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The T carrier services use TDM (time-division multiplexing) techniques to transmit multiple voice or data over digital trunk lines. An example is to connect a multi-line enterprise PBX to the phone company over a single line. The most common T carriers are T1 (1.544 Mbits/sec) and T3 (45 Mbits/sec). Note that the "T" stands for terrestrial (as opposed to satellite transmissions), and the 1 is an abbreviation for the 1.544-Mbit/sec signal rate.

A T1 or T3 is a digital circuit that is based on the PDH (plesiochronous digital hierarchy), which is a system of multiplexing numerous individual channels into higher-level channels. In North America, this hierarchy is called NADH (North American Digital Hierarchy). The base channel rate is called DS-0 and it holds one digitized voice call at a data rate of 64 Kbits/sec. The higher rates are DS-1, which carries 24 channels, and DS-3, which carries 672 channels.