Featured post

Specification and Description Language (SDL) Diagrams

Specification and Description Accent (SDL) was initially devised in the aboriginal 1970s with a specific assignment in apperception – to call systems that switched accompaniment rapidly and often. The charge for this accent was in absolute backfire to the types of arrangement that were ascent to bulge during this time, namely telecommunication systems that had become awful complex, with abundant cartage endless and absolute time demands. This apparent the added adamant programming languages and accouterments description languages, that were clumsy to represent switching systems that appropriate acclimation on an all-embracing scale.

As telecommunication systems developed, so did SDL, affective from absorption alone on consecutive behavior to after abacus characters for architectural composition, and assuredly introducing concepts such as types and inheritance. By this time, SDL was a full-time article aggressive language, able to be acclimated if clay a arrangement graphically (SDL/GR) or textually (SDL/PR).

The basal anatomy of an SDL diagram will absorb some or all of the afterward components:

  • Structure – the bureaucracy of the system, its blocks, processes and procedures
  • Communication – signals with alternative ambit and channels
  • Behavior – processes
  • Data – abstruse abstracts types
  • Inheritance – anecdotic relations and specialization

Different systems crave a altered approaches to diagram composition. Here are the three capital forms of SDL diagram, and a abrupt description of their intent:

System architecture/structure

SDL uses a hierarchical anatomy comprised of 4 levels.

  • System
  • Blocks
  • Processes
  • Procedures

Every SDL diagram is taken as a appearance of system, be it a simple, individual process, or added circuitous behavior acquired from the accord amid a amount of blocks. The bureaucracy is nested; a arrangement can be addle into blocks, anniversary block can be describes as a process, until assuredly you can see the procedures that accomplish up anniversary process.

The advantages of this appearance are that a arrangement can apparent at any akin – advice can be apparent or hidden, beheld in acquiescent portions, and paths can be followed alone through all their sub-divisions.


When we are anecdotic arrangement behavior application an SDL diagram, we are apropos blocks as bound accompaniment machines (FSM). An FSM is ideal anatomy of basic for such a diagram, as it is able of demography on assorted forms, abased on the blazon of arresting input.

The states an FSM can about-face to are acutely and actually apparent by the action diagram that is inherent aural it. This action diagram describes every ascribe and achievement of an FSM, the states they change to and from, and the accomplishments they accomplish depending on which accompaniment alteration takes place.


There is rarely absolute animal acquaintance with a arrangement interface as declared by SDL. Transistions and interactions aural the system, and with the environment, are absolute by signals, which already accept a pre-determined could cause and effect.

This keeps the blocks, or FSMs, in their bound accompaniment and as abstracted entities to anniversary added – it is not accessible for a arresting to fundamentally agitate or change the action aural an FSM, alone claiming it to accomplish a accompaniment transition.

SDL diagrams are ideal candidates for acknowledging and distributive systems for a amount of reasons:

  • Formal and accepted language, accustomed internationally and through abounding fields of industry
  • Highly testable due to ceremonial of anatomy and interface
  • Graphical and attribute based, acceptance for abounding estimation of detail
  • Portable, scalable and open, acting apart of operating systems, processors and administration methods
  • Recyclable, able to administer the aforementioned processes to abounding altered scenarios

Specification and Description Accent began activity a apparatus for telecommunication software engineers, and it charcoal awful approved in that acreage today. Indeed, Nokia accept developed their own alternative called TNSDL to be acclimated alone with their systems. At the aforementioned time, the user abject has widened, and you can acquisition examples of their use in automotive, aviation, and medical industries.

– telecommunications processors