V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

T1
       [digital] Transmission link 1 [1.544 Mbit/s] (DS1, BIT)


The Free On-line Dictionary of Computing (30 December 2018):

T1
T1 line
T1 rate

    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)