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 (and codes) were:
  • Birmingham (021)
  • Edinburgh (031)
  • Glasgow (041)
  • Liverpool (051)
  • London (01)
  • Manchester (061)
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 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 uses 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 mechanisation & Subscriber Trunk Dialling:

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

Under All-Figure Numbers (AFN):

Telephone numbers became 7 digits even for own exchange calls, thus Whitehall 1212 became 930-1212

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

All callers within the London Director Area dialled the full number to be connected. i.e. 9301212

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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