The Specification and Description Language (SDL) is a specification and description language of the International Telecommunication Union (ITU) with which the behavior of switching systems and their development are supported. The SDL description language dates from the 1970s and was standardized by the ITU under standards Z.100ff.
The Specification and Description Language is used to describe functions, protocols and services within and between exchanges and terminal equipment. The description language is based on graphical elements with which real-time systems are described. There are graphical elements for describing the architecture and others for describing the behavior. The use of the graphical symbols and their sequence are defined by rules. There are two syntactic forms, the Graphical Representation (SDL/GR) and the Phrase Representation (SDL/PR). These are different forms of representation of the common semantics. SDL has been constantly developed further and has, among other things, included object-oriented programming and the abstract syntax ASN.1 in OSDL.
The SDL description language can be used to describe the hierarchical system structure, as well as the system states and flow control. The system structure from the SDL language consists of blocks in which processes run. The blocks are interconnected and exchange information with each other via the connection. The running processes are independent of each other and can run simultaneously.