T-lines are connecting lines between trunk exchanges in North America for the transmission of PCM-codedvoice. In addition, they are used for connection for large customers. T-lines are multiplex lines standardized by the International Telecommunication Union( ITU).
T1 is a physical transmission standard in North America that uses two TP cables and related components to transmit a DS-1 signal at 1.544 Mbps. The ITU version of T1 defines 24 64-kbit/s transmission channels with a control channel of 8 kbit/s. A T1 frame is 193 bits long and lasts 125 µs.
T2 characterizes one level of a multiplex hierarchy. The transmission characteristics specified in T2 consist of 96 64-kbit/s transmission channels and 21 control channels of 8 kbit/s, corresponding to a DS-2 signal of 6.312 Mbit/s
T3 is an informal designation for all transmission systems capable of transmitting a DS-3 signal at 44.736 Mbit/s. T3 includes 28 T1 lines and has a transmission speed of 44.736 Mbit/s, which is equivalent to a DS-3 signal. There is no transmission standard with the designation T3.
T4 provides 168 T1 channels or 4,032 user channels of 64 kbit/s. The resulting transmission rate of T4 is 274.176 Mbit/s.