The T1 terminology that is known as a "T-1" or "T1" is abridged for Trunk Level 1, which is a telecommunications standard for digital transmission under the Digital Signal Level hierarchy. A T1 over copper is delivered by connecting two pairs of twisted copper wires between the subscriber location and the telco providers central office also known as the (CO). One of the 2 twisted copper pairs is for transmitting data and the other pair is for receiving data. In conjunction, the 4-copper-wire transmission media carrying the digital signaling standard produces a full duplex symmetrical or bi-directional circuit. When a T1 delivered over fiber, a fiber pair is used as opposed to the 4-wire copper configuration as stated above.
When a T1 Line of High-Speed Internet is connected to the World Wide Web/Internet over fiber, dedicated copper or Ethernet using a telecommunications standard that was developed by AT&T/Bell Labs for North America/United States/Canada and Japan it transmits data packets at a collective speed of 1.544 megabits per second (Mbps) and usually connects from the customer premise (CP) directly to an extensive fiber optic Internet Service Provider (ISP) backbone.
The effectiveness of an Internet T1 over cable or DSL Internet service is that the total amount of 1.544 Mbps minus the .008 Mega bits per second for framing of available bandwidth is delivered by a T-1 connection that is guaranteed 24 x 7 x 365 by the Internet Service Provider under a Service Level Agreement (SLA) that dictates service reliability in precise terms.
The end result is that a T1 is used to provide last mile Internet Access to an ISP's (Internet Service Provider's) Point of Presence also known as their PoP, a local loop is then delivered and T1 access is provisioned as 1.536 Mbps of bandwidth and a full 1.536 Mbps bi-directional plus the .008 Mbps of framing bits which equals the 1.544 Mbps that is traditionally sold by ISP's.