Methodological Framework for Freeband Services Development

platform characteristics and the characteristics of modelling languages adopted for a design are interrelated. For example, let us suppose a designer chooses to use SDL [41] to represent platformindependent designs. This language provides the “agent” structuring construct: an “agent” is an entity that can exhibit reactive behaviour and communicates with other “agents” by exchanging “signals” asynchronously. If a designer models application parts as “agents” that interact with other application parts through “signals”, this use of SDL implicitly defines an abstract platform that supports reliable asynchronous message exchange. One might be tempted to believe that all relevant characteristics of a design’s abstract platform can be derived from the concepts underlying the modelling language adopted for the design. However, abstract platform characteristics may depend on restrictions on the use of particular constructs in a modelling language or the use of certain modelling styles or patterns. In the example above, several modelling choices have been made by the designer with respect to which constructs to use for modelling application parts and their interaction. An alternative and equally valid usage of SDL might define that all application parts modelled using “agents” must interact by broadcasting “signals” to all other application parts. Furthermore, in the general case, it is not possible to derive the set of modelling constructs that can actually be used by observing specific models of applications. Specific models only reveal the set of constructs that are used in particular combinations. For example, in the SDL examples above, it is impossible to know what “types” of “signals” may be exchanged between application parts. A model of a specific application will reveal the “types” actually used in the application. Without further definition of the abstract platform, one may have to assume that all “types” allowed by the language can be used (without restriction), and, hence, that all “types” are supported by the abstract platform. We conclude that even using the same modelling language, with the same set of underlying concepts, a designer might implicitly define different abstract platforms. Therefore, it is necessary that the designer clearly document the styles and restrictions applied on the language, so that the intended abstract platform can be determined explicitly and unambiguously. We call this approach language-level abstract platform definition. This approach is illustrated schematically in Figure 3.11, where concepts are represented as geometric forms.