T1
T1
(communications)T1 transmission uses a bipolar Return To Zero alternate mark inversion line coding scheme to keep the DC carriercomponent from saturating the line.
Although some consider T1 signaling obsolete, much equipmentoperates at the "T1 rate" and such signals are eithercombined for transmission via faster circuits, ordemultiplexed into 64 kilobit per second circuits fordistribution to individual subscribers.
T1 signals can be transported on unshielded twisted pairtelephone lines. The transmitted signal consists of pips of afew hundred nanoseconds width, each inverted with respect tothe one preceding. At the sending end the signal is 1 volt,and as received, greater than 0.01 volts. This requiresrepeaters about every 6000 feet.
The information is contained in the timing of the signals, notthe polarity. When a long sequence of bits in the transmittedinformation would cause no pip to be sent, "bit stuffing" isused so the receiving apparatus will not lose track of thesending clock.
A T1 circuit requires two twisted pair lines, one for eachdirection. Some newer equipment uses the two lines at halfthe T1 rate and in full-duplex mode; the sent and receivedsignals are separated at each end by components collectivelycalled a "hybrid". Although this technique requires moresophisticated equipment and lowers the line length, anadvantage is that half the sent and half the receivedinformation is mixed on any one line, making low-tech wiretapsless a threat.
See also Integrated Services Digital Network.
T1
(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