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A message sequence chart (MSC) is a notation for specifying or describing how system components and their environment communicate by exchanging messages. It represents a selected communication scenario as a trace, using textual or graphical syntax defined by ITU-T Recommendation Z.120.
What does a message sequence chart show?
An MSC lays out the participants in a communication and the messages exchanged among them over the course of a scenario. It can describe a typical flow, an exception, or another behavior worth specifying or examining. The chart is a trace of a scenario, not necessarily a complete account of every behavior the system can exhibit.
ITU-T Z.120 defines MSC syntax in both textual and graphical forms. Its purpose is to describe communication behavior through message interchange, rather than to depict a system’s structure in general. ITU-T Recommendation Z.120
How do you read an MSC?
Read the chart as a timeline of message exchanges among entities. In the graphical notation described by ETSI, each vertical line represents an autonomous execution entity, and an arrow represents an asynchronous message. Follow the messages in the order shown to understand the scenario’s communication trace.
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MSC notation can express more than a single straight-line exchange. It includes ways to represent timing, repetition, optional or alternative behavior, and exceptional behavior. A larger pattern can be expressed with a High Level MSC (HMSC), which sequences or selects among named MSC scenarios; the referenced MSCs then provide the detailed message exchanges. ETSI technical overview of MSC
What are message sequence charts used for?
Teams use MSCs to make communication behavior explicit. Depending on the project, a chart can support:
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- Specifying system requirements and interfaces
- Showing simulation or validation traces
- Defining test cases
- Documenting real-time systems
Because a chart focuses on a chosen scenario, it can help clarify what participants send and when, including exceptional paths. It does not replace a broader system specification when other aspects of behavior also need to be described. ETSI outlines these application areas in its MSC overview.
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How is an MSC different from a UML sequence diagram?
MSC and UML sequence diagrams both show interactions over time, but their conventions are not interchangeable. ETSI’s overview distinguishes them as follows:
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| Aspect | Message sequence chart | UML sequence diagram |
|---|---|---|
| Vertical lines | Represent autonomous execution entities | Usually represent objects |
| Arrow meaning | Denotes an asynchronous message | Usually denotes a synchronous operation call |
These are orientation-level distinctions, not a substitute for the full semantics of either notation. Choose based on what the model needs to express: communication traces between autonomous entities, object interactions and operation calls, or MSC-specific composition of scenarios through an HMSC. ETSI summarizes the notation differences in its technical overview.
Which version of the MSC standard is current?
The ITU-T database lists Recommendation Z.120 (02/2011) as in force. It also lists Z.120 Annex B (04/1998), concerning formal semantics, as in force. The status listing identifies earlier main editions as superseded or withdrawn. Check the ITU-T record when relying on the standard, since status can change. ITU-T Z-series component-status listing
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How can you create an MSC?
ETSI notes that MSCs can be drawn with ordinary graphics applications. A specialized MSC tool may help authors use standardized language elements and maintain diagrams, but the cited material does not establish a particular product recommendation. If conformance to Z.120 matters, use tooling and review practices that can check the notation rather than relying only on a free-form drawing.
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