Director telephone system
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The Director System was introduced to six cities in the UK from 1922 following the introduction of the automatic telephone exchange in the UK in 1912. [1]. It involved a device (the director) which received dialled digits and automatically translated them to route calls between exchanges in the city; in modern parlance a director incorporated a register-translator and a digit store. Directors were applied to step-by-step switching equipment; crossbar and, later, electronic switches of necessity had such capabilities built into them.
Each subscriber was given a seven digit number where the first three digits corresponded to the local exchange name, and were chosen to give the name a meaningful mnemonic. This was done by linking each number on the telephone dial to letters:
- 1
- 2 ABC
- 3 DEF
- 4 GHI
- 5 JKL
- 6 MN
- 7 PRS
- 8 TUV
- 9 WXY
- 0 OQ
Thus a subscriber in Wimbledon could be allocated the number WIMbledon 1234; the first three letters, written in capitals, indicated the code to be dialled. The actual trains of pulses from the subscriber's dial would, of course, be 946 1234. As the code (946 in this example) was the same from any telephone in the London director area, this uniformity is an example of a linked numbering scheme.
The Director system was adopted by the GPO as a solution for the reorganisation of the London telephone area which would use the existing expertise in step-by-step switching. Western Electric in the U.S. had produced the common-control Panel system for equipping cities, but its basic switching module (the Panel) was comparatively large and the system was for economic reasons far better suited to business than to residential areas. Director switching, by contrast, had much smaller switching modules with distributed control; these could be used economically in suburban areas where the rate of line provision was comparatively light and calling rates were low, as well as in the central business district, which in London meant the City of London.
A director translated the first three digits of the subscriber number to a much longer string, which could consist of from one to six digits. These stepped the selectors at the intermediate exchanges on the route giving access to the target exchange. The remaining digits were then forwarded unchanged, to step the local numerical selectors at the target exchange.
With the introduction of Subscriber Trunk Dialling (STD) each city with a Director system was given a 3 digit code where the second digit corresponded to the first letter of the cities name on the telephone dial, with the exception of London which was given a 2 digit code "01":
- 01 London
- 021 Birmingham
- 031 Edinburgh
- 041 Glasgow
- 051 Liverpool
- 061 Manchester
It became increasingly difficult to make meaningful names from number combinations, and names of notable local figures often substituted for abbreviated placenames. The use of letter mnemonics had to be dropped in favour of all figure numbering in 1966.
Calls from Ireland
Until 1992, calls to these cities from Ireland were made using the following codes:
- 031 London
- 032 Birmingham
- 033 Edinburgh
- 034 Glasgow
- 035 Liverpool
- 036 Manchester
In that year, this changed to dialling in the international format 00 44, and the 03 range was withdrawn from use.
Director systems in the US
In the United States, most large cities did not use step-by-step equipment, but Los Angeles was a major exception. Before the advent of electronic switching, directors were commonly used in areas of the city served by GTE.