Jens Grabowski

Message Sequence Chart: Composition Techniques versus OO-Techniques – 'Tema con Variazioni' –

Structural concepts for Message Sequence Charts (MSCs), i.e., composition, types, inheritance, and virtuality, are applied to a telecom example provided by the public switching systems division of the Siemens AG. The example contains several variations of the peripheral parts of an initial MSC which may be combined independently. The independent combinations of the peripheral variations are described by means of several new composition operators and by using object-oriented techniques (OO-techniques), i.e. types, inheritance, and virtuality. A comparison of both techniques shows that composition operators may provide a compact, easy, but abstract description, whereas some OO-techniques allow a graphical, intuitive, but not compact specification. Typical OO-techniques like inheritance and virtuality seem to be less fruitful for the description of at least the provided example. A combination of composition operators and OO-techniques, e.g., a variant type concept employing the alternative composition operator, may combine the advantages of both techniques.

Component interface description using HyperMSCs and connectors

Modelling of complex systems with message sequence charts requires several extensions in order to arrive at sufficiently transparent and manageable descriptions. Two extensions of major importance are introduced in this paper: 1) extended high level MSCs denoted as HyperMSCs are allowed to contain MSC references with hypertext-like inscriptions or in expanded form as detailed MSCs; and 2) MSC connectors are introduced in form of MSCs representing high level communication patterns between MSC components. The introduction of MSC connectors may be viewed as a generalisation of the gate concept and as a completion of the MSC language for communicating operator expressions. Moreover, MSC connectors can be employed quite generally as a communication description on a higher level of abstraction - a structural language construct which is still missing in the MSC standard. These new concepts allow a system modelling based on stepwise refinement starting with HyperMSCs, decomposed instances and MSC connector communication.