 data flow interfaces, considering different types of inter- faces (switched, non-switched, connection-oriented, connection-less, radio and fixed) Special notations for connectivity structures. Support for layering of communication functions network-wide, based on a refinement of the OSI concept and on generic patterns for structuring layers. 1.6.2 Syntactical Style The extraordinary development in graphical user interfaces, which today brings features like colours, 3D, animation and multimedia representation into the hands of every PC user, tends to obscure the fact that not everything must be, or is best represented as a graphical model, at least not in design activities. There is nothing wrong with using graphics to produce expressive models, but beware of the decep- tiveness of nice graphics: the fact that the model you are designing looks nice may fool you into assuming that it is also logically correct and exhaustive; i.e. your nor- mal instinct as an engineer to carefully check your design before releasing it may become blocked. Figure 1.16 gave an example where the use of graphic representation is of ques- tionable value. A sysnet model is not just a structure of units and communication 1.6 Requirements on a Modelling Language for Systems Modelling relations between units. This is, in fact, the smaller part of all information that is part of the model. The bulk of information is attribute specifications that are associ- ated with elements in the structure, such as requirements and specifications of pro- tocols, data, messages, events, operations, behaviour and constraints. For example, the specification of an abstract service primitive (a description of a type of commu- nication event) in AML is a textual information element that may look as shown in Fig. 1.17. There is very little to gain, in this case, in trying to provide an exhaustive graphical representation. PRIMITIVE CAS/callnfoConfirm (CHOICE{Time Day, Date}) Time ::= SET hour,minute OPTIONAL} Day ::= {"Monday", "Tuesday", "Wednesday", "Thursday",