A CDE Definition
(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.
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.
T-Carrier Total Speed Channels
T1 1.544 Mbps 24
T2 6.312 Mbps 96
T3 44.736 Mbps 672
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