History of SDL
1968 ITU study of stored program control systems
1972 Specification, programming and HMI studies started
1976 Orange Book SDLBasic graphical language
1980 Yellow Book SDLProcess semantics defined
1984 Red Book SDLStructure, data added. Definition more rigorous.Start of tools. User guide.
1988 Blue Book SDL (SDL-88)Effective tools. Syntax well defined - formal definition. Language much as 1984.
Although the SDL language is today a language applicable to the specification and implementation of any real time system, it has its origins in telecommunications. The development of SDL arose out of an ITU study started 1968 of the way to handle stored programme control switching systems. The result of this study was to agree in 1972 that languages were needed for specification, programming and human machine interaction. The first, small SDL standard was produced in 1976 as the language for specification. SDL has been updated every four years, with some updates being larger than others.
All ITU standards (called Recommendations – they recommend norms to national bodies) are the result of collaborative work, and it is widely recognised that the early 1980 SDL language owed much to earlier work done by Göran Hemdal, Nils Lennmarker and Ivar Jacobson, then of Ericsson. Much other input was included from ITU members throughout the world. For example, in 1970 the main editor1 for SDL-2000 was working at GEC Telecommunications with state transition diagrams with “send/no wait” signal semantics very similar to SDL, and GEC contributed to the ITU study.
Before 1984 SDL lacked data, which prevented it from being used formally for most models, as these depended on data for decisions and passing information. The initial data model added to SDL was based on an algebraic approach from Australian contributions, notable from Bob Pascoe then of RMIT. The 1984 language also had a more formal definition with the separation of the concrete syntax, the abstract syntax and a semantic model based on mathematical graphs, though the model was described in natural language only at that time.
1988 saw the introduction of a formal definition for SDL in VDM (alias Meta IV) to underpin the natural language description. Anders Olsen, then of TeleDanmark and others did the bulk of this work.
1Rick Reed, TSE Limited, UK. One of the authors of this course material.