Hardware
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, 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. 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 and number portability.
Read more about this topic: Direct Distance Dialing
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