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.
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].