EWSD (Elektronisches Wählsystem Digital in German, Electronic Digital Switching System/Electronic World Switch Digital in English) is one of the most widely installed telephone exchange systems in the world. EWSD can work as a local or tandem switch or combined local/tandem, and for landline or mobile phones. It is manufactured by Siemens AG, who claims that EWSD switches perform switching for over 160 million subscriber lines in more than 100 countries.
DeTeWe bought its first EWSD under license in 1985 for remote switching. Bosch built its first EWSD as a local exchange in 1986. Deutsche Telekom, formerly Deutsche Bundespost, the largest German telephone company, uses EWSD and System 12 (Alcatel), the former more than the latter.
EWSD was introduced in 1975 as a sequel of the Elektronisches Wählsystem (Analog), which was using analogue technology only.
Main subsystems are:
- CP (Central Processor)
- MB (Message Buffer)
- CCNC (Common Channel Network Control)
- LINE (Analog Line Group)
- LTG (Line Trunk Group)
- DLU (Digital Line Unit)
- SN (Switching Network)
- PA (ISDN Primary Access) 
- PDC (Primary Digital Carrier)
All system units are redundant so the inactive side can take over immediately in case of an error.
DLU handles analog and ISDN lines and includes codecs for analog lines, one of the BORSCHT functions for subscriber lines. Digital signals are assigned a time slot. DLU concentrates traffic onto a Line B unit, as well as Primary ISDN and V5.2 connections. Supervision and address signaling (dial pulse, DTMF) are also integrated in the DLU. For PCM30 (E-1) connections to other exchanges, Line C Units are used, which also handle signaling including SS7, MFC R2 signalling, IKZ (dial pulse), and E&M.
The Switching Network consists of 4 space division stages of 16x16 switches, and a time division section with 16 stages of 4x4 switches. Control is provided by the CP Co-ordination Processor.
There are the following kinds of Co-ordination Processors:
- CP103 with max 22,000 call attempts in the busy hour
- CP112 with max 60,000 call attempts in the busy hour
- CP113D with max one million call attempts in the busy hour
- CP113C with max six million call attempts in the busy hour
- CP113E with max ten million call attempts in the busy hour
The software of EWSD is called APS (Automatic Program System). The APS is on a hard drive and includes the operating system, developed by Siemens in cooperation with Bosch. It is predominantly written in the CHILL language. Application software is switch specific and serves among other things traffic management, path search, and call charging. Support software serves translating programs, binding modules as well as administration of libraries for generating data. Operating and data communication software serve for co-operation of maintenance centers and switching centers.
- Number of access lines: to 250,000
- Number of feeder lines: 240,000
- Traffic connection: 25,200
- Call attempts in busy hour: 10 million
- Operating voltages: -48V -60V -90V
- Rate zones: 127, for each zone of 6 tariffs
- Tariff change-over at 15 minute intervals
- Space requirement with 10,000 access lines: 35 square meters