T-carrier

The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls. The first version, the Transmission System 1 (T-1), was introduced in 1962 in the Bell System, and could transmit up to 24 telephone calls simultaneously over a single transmission line of copper wire. Subsequent specifications carried multiples of the basic T1 (1.544 Mbit/s) data rates, such as T2 (6.312 Mbit/s) with 96 channels, T3 (44.736 Mbit/s) with 672 channels, and others.

T-1 is a hardware specification for telecommunications trunking. A trunk is a single transmission channel between two points on the network: each point is either a switching center or a node (such as a telephone).

Initially, T-1 trunks were used only to connect major telephone exchanges, via the same twisted pair copper wire that the analog trunks used. One transmit pair, one receive pair. If the exchanges were too far apart, a repeater boosted the signal.

Before the digital T-1 system, carrier wave systems such as 12-channel carrier systems worked by frequency division multiplexing; each call was an analog signal. A T-1 trunk could transmit 24 telephone calls at a time, because it used a digital carrier signal called Digital Signal 1 (DS-1). DS-1 is a for multiplexing the bitstreams of up to 24 telephone calls, along with two special bits: a framing bit (for frame synchronization) and a maintenance-signaling bit. T-1's maximum data transmission rate is 1.544 megabits per second.

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