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.