T1
<communications> 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)
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