Scope-objective This Recommendation defines SDL (Specification and Description Language) intended for unambiguous specification and description of telecommunications systems. The scope of SDL is elaborated in clause 1. This Recommendation is a reference manual for the language. Coverage SDL has concepts for behaviour, data description and (particularly for larger systems) structuring. The basis of behaviour description is extended finite state machines communicating by messages. Data description is based on data types for values and objects. The basis for structuring is hierarchical decomposition and type hierarchies. These foundations of SDL are elaborated in the respective main clauses of this Recommendation. A distinctive feature of SDL is the graphical representation. Applications SDL is applicable within standard bodies and industry. The main applications areas, which SDL has been designed for are stated in 1.2, but SDL is generally suitable for describing reactive systems. The range of application is from requirement description to implementation. Status/Stability This Recommendation is the complete language reference manual supported by guidelines for its usage in Supplement 1. Annex F gives a formal definition of SDL semantics. The main text of this Recommendation is stable and needs to be issued immediately to meet market needs, but further study is required to complete Annex F. Appendix I records the status of Z.100, and should be updated as further studies are completed. Although further language extensions are anticipated in the future, SDL-2000 as defined in this Recommendation should meet most user needs for some years. The current version is based on wide user experience of SDL and recent additional user needs. The main text is accompanied by annexes: - Annex A Index of non-terminals. - Annex B Reserved for future use - Annex B (03/93) is no longer valid. - Annex C Reserved for future use - Annex C (03/93) is no longer in force. - Annex D SDL Predefined data. - Annex E Reserved for examples. - Annex F Formal Definition (further study needed for SDL-2000). - Appendix I Status of Z.100, related documents and Recommendations. - Appendix II Guidelines for the maintenance of SDL. - Appendix III Systematic conversion of SDL-92 to SDL-2000. ITU-T Z.100 has also an independently published supplement: - Z.100 Supplement 1 SDL+ Methodology: Use of MSC and SDL (with ASN.1).