Stands for "Transmission System Level 3."
A T3 line is a telecommunications standard that provides the equivalent bandwidth of 28 T1 lines, with 672 individual voice channels and 45 Mbps of Internet bandwidth. A T3 connection operates over a coax cable, but limited segments can also use a fiber optic cable. A T3 line is also known as a DS3 (Digital Signal Level 3) line.
T3 lines serve as telephone network trunk connections, connecting telephone switching centers to transmit digitized voice calls and other data from place to place. ISPs also offer T3 lines as dedicated Internet connections for businesses, universities, and other institutions.
Newer network technologies have led to a decline in the popularity of T3 lines. Their copper wire backbone is not as fast as the fiber-optic cables replacing them, with some ISPs retiring their copper wire networks entirely.
NOTE: While 45 Mbps is a modest Internet speed now, a T3 line was the fastest Internet connection available during the dial-up era of the 1980s and 1990s.