2 An Algebraic Semantics for Message Sequence Chart

Message Sequence Charts (MSCs) are a graphical and textual language for the speciication of message passing systems, in particular telecommunication systems. MSCs are standardised by the Internal Telecommunication Union in standard Z.120. Included in the standard is a formal semantics for MSCs by means of a process algebra. This semantics covers the complete language of single MSCs but lacks an interpretation for conditions which are used as continuation points of MSCs within an MSC document (a collection of MSCs). In this paper, we give a process algebraic semantics for basic MSCs including conditions, enabling the formal interpretation of entire MSC documents.