资料来源 : Free On-Line Dictionary of Computing

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} or
        {bit-robbing}, 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].
     
        (2001-04-08)