1962

Digital Long Distance

Picture this: It’s Mother’s Day around 1960. All over the country, sons and daughters who live far away from their mothers are calling to wish them a happy day and say thank you for all they do. Except many of them can’t, because they can’t get their calls to go through. All they hear when they dial is a busy signal or an automated voice saying to try again later. That was because there was a relatively small number of copper wire pairs crisscrossing the country as part of the telecommunications network, and each pair could carry just a single conversation.

With the introduction of AT&T’s digital T1 carrier service, the capacity of each pair of copper wires dramatically increased. Rather than one conversation per pair of twisted wires, two pairs could carry 24 conversations simultaneously. The T1 service did this by converting all the analog voice data to digital format and sequencing or organizing that data to travel together on a copy pair and then get accurately separated for delivery to the intended residence or phone line. In essence, there was suddenly more than 10 times the capacity on each copper pair. (For technical reasons, the T1 required a copper pair to carry data in each direction.) The first T1 was installed in Chicago, where the city had run out of space in places to add more buried cable under the city streets.

The digital long-distance service required three things: the T1 digital communication protocol, a technology called a multiplexer to combine the 24 conversations into a single data stream, and a converter that changed analog data to digital and digital back to analog.

The T1 created the possibility of connecting two computers with a high-speed digital network ordered from the phone company. The evolution and maturation of the specifications and standards surrounding the T1 carrier service, popularly referred to as a T1 line, was fundamental to a lot of other innovation occurring, including both the early internet and the eventual computerization of the local telephone network with the invention of the 5ESS switch.

Engineers at Bell Telephone replace the T1 interface deep within a telephone switch.