Editing Direct distance dialing (section)

==Hardware==
The No. 4 Crossbar switching system had been introduced in the early 1940s to switch [[four-wire circuit]]s and replace the incoming operator. With semiautomatic operation analogous to the early days of the [[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 [[telephone exchange|central office]] code number dialed by the customer. This determined the proper [[Trunking#Telecommunications|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. 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 [[Automatic Number Identification]] (ANI). [[Common control]] switches, such as the [[1XB switch]], were fairly quickly retrofitted to provide ANI, and most [[5XB switch]]es were initially installed with ANI services. [[Panel switch]] were eventually retrofitted, as were some [[Strowger switch|step-by-step systems]] that were not scheduled for immediate replacement. Even if a switch had ANI, it could not identify callers on [[Party line (telephony)|party lines]]. This was only partly overcome by tip-party identification for two-party lines.<ref>{{cite web |url=https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/4862495 |website=[[United States Patent and Trademark Office]] |title=Telecommunications Subscriber Line Interface Circuit and Party Identification Facility}}</ref> 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 system]]s 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 exchange]]s 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 carrier]]s, [[number pooling]] and [[local number portability|number portability]].