Subscriber trunk dialling

Subscriber trunk dialling (STD, also known as subscriber toll dialing) is a telephone system allowing subscribers to dial trunk calls without operator assistance. The term was introduced when it first became possible for long-distance calls to be dialed directly, and is now rarely used where calls to any destination can be dialed.

Terminology

The term subscriber trunk dialling is used in the United Kingdom, the Republic of Ireland, Australia, India and South East Asia. The corresponding term in the North American Numbering Plan (i.e. in the United States and Canada) is direct distance dialing.

The term was extended when, on 8 March 1963, subscribers in London were able to directly dial Paris using international direct dialing.

Explanation
Typical signboards of STD booths (kiosks from where STD calls can be made) in India

The introduction in the UK of subscriber dialing of long-distance calls removed the distinction that had existed between trunk and toll calls. This term however, is still widely prevalent in India to describe any national call made outside one's local unit. A "subscriber" is someone who subscribes to, i.e. rents, a telephone line and a "trunk call" is one made over a trunk line, i.e. a telephone line connecting two exchanges a long distance apart. Now that all calls may be dialled direct, the term has fallen into disuse.

History

When telephone systems were first introduced, subscribers called a telephone exchange and asked a human operator to connect the call to another subscriber on the same exchange; calls to other exchanges were originally not possible. Later it became possible to dial numbers on the same exchange; calls to other exchanges (trunk calls) were possible, but had to be connected by an operator. When subscribers in one area became able to dial non-local subscribers, the term used for the innovation was subscriber trunk dialling.

In the UK, STD started before 5 December 1958 when the Queen, who was in Bristol, publicised it by dialling Edinburgh – the farthest distance a call could be directly dialled.[1] The STD system was completed in 1979,[2] though most of the country was covered well before then. The system required that a new STD code, which could be dialled by subscribers, be allocated to each area; in the UK area codes are still sometimes called STD codes.

Numbering plan

With the introduction of subscriber trunk dialling, each city with a director system was assigned a 3-digit code, in which the second digit corresponded to the first letter of the city name on the telephone dial (except London which had the two-digit code 01). Codes were later changed (e.g., London became 020, and Manchester 0161).

  • 01 London
  • 021 Birmingham
  • 031 Edinburgh
  • 041 Glasgow
  • 051 Liverpool
  • 061 Manchester

Calls from and to the Republic of Ireland

Because of the high volume of calls between the Republic of Ireland and the UK, international direct dialling was possible before the formal introduction of International Subscriber Dialling in either country. Calls were processed through the domestic STD networks and passed between the two networks as trunk traffic, without the need for international gateway exchanges.

From the Republic of Ireland

Calls to Northern Ireland were made by dialling 08 and the Northern Irish STD code, for example Belfast was reached by dialling 08 0232.

Calls to Britain were made by dialling 03 and the British STD code, e.g. 03 0222 XXX XXX or 03 061 XXX YYYY.

Calls to cities with director area codes could also still be made with the following codes; this was an older arrangement but the numbering remained in service until the 03 code was closed:

  • 031 London
  • 032 Birmingham
  • 033 Edinburgh
  • 034 Glasgow
  • 035 Liverpool
  • 036 Manchester
  • Calls to Belfast could also be dialled with 084 and the local six-digit number. Belfast was not a director area.

In 1992, Ireland adopted the harmonised European international access code 00, replacing the 16 prefix for international calls and the legacy arrangements for calling Britain. From that year, calls were made in the standard international format i.e. 0044, and the 03 range was withdrawn from use.

Calls to Northern Ireland are now made by dialling 048 and the eight-digit local number, omitting the 028 STD code. This ensures calls are charged at lower rates. Alternatively, the full international code +44 28 can be used.

Calls to Ireland from the UK

These were dialled using the full international code 010 353, or using legacy short codes. Examples were:

  • Dublin 0001
  • Cork 0002
  • Limerick 0006
  • Galway 0009

These legacy codes dialled directly into Irish cities that had crossbar switching in the 1950s and 60s, and predated the introduction of ISD in the UK. The Irish STD system evolved around the introduction of LM Ericsson ARM and ITT Pentaconta crossbar trunk/tandem switches, and did not use the UK's director approach. While these calls were international, they were processed within the UK STD infrastructure, without passing through an international gateway exchange.

Calls to Ireland are now made in the standard international format +353 (or 00 353) and special codes are no longer used.

See also

References
  1. "Events in Telecommunications History: 1958, 'BT's history', btplc.com/". 2008-06-18. Retrieved 2008-08-12.
  2. "Events in Telecommunications History: 1979, 'BT's history', btplc.com/". 2008-06-18. Retrieved 2008-08-12.


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