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)
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