Specification and Description Language (SDL) is a specification language targeted at the unambiguous specification and description of the behaviour of reactive and distributed systems. Overview The ITU-T has defined SDL in Recommendations Z.100 to Z.106. SDL originally focused on telecommunication systems; its current areas of application include process control and real-time applications in general. Due to its nature it can be used to represent simulation systems without ambiguity and with a graphical notation. The Specification and Description Language provides both a graphical Graphic Representation (SDL/GR) as well as a textual Phrase Representation (SDL/PR), which are both equivalent representations of the same underlying semantics. Models are usually shown in the graphical SDL/GR form, and SDL/PR is mainly used for exchanging models between tools. A system is specified as a set of interconnected abstract machines which are extensions of finite state machines (FSM). The language is formally complete, so it can be used for code generation for either simulation or final targets. The Specification and Description Language covers five main aspects: structure, communication, behavior, data, and inheritance. The behavior of components is explained by partitioning the system into a series of hierarchies. Communication between the components takes place through gates connected by channels. The channels are of delayed channel type, so communication is usually asynchronous, but when the delay is set to zero (that is, no delay) the communication becomes synchronous. The first version of the language was released in 1976 using graphical syntax (SDL-76). This was revised in 1980 with some rudimentary semantics (SDL-80). The semantics were refined in 1984 (SDL-84), the textual form was introduced for machine processing and data was introduced. In 1988, SDL-88 was released with a formal basis for the language: an abstract grammar as well as a concrete grammar and a full formal definition. The version released in 1992 (SDL-92) introduced object-oriented concepts such as inheritance, abstract generic types etc., with the object-oriented features described by transformations into non-object oriented ones. SDL-2010 is the latest version, an updated version of SDL-2000 that was completely based on object-orientation, rather than description by transformations. This version is accompanied by a UML-Profile: ITU-T Recommendation Z.109 (04/12), SDL-2010 combined with UML. SDL-2010 also introduced the support of C data types as initially introduced by SDL-RT. Organization Hierarchy level The Hierarchy level of SDL is structured as follows. Library package System agent Block agent Process agent Procedure type Remote procedure Architecture