Direct Distance Dialing - IDDD

IDDD

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 signaling, 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.