Inland Switching: Exchange Types
Exchange Types
Once, all switching equipment used electro-mechanical switches to connect telephone calls. The Strowger system used a step-by-step process to route calls through the exchange. The Crossbar system used cross-point switches with marker control and lots of relays. Even the replacement electronic systems used reed-relays, until the solid-state System X and System Y switches were introduced in the late 1970s.

Photo: A large Strowger exchange © Dave Milner.
| EXIT | Introduction | Strowger | Crossbar | Electronic | Digital | Signalling | Abbreviations | TE Buildings | References |

Telephone Exchanges (TEs)

Automatic Telephone Exchanges (ATEs) are (or used to be) categorised into two kinds:  
The two types analyse the subscriber's number in different ways and use this to complete the switching of the call.


Director Exchanges grew from the requirement for an automatic switch that could route calls between a large number of exchanges that were located within in a small area (e.g within a city such as London).

The original Director areas had dialling codes beginning:

  • 01 London
  • 021 Birmingham
  • 031 Edinburgh
  • 041 Glasgow
  • 051 Liverpool
  • 061 Manchester
In summary, the switching equipment was configured in a special way and used routing techniques not found outside of the big cities.

Learn more about Director Exchanges.


All other exchanges and codes are assumed to be Non-Director. This type of switching can be found in the smallest exchange of just a few hundred (or fewer) subscribers and was used in large towns as well.

Telephone eXchange (Types)

The different exchange types can then be split according to size and type of switching equipment in use:

TXS Telephone eXchange Strowger
TXK Telephone eXchange Crossbar  [Exceptionally the C is changed to a K]
TXE Telephone eXchange Electronic
TXD Telephone eXchange Diigital
TXS, TXK and TXE types are now all obsolete.

TXS or Strowger exchanges, were electro-mechanical and known as step-by-step, as the switching stages followed a pre-wired start to finish path through the equipment.

TXS or Strowger Exchanges
Throughout the Strowger era there have predominantly been only three different versions of the 2-motion selector, some older exchanges using a mixture of all three, which were:

Pre-2000 type- The original and all designs prior to 1936.
2000 type- a greatly improved more compact design introduced in 1936.
4000 type- a slightly modified design, less durable, introduced in 1958.

Learn more about
Strowger Exchanges.
TXK or Crossbar used common control and end to end selection, but were still electro-mechanical using a matrix pattern of switches and relays. There are some shared or common control units which are only used during the setting up of the call which are then released for use on other calls in progress at the same time. The switching does not follow the same path through the exchange each time. Very simply, each end of the circuit is marked and the exchange then selects a path between between the two ends. There were four common types of switch, plus TXK5 and 6.

TXK or Crossbar Exchanges
The first Telephone eXchange Crossbar no.1 TXC1 to be installed by the Post Office was in Broughton, Lancashire in 1964. TXC sounded too much like TXE, so the designation was later changed to TXK.

Learn more about Crossbar Exchanges.

Photo: TXK at Sudbury © Martin Loach.
TXE or Electronic exchanges also used common control for call set up with solid-state (no moving parts) processors. However, this was far from being anything but a hybrid of technologies. The actual switching circuits comprised of many hundreds of reed relays and various add-on units to interface with existing signalling systems.
TXE or Electronic Exchanges
One of the first model electronic exchanges opened in Dollis Hill in 1959, followed by an 800 line unit at Highgate Wood, London in 1962. Both used TDM (Time Division Multiplexing) and PAM (Pulse Amplitude Modulation) systems. However, as integrated circuits and microprocessors had not yet been developed, the costs were too high and therefore SD (Space Division) techniques using reed relays were chosen.

Learn more about Electronic Exchanges.

Photo: TXE4, Liverpool Central © Nigel Pope 1981.
TXD or Digital exchanges are software controlled using both digital signalling and transmission to connect the calls. The first fully digital exchange to be brought into service in the UK was a UXD5A (Unit eXchange Digital), a modified Monarch 300 line PBX at Glenkindie, Aberdeenshire in 1979. And from 1983, an updated version UXD5B was installed in further rural locations.

Digital Exchanges
The first exchange to switch Pulse Code Modulation (PCM) signals from one group of lines to another, in digital form, was demonstrated at the Post Office Research Station on 15th February 1968.

Learn more about Digital Exchanges.

Photo: TXD (System X) © LSA Nov 2008.

Inter-exchange signalling on the junction, trunk and international networks has evolved over the years from simple digital techniques on copper cables, through analogue to high speed data and voice capability at ever more flexible bandwidths over fibre.

Learn more about Signalling.

Photo: PCM 30 racks © LSA July 2013.


Here is a list (circa 1980) of the letter designations of all the main types of exchanges in use at the time:

D Director, Strowger
DE Director, Electronic
DK Director, Crossbar
DY Director, mixed Strowger & Electronic
DZ Director, mixed Strowger & Crossbar
ND Non-Director, Strowger
NDE Non-Director, Electronic
NDY Non-Director, mixed Strowger & Electronic
NDZ Non-Director, mixed Strowger & Crossbar
U Unit Automatic Exchange, Strowger
LC Line connector with an exchange name
IAX Island Automatic Exchange 
SAX Small Automatic Exchange, Strowger
(M) Mobile Exchange
DDI Exchange codes allocated to PBX groups with Direct Dialling In facilities.

Over the years, quite a variety of letter codes have been (wrongly?) used, most commonly the D or ND being omitted and the letter X being used for eXchange, to free up the letter E to be used for other abbreviations.

Thus A.T.E. is generally an Automatic Telephone Exchange and an N.D.X. is a Non-Director eXchange.
TXK was used for Crossbar as C and E sound similar.

And the list for A.M.C.s (Auto-Manual Centres ) e.g. Operator Services:
  • AB Automanual (Bridge Control)
  • AS Automanual (Sleeve Control)
  • AC Automanual (Cordless Type)
And the abbreviations for Dedicated Main Network Switching Centres:

GSC Dedicated Group Switching Centre
SSC Sector Switching Centre
DSC District Switching Centre
D Director
ND Non-Director
E Electronic
K Crossbar
I/C Incoming
O/G Outgoing
RT Register Translator
Exchange Buildings

For Telephone Exchanges, the abbreviation is TE, or ATE for Automatic Telephone Exchange, but seldom TX, which usually refers to the equipment rather than the place or the building.

Red Book Green Book THE code DECODER
The 'Red and Green' books A1360 D Parts 1 and 2 and 'The code Decoder'.
Design, images and text compiled by © Light-Straw. Page last updated 22nd Jan 2015.

All logos and trade marks are the property of their respective owners and are used on the Light Straw site(s) for review only. Students and researchers are recommended to make their own independent enquiries as to the accuracy of the information contained therein.