Director Exchanges
Pre-2000 S50 Director Switch
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).


Image: An electro-mechanical Siemens Director switch © John Evans.[ Click photo for larger image.]
| EXIT | Director Exchanges | Belgravia | Holborn | Whitehall |

The Original Director Areas

The five original Director Areas developed from 1927 onwards, starting with the opening of Holborn (London) exchange.
  • Birmingham (021) from 1931
  • Edinburgh (031) from 1950
  • Glasgow (041) from 1937
  • Liverpool (051) from 1941
  • London (01) from 1927
  • Manchester (061) from 1930
Director Switching Explained

Numbering

In ND (Non-Director) exchanges (for local calls) the digits dialled by the subscriber are used to directly step the switches. Linked numbering schemes (LNS) minimise the number of  separate junctions between exchanges. A  large LNS might include as many as 20 exchanges.

In Director exchanges (for local calls) the digits dialled by the subscriber are translated into routing digits which identify the destination exchange. This allows a very large local call area with a single access code, e.g. London (01) which covers nearly 150 (check this number) exchanges. This has evolved historically from the large number of manual exchanges and the concentration of lines in a comparatively small geographical area.

To do this, the Director system used the exchange code as part of the telephone number...

For example, Whitehall 1212

Under the manual system:

Whitehall was the exchange name and 1212 was the telephone number.

Under auto exchange working

Letter and Number Dial
Callers dialled WHI to reach Whitehall exchange and 1212 for the telephone number.


Dialling W H I on a lettered dial gave the code 9 4 4, then the telephone number 1212.


Unlike Non-Director exchanges, both the code and telephone number were dialled, even on own exchange calls.
   
Under Subscriber Trunk Dialling (STD 1958 onwards):

As STD became available from 1958 onwards, before AFN, callers outside of London dialled

01 WHI 1212

STD Dialling Instruction Booklet 1964
Extract from DIB - Dialling Instruction Booklet 1964.

This shows a mix of Director city codes together with pre-AFN exchange codes and telephone numbers e.g.
Birmingham ACOcks Green 8000; dial 021 ACO 8000
   
Under All-Figure Numbers (AFN 1965 onwards):

With the progressive roll out of All-Figure Numbers (AFN) from 1965 onwards, London telephone numbers became 7-digits.
Some of the codes previously used under the mixed letter and number scheme were changed to give greater flexibility of codes. The code for Whitehall was changed from 944 to 930.

Thus WHI 1212 became 930 1212

With AFN this was then shown as 01-930 1212.

Callers within the London Director Area dialled the 7-digit number 930 1212, and callers outside of the LDA dialled 9-digits 01-930 1212.

The 930 code was translated to switch the call to the Whitehall exchange (from any location in London) and the 1212 was the actual number switched to.

See London Codes for more details.

   
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