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Direct Distance Dialing

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Title: Direct Distance Dialing  
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Subject: List of original NANP area codes, Push-button telephone, Mayor of Englewood, New Jersey, CTS, Telephone switchboard
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Direct Distance Dialing

Direct distance dialing (DDD) or direct dial is a telecommunications term for a network-provided service feature in which a call originator may, without operator assistance, call any other user outside the local calling area. DDD requires more digits in the number dialed than are required for calling within the local area or area code. DDD also extends beyond the boundaries of national public telephone network, in which case it is called international direct dialing or international direct distance dialing (IDDD).

DDD is a North American Numbering Plan term considered obsolete since completing a call in any manner other than direct dialing became rare. In the United Kingdom and other parts of the Commonwealth of Nations, the equivalent terms are or were "STD", for subscriber trunk dialing, and "ISD" for international subscriber trunk dialing.


The telephone industry made a United States "first" in the New Jersey communities of Englewood and Teaneck with the introduction of what is known now as direct distance dialing. Starting on November 10, 1951, customers of the ENglewood 3, ENglewood 4 and TEaneck 7 exchanges (who could already dial New York City and area) were able to dial 11 cities across the United States, simply by dialing the three-digit area code and the seven digit number (which at the time consisted of the first two letters of the exchange name and five digits).

The 11 cities and their area codes at that time were:

Many other cities could not yet be included as they did not yet have the necessary switching equipment to handle incoming calls automatically on their long-distance calling circuits. Other cities still had either a mixture of local number lengths or were all still six-digit numbers; Montreal and Toronto, Canada, for example, had a mix of six- and seven-digit numbers from 1951 to 1957, and did not have DDD until 1958. Whitehorse, Yukon, had seven-digit numbers from 1965, but the necessary switching equipment was not in place locally until 1972.


The Number 4 Crossbar, or 4XB switch, had been introduced in the 1940s to switch four-wire circuits and replace the incoming operator. With semiautomatic operation analogous to the early days of panel switch, the operator in the originating city used a 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 central office code number dialed by the customer. This determined the proper trunk circuits to use, where separate circuit groups were used for different cities in the same area code, as in the case of Oakland and San Francisco. The new device used metal cards similar in principle to computer punched cards, and they were rapidly scanned as they fell past a light beam. On busy days, it sounded like a machine gun firing. 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. One obstacle was that the majority of switching gear did not provide Automatic Number Identification (ANI). Common control switches such as 1XB switch were fairly quickly retrofitted to provide ANI, and most 5XB switches were initially installed with ANI services. Panel switch were eventually retrofitted, as were some step-by-step that were not scheduled for immediate replacement. Even if a switch had ANI, it could not identify callers on party lines. This was only partly overcome by Tip Party Identification.[2] As the cost of subscriber line carrier declined, party lines were gradually phased out.

As this and other improved technologies became available, as well as Automatic Message Accounting (AMA) computers to process the long distance records into customer bills, the reach of DDD was slow in the 1950s, but quickened in the early 1960s. Electronic switching systems with stored-program capability allowed electronic processing of the dialed digits, referring to electronic memories to determine call routing, and this has reached the state of the art, with digital telephone exchanges which are basically specialized computers that route voice traffic from one "peripheral" to another as digital data. Call routing can now be done based on the area code, central office code and even the first two digits of the line number, although routing based on digits past the central office code is usually limited to cases of competitive local exchange carriers, number pooling and number portability.


In the 1960s, with the domestic conversion still underway, plans were laid to extend Direct Dialing beyond North America and its nearby islands. With so much new equipment already working that could only handle ten digit phone numbers, a system was devised by which most toll offices did not have to store and forward the whole international phone 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 32 Avenue of the Americas. The new LT1 5XB switch on the tenth floor of 435 West 50th Street received new Originating Registers and Outgoing Senders able to handle 15 digit telephone numbers, with appropriate modifications to Completing Markers and other equipment. Other 5XB 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 multi-frequency pulsing. The outgoing sender sent its Class 4 toll center an off-hook signal as usual, received a wink as usual as a "proceed to send" signal, and outpulsed only a special 3-digit (later 6-digit) access code. The toll center picked a trunk through the long distance network to the gateway office, which sent a second wink to the originating office, which then sent the whole dialed number. Thus the toll switching system needed no modification except at the gateway. The international trunks used Signaling System No. 5, a "North Atlantic" version of the North American multi-frequency signaling system, with minor modifications including slightly higher digit rate. European MF systems of the time used compelled signalling, which would slow down too much on a long transoceanic connection.

In the 1970s, toll centers were modified by adding 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.


  1. ^ 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 in order to distinguish between the two areas. Area code 318 was temporarily used to specify San Francisco and areas north of the Golden Gate, while calls with destinations in Oakland and the East Bay continued to use area code 415. When the electromechanical card-translator box became available sometime during 1952–53, 6-digit translation became possible and the use of area code 318 was no longer required. Area code 318 was reclaimed for future use and the entire San Francisco Bay Area returned to using area code 415.[1]
  2. ^ Free Patents Online Tip Party Identification


External links

  • 4XB switch
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