Direct distance dialing

Source: Wikipedia, the free encyclopedia.

Direct distance dialing (DDD) is a

caller may, without operator assistance, call any other user outside the local calling area. Direct dialing by subscribers typically requires extra digits to be dialed as prefixes to the directory telephone number of the destination. International Direct Distance Dialing (IDDD) extends the system beyond the geographic boundaries of the North American Numbering Plan
(NANP).

History

The first direct-dialed long-distance telephone calls were possible in the

long-distance call, to Mayor Frank Osborne of Alameda, California.[1][2]

The destinations, and their area codes, equipped with a long-distance toll-switch at that time were:

Other areas could not yet be included in DDD as they did not have the necessary toll switching equipment, or because they still did not use a seven-digit local numbering plan.

Whitehorse, Yukon
, had seven-digit numbers starting in 1965, but the necessary switching equipment was not in place until 1972.

San Francisco required the special area code 318 due to temporary routing requirements. San Francisco and Oakland each had their own separate toll-switches, so calls had to be routed accordingly depending on the final destination. As the telephone equipment used at the time could only handle three-digit translation, the temporary use of area code 318 was required to distinguish between the two areas. Area code 318 was temporarily used to specify San Francisco and areas north of the

East Bay continued to use area code 415. When the electromechanical card-translator box became available sometime during 1952–53, six-digit translation became possible and the use of area code 318 was no longer required. Area code 318 was reclaimed for future use (now used as an area code for northern Louisiana), and the entire San Francisco Bay Area returned to using area code 415.[3]

Hardware

The No. 4 Crossbar switching system had been introduced in the early 1940s to switch

multifrequency
keypad to dial an access code to connect to the correct city and to send the seven-digit number to incoming equipment at the terminating city. This design was further refined to serve DDD.

The card sorter of the 4A/CTS (Number 4A Crossbar / Card Translator System) allowed six-digit translation of the

punched cards, and they were rapidly scanned as they fell past a light beam. CTS machines were called 4A (Advanced) if the translator was included in the original installation, and 4M (Modified) if it was added later. A 1970s version of 4XB, the 4A/ETS, used a computer to translate. For international dialing, Traffic Service Position System
(TSPS) provided the extra computer power.

The reach of DDD was limited due to the inefficiency and expense of switching equipment, and the limited ability to process records of completed calls. An early obstacle was that the majority of switching systems did not provide

subscriber line carrier
declined, party lines were gradually phased out.

As this and other improved technologies became available, as well as

number pooling and number portability
.

IDDD

AT&T Long-Distance Building

In the 1960s, with the domestic conversion still underway, plans were laid to extend Direct Distance Dialing beyond North America (including a number of the Caribbean Islands). Some subscribers could already directly dial transatlantic telephone calls to certain destinations as early as in 1957 over the recently completed Atlantic cable to England. A new systematic extension of Direct Distance Dialing was developed and was introduced as International Direct Distance Dialing (IDDD) in March 1970.[5]

With so much new equipment already working that could not handle more than the requisite ten-digit telephone numbers of DDD, the new system was based on designs by which most toll offices did not have to store and forward the whole international telephone number. Gateway offices were set up in New York, London and Paris, connected to the ordinary automatic toll network. The New York gateway was at

5XB switch on the tenth floor of 435 West 50th Street received new originating registers and outgoing senders able to handle fifteen-digit telephone numbers, with appropriate modifications to completing markers
and other equipment. Other 5XB switches in the next few years were installed with IDDD as original equipment, and in the 1970s ESS offices also provided the service.

The key to the new system was two-stage

compelled signaling
, which would slow down too much on a long transoceanic connection.

In the 1970s, toll centers were modified by adding the Traffic Service Position System (TSPS). With these new computers in place, digit storage in the toll system was no longer a problem. End offices were less extensively modified, and sent all their digits in a single stream. TSPS handled the gateway codes and other complexities of toll connections to the gateway office.

Equivalent service in the United Kingdom

In the United Kingdom and other parts of the

Lord Provost.[citation needed
]

See also

References

  1. ^ "Englewood Begins Long Distance Customer Dialing" (PDF). Bell Laboratories Record. 29 (12): 571–572. December 1951.
  2. ^ "Who's on First? Why, New Jersey, of Course". The New York Times. July 22, 1979. Retrieved May 28, 2017. More recently, on Nov. 10, 1951, Mayor Leslie Denning of Englewood telephoned Mayor Frank Osborne of Alameda, Calif., without the help of an operator and Englewood became the first city in the nation whose residents had direct‐dial coast‐to‐coast service.
  3. ^ Townson, Patrick A., ed. (August 15, 1996). "When Was Direct Distance Dialing Cut In?". Telecom Digest. 16 (401).
  4. ^ "Telecommunications Subscriber Line Interface Circuit and Party Identification Facility". United States Patent and Trademark Office.
  5. ^ AT&T, Notes on Distance Dialing (1975)

External links

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