Overview

The T-carrier system, introduced by the Bell System in the 1960s, was the first system that supported digitized voice transmission. It was developed by the telcos to allow more voice calls to be transported over copper wires between their local offices. T-1 rapidly became the backbone of long distance toll service and is the primary transport method for local transmission between central offices.

T-1 is full-duplex, using two pair (four) wires; one pair to receive and one pair to send. Dedicated copper T-1 circuits terminate using a RJ-48C connector.

The term T-1 originated as a description of a very specific type of physical equipment: selected cable pairs and digital regenerators at 6,000-foot intervals. Later developments improved on the capacity of the original T-1 repeaters. Placing the regenerators at 3,000-foot intervals doubled the bit rate to 3.152 Mbps. (called DS-1C).

Copper-based T-1 circuits are billed based on the additional cost of repeaters; the longer the circuit, the higher the price. Despite the largely fiber optic physical plant installed today, the mileage-sensitive tariff structure remains.

T-1 (Transmission One) signaling reduces analog voice traffic to a series of binary coordinates that represent sound.

A single voice signal in digital form takes 64,000 bps. This is known as digital signal level zero, or DS-0. When T-1 originated, 1.5 Mbps was about the highest rate that could be supported reliably over the one mile distances between manholes in large cities. (Manholes were used for locating cable splices, and were sites for signal boosters or...