T-carrier system
T-carrier system
(communications)The basic unit of the T-carrier system is the DS0, which hasa transmission rate of 64 Kbps, and is commonly used for onevoice circuit.
Originally the 1.544 megabit per second T1 format carried 24pulse-code modulated, time-division multiplexed speech signalseach encoded in 64 kilobit per second streams, leaving 8kilobits per second of framing information which facilitatesthe synchronisation and demultiplexing at the receiver. T2and T3 circuits channels carry multiple T1 channelsmultiplexed, resulting in transmission rates of up to 44.736Mbps.
The T-carrier system uses in-band signaling, resulting inlower transmission rates than the E-carrier system. It usesa restored polar signal with 303-type data stations.
Asynchronous signals can be transmitted via a standard whichencodes each change of level into three bits; two whichindicate the time (within the current synchronous frame) atwhich the transition occurred, and the third which indicatesthe direction of the transition. Although wasteful of linebandwidth, such use is usually only over small distances.
T1 lines are made free of direct current signal components byin effect capacitor coupling the signal at the transmitter andrestoring that lost component with a "slicer" at the receiver,leading to the description "restored polar".
[Telecommunications Transmission Engineering, Vol. 2,Facilities, AT&T, 1977].