Director telephone system
Encyclopedia
The Director System was a system which made it possible to call subscribers at other telephone exchange
s 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.
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":
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.
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 switch
ing, 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.
(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":
were made using the following codes:
In that year, this changed to dialling in the international format 00 44, and the 03 range was withdrawn from use.
, 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 system
s, directors were commonly used in areas of the city served by GTE
.
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
s 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
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
from 1927, starting with London, following the introduction of the automatic telephone exchange in the UK in 1912.
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; crossbarCrossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....
and, later, electronic switches
TXE
TXE, which stands for Telephone eXchange Electronic, was the designation given to a family of telephone exchanges developed by the British General Post Office , now BT, designed to replace the ageing Strowger systems....
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
Mnemonic
A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...
. 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
Wimbledon, London
Wimbledon is a district in the south west area of London, England, located south of Wandsworth, and east of Kingston upon Thames. It is situated within Greater London. It is home to the Wimbledon Tennis Championships and New Wimbledon Theatre, and contains Wimbledon Common, one of the largest areas...
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
Linked numbering scheme
A Linked Numbering Scheme is a telephone numbering plan applied to an area of the country where calling between lines on a number of adjacent exchanges is done without using a dialing code....
. 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 ElectricWestern Electric
Western Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...
in the U.S. had produced the common-control
Common control
In telecommunication, a common control is an automatic telephone exchange arrangement in which the control equipment necessary for the establishment of connections is shared by being associated with a given call only during the period required to accomplish the control function for the given call...
Panel switch
Panel switch
The panel switching system was an early type of automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s...
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
City of London
The City of London is a small area within Greater London, England. It is the historic core of London around which the modern conurbation grew and has held city status since time immemorial. The City’s boundaries have remained almost unchanged since the Middle Ages, and it is now only a tiny part of...
.
Western Electric had proposed the Rotary system for London, a machine-switching system developed by Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
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 switch
Strowger switch
The Strowger switch, also known as Step-by-Step or SXS, is an early electromechanical telephone switching system invented by Almon Brown Strowger...
ing, 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 DiallingSubscriber trunk dialling
Subscriber trunk dialling is a term for a telephone system allowing subscribers to dial trunk calls without operator assistance.- Terminology :...
(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 IrelandRepublic of Ireland
Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...
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 StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, most large cities used the Panel switch
Panel switch
The panel switching system was an early type of automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s...
rather than step-by-step equipment. Los Angeles
Los Ángeles
Los Ángeles is the capital of the province of Biobío, in the commune of the same name, in Region VIII , in the center-south of Chile. It is located between the Laja and Biobío rivers. The population is 123,445 inhabitants...
being a small town early in the 20th century (and partly served by an independent telephone company
Independent telephone company
An Independent telephone company in the United States was a telephone company providing local service that was not part of the Bell System group of companies, "Ma Bell", before the 1984 Bell System divestiture or breakup of the Bell system...
) grew up to be a major exception. Before the advent of electronic switching system
Electronic switching system
In telecommunications, an electronic switching system is:* A telephone exchange based on the principles of time-division multiplexing of digitized analog signals. An electronic switching system digitizes analog signals from subscriber loops, and interconnects them by assigning the digitized...
s, directors were commonly used in areas of the city served by GTE
GTE
GTE Corporation, formerly General Telephone & Electronics Corporation was the largest independent telephone company in the United States during the days of the Bell System....
.