Source : Free On-Line Dictionary of Computing

T1
     
         An {AT&T} term for a {digital carrier}
        facility used to transmit a {DS1} formatted digital signal at
        1.544 megabits per second.
     
        T1 transmission uses a bipolar {Return To Zero} {alternate
        mark inversion} line coding scheme to keep the DC carrier
        component from saturating the line.
     
        Although some consider T1 signaling obsolete, much equipment
        operates at the "T1 rate" and such signals are either
        combined for transmission via faster circuits, or
        demultiplexed into 64 kilobit per second circuits for
        distribution to individual subscribers.
     
        T1 signals can be transported on {unshielded twisted pair}
        telephone lines.  The transmitted signal consists of pips of a
        few hundred nanoseconds width, each inverted with respect to
        the one preceding.  At the sending end the signal is 1 volt,
        and as received, greater than 0.01 volts.  This requires
        repeaters about every 6000 feet.
     
        The information is contained in the timing of the signals, not
        the polarity.  When a long sequence of bits in the transmitted
        information would cause no pip to be sent, "{bit stuffing}" is
        used so the receiving apparatus will not lose track of the
        sending clock.
     
        A T1 circuit requires two twisted pair lines, one for each
        direction.  Some newer equipment uses the two lines at half
        the T1 rate and in {full-duplex} mode; the sent and received
        signals are separated at each end by components collectively
        called a "hybrid".  Although this technique requires more
        sophisticated equipment and lowers the line length, an
        advantage is that half the sent and half the received
        information is mixed on any one line, making low-tech wiretaps
        less a threat.
     
        See also {Integrated Services Digital Network}.
     
        (1994-11-23)