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I have removed the UML Sequence Chart picture that actually does not exist. And I have listed the usual misunderstanding between UML Sequence Diagram and ITU-T MSC. |
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[[File:Typical MSC (Message Sequence Chart).png|thumb|The diagram shows three entities. At start the phone is disconnected. A user tries to establish a connection. A connection request is sent to the switch and a timer is started. An alternative deals with two possibles responses: 1 - The timer goes off because the switch did not reply and the phone goes back to the disconnected state.2 - The switch grants the connection and the call is established.]]
A '''message sequence chart''' (or '''MSC''') is an interaction diagram from the [[Specification and Description Language|SDL]] family
The main area of application for message sequence charts is as for communication behaviour in [[Real-time computing|real-time systems]], more specifically telecommunication [[electronic switching system]]s.
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==Comparison to UML==
UML Sequence Diagram looks very similar to the ITU-T MSC but the default basic principles are quite different:
* Lifelines
:* In an MSC, the vertical lines are autonomous execution entities. They usually represent state machines executing in parallel. The state machines might not even be on the same machine.
:* In a Sequence Diagram, a vertical line is usually an object. The object can be active (in its own thread of execution) or passive (in the execution context of an active object).
* Arrows
:* In an MSC an arrow is usually an asynchronous message sent from one entity to another one. Once the message is sent the sending entity resumes its execution.
:* In a Sequence Diagram an arrow is usually understood as an operation call on a class. It is therefore synchronous and the calling entity hangs until the operation returns.
It has been said that MSC has been considered as a candidate for the [[Unified Modeling Language#Interaction diagrams|interaction diagrams]] in UML.<ref>{{cite web
| url=http://www.swe.informatik.uni-goettingen.de/publications/ER_JG_PG/SDL99-Harmonization.ps.gz
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