Message Sequence Chart (MSC) formalism is primarily used for description of design requirements for concurrent systems such as networks or telecommunications software. Description expressed by MSC is given with formal notation and can be subjected to analysis of system behaviour. MSC depicts a potential exchange of messages among communicating entities in a distributed system and corresponds to a single (partial-order) execution of the system. This formalism consists of two parts : Basic MSC (BMSC) and High-Level/Hierarchical MSC (HMSC). To represent the simple communication (scenario) Basic MSC is used. HMSC stands for a hierarchical structure of scenarios, where Basic MSCs can be combined to more complete system description.
Basic Message Sequence Chart (BMSC) consists of final set of system components communicating via messages. For each component exists a single instance that stands for a time axis. The instance contains events representing either receipt or sending of a message. The events can be situated in two kinds of areas: strict order area and coregion area. In strict order area the events are ordered totally, unlike the coregion area, where the events are unordered in default. The ordering is also denoted with messages. The send event of a message is ordered before receive event of the same message. Message that is not connected to any destination instance is represented with lost message and message without source is defined as found message. The message’s description is situated above the message in message label. For the purpose of describing events’ time stamps and time restrictions there are components absolute time and time interval in the MSC structure. The absolute time represents event’s timestamp related to start of communication. There are two types of time intervals. Absolute interval represents a time constraint between the start of a system communication and the event, while relative interval denotes the elapsed time with respect to the previous events.
Hierarchical Message Sequence Charts stands for a hierarchical structure of scenarios, where Basic MSCs can be combined to more complete system description. One HMSC can consist from many levels and each of them could contain nodes referencing to another HMSC or BMSC.