SDL (Specification and Description Language) is a graphical specification
language standardized by ITU (International
SDL, defined in Z.100, has been evolving since the
first recommendation in 1980. Every fourth year an updated revision of the
language standard has been adopted. In 1992 Object Oriented features were
included in SDL. The standard from1996, called SDL-96, introduced only minor
updates. The current standard is SDL-2000, and it introduces a
number of new features, including exception handling, a new data model, and
Although SDL is widely used in the telecommunications field, it is
not designed specifically for describing telecommunications
services. Rather SDL is a general purpose specification language for
communication systems and embedded systems.
The graphical notation, the formal semantics, and object-oriented concepts
makes SDL a powerful and versatile language both for systems specification
and their implementation.
The basis for description of behaviour in SDL is communicating extended
finite state machines, represented by processes. A process consists of
a number of states and a number of transitions connecting the states.
Communication between processes is done by signal exchange. Signals can be
exchanged between two processes in a system or between a process and the
environment of the system. The remote procedure and remote variable paradigms
for information exchange between entities in an SDL system are also supported.
In addition to the SDL recommendation, a number of recommendations has been
defined to complement and ease the use SDL in combination with other
notations. E.g. recommendations on Message Sequence Chart (MSC) Z.120, on the
combined use of SDL and ASN.1 (Abstract Syntax Notation 1) Z.105, and an layout
preserving interchange format, CIF (Common Interchange Format) Z.106 has been
For more information on new features in SDL-2000
and information on upcoming events related to SDL, see also