rlay fashion. For example, suppose you want to introduce a new service feature in a network. The physical network structure (or some kind of logical structure of major network functions) is already decided. In that case, you want to see the physical (or logical) structure as a write-protected background model, and to do your modelling of how to allocate your feature functionality in the foreground. Sometimes two, or even three, background models might be required. To support views that are built on overlaid fragments in this way is not an easy task for a tool, but nevertheless is an important requirement for sysnet modelling. A more thorough discussion on views is given in Sect. 2.5. 1.6.4 Expressive Value Considering the popularity of some general purpose drawing tools, such as Micro- soft Power Point, one of the most important aspects to the success of a (graphical) modelling language is how expressive models are to users. As an example, we can look at a model over the logical resources of a particular switching system, as depicted in Fig. 1.19. This model was created by using an entity-relation-attribute (ERA) type of modelling language. Such a language offers basically only two graphical symbols: one for entities and one for relations between entities. 1 Introduction to Systems Modelling SwitchingSwitchView Conference TwoPartyConnection TwoPartyConnection TwoPartyConnection LogicalChadnel LogicalChannel poicalChannel LogicalChannel LovicalChannel LogicalChannel LogicalChannel LogicalChannel LogicalChannel AccessSwitchView AccessSwitchView LogicalObject PhysicalChannel PhysicalChannel LogicalObject Fig. 1.19 A model with low expressive values The model is built by five different entity types and eleven types of relations. Since I regarded the expressiveness of the model less satisfying, I introduced spe- cial symbols for each type of entity, and for two of the relation types. The remain- ing relations are all some kind of consistsOf relations, for which I defined overlay drawing rules, such as "An AccessSwitch