present case, the tasks involved here are those identified in part A of this study. 
9 Since terminators are not a component of the system concerned