The T-carrier system is a fully digital transmission system developed by Bell Labs and introduced in the US in 1962 with the T-1 line, which supported digitized voice transmission through pulse code modulation (PCM) and time-division multiplexing (TDM), which greatly increased the number of telephone calls that a given telephone network was capable of handling at one time.
Techopedia explains T-Carrier
T-carrier systems are commonly used by telecommunications companies, most commonly the T-1 line, with a transfer rate of 1.544 Mbps, and the T-3 line, with a transfer rate of 44.736 Mbps, for providing Internet access to homes and businesses. The T-carrier system makes use of four wires: a pair is used for receiving data and the other pair is for transmission, making it a full-duplex transmission system.
The T-1 digital stream is made up of twenty-four 64-Kbps channels, which are further multiplexed. The four wires being used for the T-1 system originally made use of twisted pair wires, but now some variations make use of coaxial cables or even optical fiber.
T-carrier technology was already in use in the US by 1962, but it was not until 1983 that AT&T introduced it to the general public as a communications product, which was initially aimed at voice data but increasingly found use in general data transmission. Because of its good qualities, T-carrier technology is still being used today by Internet service providers (ISPs), specifically the T-1 and the T-3 lines.
Variations of the T-carrier system are as follows: