Director telephone system

The Director System was a system which made it possible to call subscribers at other telephone exchanges without operator intervention in large multi-exchange cities, and to have a mixture of automatic and manual exchanges within these cities. It was introduced to six cities in the UK from 1927, starting with London, following the introduction of the automatic telephone exchange in the UK in 1912.^[1]

History

Mechanics and organisation

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. The letter arrangement was similar to American dials, except that the letters "O" (and also "Q") were at digit "0" not "6":

• 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 code was written in bold capitals if the caller should dial all seven digits. If written merely in capitals it indicated that the desired number was on an exchange which had not yet been converted to automatic working, and that the caller should dial only the initial three code digits, and expect to be connected by an operator. As conversion was completed (and as the remaining manual exchanges were equipped with CCI or Coded-Call Indicators which displayed the local digits dialled by the caller) this difference gradually disappeared.

Implementation

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 switch 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.

Western Electric had proposed the Rotary system for London, a machine-switching system developed by Bell Labs with many features in common with the Panel system, but with smaller switches. However, as the GPO had extensive experience of step-by-step switching, it favoured the Director system which would have much in common with the existing non-director exchanges, and which would be manufactured in Britain from the outset.

The Director system worked well for individual lines, but for offices and shops which had multiple lines with a common directory number and usually terminating on a switchboard it was necessary to provide elaborate final selectors to permit access to all of the lines. Although this problem arises for any area with Strowger switching, the nature of an area with a Director system meant that large installations were much more common. Each final selector had to be able to search every line to find a free one, and as premises could have up to 200 lines the selector elaboration was considerable. This meant more expensive final selectors, fewer selectors per exchange rack as their relay sets were larger, and a longer wait for the caller while the selector found a free line. In a common control system line hunting is a fundamental part of its design, so this deficiency did not exist.

A director translated the first three digits of the subscriber number to a variable length string, of from one to six digits. A single digit translation was used for local calls within the exchange or for calls to adjacent exchanges on busy routes; this minimised call setting-up time, and minimised the number of switches required within the exchanges as busy routes required only one group selection stage preceding the local exchange numerical selectors. Up to six digits could be allocated to calls to distant exchanges; this gave the ability to pass calls through intermediate tandem exchanges, so that small amounts of traffic could be concentrated into busy routes rather than requiring lightly loaded direct routes to many exchanges. This routing could be changed as more exchanges were automated or new local and tandem exchanges introduced.

The remaining four dialled digits were then forwarded unchanged, to step the local numerical selectors at the terminal (local) exchange (or to actuate the CCI or Coded-Call Indicator equipment at manual local exchanges equipped with it).

For many years the code for the operator was '0', and special provision was made to allow certain of the directors to dial up to four digits for operator access after receiving only the single '0'. With the decision to use '0' as the STD prefix digit this special access could no longer be maintained, and the operator assistance code was changed to '100'.

Usually ten groups of Directors were required (selected by a group selector which operated on the first Exchange digit). But some smaller Director areas could combine some first-digit levels, although this further restricted the names which could be used for exchanges. Before the exchanges were replaced, some Director exchanges had the Director equipment replaced by electronic directors using CMOS technology which controlled the step (Strowger) switches in the exchange, with economies in space and maintenance.

Subscriber Trunk Dialling

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 city's 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

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 used the Panel switch rather than step-by-step equipment. Los Angeles being a small town early in the 20th century (and partly served by an independent telephone company) grew up to be a major exception. Before the advent of electronic switching systems, directors were commonly used in areas of the city served by GTE.

See also

• Subscriber trunk dialling
• Telephone number
• Sender

References

1. ^ Events in Telecommunications History - 1927 BT Archives

External links

• UK telephone history