Message Sequence Charts (MSC)

Message Sequence Charts (MSC) is a graphical language for the description and specification of the interactions between system components. These interactions usually include typical or exceptional message flows between communicating entities. The language is standardized by the ITU-T as ITU Recommendation Z.120.

MSCs have long been used in standardization and in industry for visualization of selected message traces. Its simplicity and intuitive understanding have made the notation quite popular.

MSCs are often used in combination with SDL where MSCs diagrams and related SDL descriptions should be regarded as different but complementary views of a system. SDL provides behaviour descriptions of individual entities, but there is no direct description of communication between communicating entities. By contrast, MSCs provide a clear description of communication traces in the form of message flows.

Despite their simplicity, MSCs are a powerful notation, with built-in mechanisms to portray timers, loops, optional, alternative and exceptional system behaviour, as is necessary in any protocol description.

Message Sequence Charts may be used for requirement specification, display of simulation and validation traces, test-case specification and documentation of real-time systems.

Use of MSC tools

MSC diagrams can be drawn using any computer application capable of handling simple graphics. However, the use of specialized MSC tools improves all aspects of their development and use. It enforces the use of standardized MSC language elements, while permitting rapid development and maintenance of diagrams. Including MSC diagrams into the text of the standard is very straightforward.

High Level Message Sequence Charts (HMSC)

The Z.120 standard also defines High Level Message Sequence Charts (HMSCs). HMSC diagrams are used to specify more complex patterns of message flows by showing sequences or alternatives of atomic MSC scenarios, shown only as MSC references. HMSC diagrams specify the composition of communication activities that are needed while the referenced MSC diagrams show how the communication is done in terms of message sequences. HMSC diagrams can conveniently express options and alternatives in using parts of a complex procedure. Expressing the same thing only in MSC diagrams is possible but less straightforward.

ETSI Guidance on the use of MSC

A guideline for the use of MSC in communication standardization is available within the guidelines for SDL, EG 202 106, drafted by ETSI technical committee Methods for Testing and Specification (MTS).

MSCs and UML sequence diagrams address the same needs and are in many ways similar, however, there are differences between the two. In MSCs the vertical lines represent autonomous execution entities while in Sequence diagrams they usually represent an object. In MSCs an arrow is an asynchronous message while in sequence diagrams it is usually a synchronous operation call on an object.

Example MSC diagram