APPROVAL No. NS/3343/3/J/600812

 The MDSS (Modular Digital Switching System) is an analogue switch that was marketed by GEC and Siemens. Field trailed in early 1989 the original GEC version had v3 software that was programmed by a Psion hand controller and did not have LCR. Siemens only installed the v4, which was programmed by use of SIM software (PC based) and had LCR compatibility (only prior to phone day). After phone day smart boxes have to be fitted to achieve LCR functions. The system can have up to 64 ports (maximum of 32 trunks, 48 POTS, 56 keysets and 14 PW's). DC5 private wire circuits are LD out but can be LD or MF in. All control card and CPU software must be the same revision.

The unit is effectively a system box within a large metal cover, which was designed to stop electromagnetic radiation.  Probably overkill!

The system supports POTS and keyphones called Analogue System Supported Featurephones (ASSF). The POTS have to be MF dialling and TBR.

The 16 button handsfree model was known as the Keyset 20

CCU dimensions:- height - 780mm, width - 800mm, depth - 350mm
CCU weight:- 50kg

Made originally by GEC, then GPT, then Siemens/GEC and finally Siemens.


Telephone terminals may have an additional suffix of SAS or SMN. SAS is for the GEC variant whilst SMN is for the Siemens variant.

The software levels are 3.11 for v3 and for v4.



BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: November 05, 2021