T-carrier system
A series of wideband digital data transmission formats originally developed by the Bell System and used in North America and Japan.
The basic unit of the T-carrier system is the DS0, which has a transmission rate of 64 Kbps, and is commonly used for one voice circuit.
Originally the 1.544 megabit per second T1 format carried 24 pulse-code modulated, time-division multiplexed speech signals each encoded in 64 kilobit per second streams, leaving 8 kilobits per second of framing information which facilitates the synchronisation and demultiplexing at the receiver. T2 and T3 circuits channels carry multiple T1 channels multiplexed, resulting in transmission rates of up to 44.736 Mbps.
The T-carrier system uses in-band signaling, resulting in lower transmission rates than the E-carrier system. It uses a restored polar signal with 303-type data stations.
Asynchronous signals can be transmitted via a standard which encodes each change of level into three bits; two which indicate the time (within the current synchronous frame) at which the transition occurred, and the third which indicates the direction of the transition. Although wasteful of line bandwidth, such use is usually only over small distances.
T1 lines are made free of direct current signal components by in effect capacitor coupling the signal at the transmitter and restoring that lost component with a "slicer" at the receiver, leading to the description "restored polar".
[Telecommunications Transmission Engineering, Vol. 2, Facilities, AT&T, 1977].