Specification and Description Language

The Specification and Description Language (SDL, Eng. Specification and Description Language ) is one of the ITU- T, ie the telecommunication standardization sector of the International Telecommunication Union, the so-called standards ( Recommendations) Z.100ff defined modeling language.

It is common in the telecommunications sector in order to describe (distributed) systems by means of extended finite state machines (processes). The different processes are connected via channels and can also exchange signals and influence thus mutually exclusive.

A specified in SDL system can be either text ( Textual Representation phrase / PR) or in graphical form ( Graphic Representation / GR ) represent.

Construction of an SDL specification

Structure and sequence

An SDL specification is structured hierarchically in general in system blocks and processes. Since SDL -2000 blocks and processes are unified into agents who differ as to the precise semantics of their concurrency. In addition, there are also libraries ( library Package) and procedures. Other concepts such as macros and services should no longer be used.

System, block and process have defined access points (gate), links to other agents are made with channels (channel). About this run (only) the signals defined for this channel and this gate.

All of these structural elements are typed, and there can be multiple instances of them in one system. Even a seemingly nichttypisierte instance always defines an implicit type.

A process ( in SDL -2000, each agent ) has a state machine. State transitions (transitions) are triggered by the reception of signals. It can also be defined as a spontaneous state transitions to map to nondeterminism. In addition, SDL has its own concept of time: It can timer (timer ) can be set and the elapsed timer as signals are received.


In SDL, there are pre-defined data types such as Pld, Integer and CharString that can be combined into structures or lists, sets, etc. can be stored.

The underlying data type concept is semantically equivalent to ACT ONE data type and the concept of LOTOS. This new data types can be defined axiomatically. But there are actually no tool support, and so the axiomatic definition is irrelevant in practice.

Example specification

The illustrations show an SDL specification for the DeamonGamem as it was first described in Turner 1993. The present version is taken Piefel 2011. In this variant, there is no explicit demons, but the change of the internal state is controlled by a timer.

Norms and Standards

  • Z.100: Specification and Description Language (SDL )
  • Z.104: Encoding of SDL data
  • Z.105: SDL combined with ASN.1 modules ( SDL/ASN.1 )
  • Z.106: Common interchange format for SDL
  • Z.107: SDL with embedded ASN.1 ( Retired )
  • Z.109: SDL -2000 combined with UML
  • Z.121: Specification and Description Language (SDL ) databinding to Message Sequence Charts (MSC )