Tutorial on Message Sequence Charts

Message Sequence Charts (MSCs) are a widespread means for the visualization of selected system runs (traces) within communication systems. They can be viewed as a special trace language which mainly concentrates on message interchange by communicating entities (such as SDL services, processes, blocks) and their environment. A main advantage of an MSC is its clear graphical layout which immediately gives an intuitive understanding of the described system behaviour. MSCs have been used informally for a long time by ITU (former CCITT) Study Groups in their recommendations and in industry. Their standardization was suggested at the 4.th SDL Forum October 1989 in Lisbon and agreed upon at the ITU-meeting Helsinki, June 1990. At the closing session of the ITU study period 1989-1992 in Geneva, May 1992, the new MSC recommendation Z.120 was approved. Within the present study period, as a major achievement, a formal semantics for MSCs based on process algebra has been standardized. A standard document on static syntax requirements is in preparation. Besides formal semantics main emphasis is put on structural concepts The reason to standardize MSCs was to allow systematic tool support, to facilitate the exchange between different tools, and to ease the mapping to and from SDL specifications. Due to the standardization, the importance of MSCs for system engineering has increased considerably. Accordingly, MSCs are used for requirement definition as an overview specification of process communication as an interface specification as a basis for automatic generation of skeleton SDL specifications for simulation and consistency check of SDL specifications as a basis for selection and specification of test cases for documentation for object oriented design and analysis (object interaction).