Message Sequence Charts (MSC)

Message Sequence Charts (MSC) is a standardized notation used for the description of typical or exceptional message exchanges between entities. MSC diagrams provide a clear description of system communication in the form of message flows.

Originally standardized as a 'sister language' to SDL, it is developed and maintained by the ITU-T as ITU Recommendation Z.120.

MSCs and SDL descriptions should be regarded as different but complementary views of a system. SDL provides behaviour descriptions of individual communicating entities, but there is no direct description of communication between several entities. By contrast, MSCs provide a clear description of system 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.

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

A set of MSC diagrams normally covers partial system behaviour. Each MSC diagram represents one scenario of either a typical or an exceptional exchange of messages between system parts. The language is particularly useful when distributed processing must be managed at several interfaces. For instance, it can be used effectively in describing basic scenarios of calls and the establishment of connections. MSCs may be used for requirement specification, displaying simulation and validation results, test purpose description and documentation of real-time systems.

Use of MSC tools

MSC diagrams can be developed 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 what communication activities 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).