Director telephone system
From Wikipedia, the free encyclopedia
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 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 code digits, and expect to be connected by an operator. As conversion was completed this difference gradually disappeared.
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.
The distributed control meant that a 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 multi-line search 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 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. For many years the code for the operator was '0', and special provision was made to allow directors to dial the access digits for the operator 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'.
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 for these areas.
However, for the rest of the country exchange names continued to be quoted by subscribers in many cases, particularly for example in local advertising where callers would not need to dial the trunk code. Although there are still isolated examples, this has largely been displaced now because of the proliferation of other telecommunications suppliers and non-geographic numbering. Nevertheless, it is still possible in most cases to narrow down or work out the rough geographical location of what is now called an "Area" code by reference to the corresponding digits on a telephone keypad, as follows:
The first figure "1" after the leading zero was an additional digit introduced to provide more allocatable subscriber numbers after PhONEday on Easter Day, 1995). Initially it was irrelevant as all numbers began "01", but with the renumbering of certain parts of the UK - London, Portsmouth and Reading, for example - "02" numbers have entered service.
The next two digits correspond with the first two letters of the main exchange area in a group within Great Britain, the Channel Islands and Isle of Man and most of the smaller offshore islands such as the Isle of Wight or the Western Islands in Scotland.
The fifth digit is either insignificant, or narrows down the area still further (01)27= BR corresponding to these letters on a keypad, so 01272= BRistol (formerly)and 01275 (outer Bristol area-still in use) 01273= BRighton 01274= BRadford 01278= BRidgwater, Somerset
and (01)48 can stand for either GU, as in 01481= GUernsey, Channel Islands 01483= GUildford, Surrey area
or HU, as in 01482 = HUll area, Yorkshire 01484 = HUddersfield area, Yorkshire
Within these areas, the STD system then further broke down the number for outlying districts, suburbs and villages surrounding the main exchange. This was determined by the first or second digits of the individual subscriber phone number following the area code. Perhaps the best way of understanding how this works is to look up main five figure area codes in one of the several "Dialling code decoders" which are available from BT and other suppliers; 01323, for instance, will show codes for Eastbourne as well as Seaford and Hailsham while, 01424 covers Hastings as well as the villages surrounding it and westwards to Bexhill.
However, it should be said that this can only be used as a rough guide and is perhaps more memorable for those using the telephone regularly in association with other data (e.g outbound call centre operators). The proliferation of telecomms operators and non-geographic numbering and now "local loop unbundling" make it likely to be a far less reliable guide in future, but a fun exercise in linking numbers and places nevertheless.
[edit] 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.
[edit] 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 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.
