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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.


(1) See Tier 1 network and Type 1 font.

(2) A 1.544 Mbps point-to-point, dedicated, digital circuit provided by the telephone companies. With the monthly cost typically based on distance, T1 lines are widely used for connecting an organization's PBX to the telephone company or a local network (LAN) to an Internet provider (ISP). They are also used for Internet access in buildings that have no DSL, cable or fixed wireless coverage.

T1 lines were widely used for connecting branch offices, but most have been supplanted by virtual private networks (VPNs) over the Internet.

It Started in the 1960s
The first T1 line was tariffed by AT&T in January 1983. However, in the early 1960s, AT&T started the move to digital transmission, and T1 lines were deployed in intercity trunks to improve signal quality and make more efficient use of the network.

T1 Anatomy
A T1 line uses two wire pairs (one for transmit, one for receive) and time division multiplexing (TDM) to interleave 24 64-Kbps voice or data channels. The standard T1 frame is 193 bits long, which holds 24 8-bit voice samples and one synchronization bit with 8,000 frames transmitted per second. T1 is not restricted to voice or 64 Kbps data streams. Channels may be combined and the total 1.544 Mbps capacity can be broken up as required. See DS, T-carrier, bipolar transmission, D4 and ESF.

64 KbpsT-Carrier   Total Speed    Channels

   T1           1.544 Mbps     24
   T2           6.312 Mbps     96
   T3          44.736 Mbps    672
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