Specification and Description Language
Specification and Description Language (SDL) is a specification language targeted at the unambiguous specification and description of the behaviour of reactive and distributed systems.
It is defined by the ITU-T (Recommendations Z.100 to Z.106). Originally focused on telecommunication systems, its current areas of application include process control and real-time applications in general.
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 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 with 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.
Hierarchy level 
The Hierarchy level of SDL is structured as follows.
Usually a system agent consists of a number of block agents. Block agent communicate with each other using channels. A block agent consists of process agents (the required structure in SDL-92; SDL-2000 is more flexible). Each process agent is a state machine that contributes to the action carried out by the system. A message stimulus from the environment or another agent to an agent is called a signal. Signals received by a process agent are first placed in a queue (the input port). When the state machine is waiting in a state, if the first signal in the input port is enabled for that state it starts a transition leading to another state. Transitions can output signals to other agents or to the environment. A process agent is allowed to contain procedure types so that the same actions can be invoked from different places. It is also allowed to call a remote procedure type to invoke a procedure in another agent (or even another system) and wait for a response.
SDL tools 
The most well-known SDL modelling tools are Telelogic Tau, PragmaDev RTDS, Cinderella, Safire-SDL, and ObjectGeode (now out of the market). PragmaDev RTDS supports both SDL and SDL-RT which is used to develop real-time and embedded software. There are also some open source projects relative to SDL modeling like JADE which is a Java based specification environment.
See also 
SDL-RT (see link below) is based on the ITU Specification and Description Language replacing the data language with C.
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- ITU Languages and general software aspects for telecommunication systems
- SDL forum
- SDL-RT for real time systems
- PragmaDev Real Time Developer Studio (COMMERCIAL)
- SDL Suite by IBM (acquired from Telelogic) an SDL Design Tool (COMMERCIAL)
- Cinderella SDL Design Tool (COMMERCIAL)
- SanDriLa SDL Design Tool (COMMERCIAL)
- SAFIRE Integrated Development & Run-Time Environment (COMMERCIAL)
- SDL tool from Humboldt University of Berlin