Tutorial on Message Sequence Charts

Abstract

An introduction to the ITU standard language Message Sequence Chart (MSC) is provided. It is pointed out that MSC in many respects is complementary to the ITU specification and design language SDL. MSC in combination with SDL or other languages, now plays a role in nearly all stages of the system development process. Since MSC has been standardized in the same study group as SDL, the language form is quite analogous, e.g. it has a graphical ( MSC GR ) and a textual ( MSC PR ) syntax form. The MSC language in the present recommendation Z.120 (MSC'92), comprises basic language elements — instance, message, environment, action, timer, process creation and termination, condition — and structural language elements — “coregion” and “submsc”. It is demonstrated how global and non-global conditions may be used for the composition of MSCs. Whereas in MSC'92 the main emphasis is put on the elaboration of basic concepts and a corresponding formal semantics, in the new MSC version (MSC'96) structural language constructs, essentially composition and object oriented concepts, will play a dominant role. With these new concepts, the power of MSC is enhanced considerably in order to overcome the traditional restriction of MSC to the specification of few selected system runs.