EWSD was introduced in 1975 as a successor of the EWSA (Elektronisches Wählsystem Analog), which was using analogue technology.
The major six subsystems of EWSD are the Coordination Processor (CP), the Common Channel Network Control (CCNC), the Line Trunk Group (LTG), the Digital Line Unit (DLU), the Switching Network (SN), and the ISDN Primary Access Unit (PH).
The Coordination Processors exists in several types: The software of EWSD is called the Automatic Program System (APS).
The APS is stored on a disk drive and includes the operating system, developed by Siemens in cooperation with Bosch.
Support software serves translating programs, binding modules as well as administration of libraries for generating data.