Editing Direct distance dialing (section)

==IDDD==
[[File:32 Avenue of Americas stormy bright jeh.jpg|thumb|[[32 Avenue of the Americas|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 [[List of Caribbean islands|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.<ref>AT&T, ''Notes on Distance Dialing'' (1975)</ref>

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 [[32 Avenue of the Americas]].{{cn|date=December 2020}} The new LT1 [[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 [[Marker (telecommunications)|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 [[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 three-digit (later six-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 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.