The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls. The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell System, and could transmit up to 24 telephone calls simultaneously over a single transmission line of copper wire. Subsequent specifications carried multiples of the basic T1 (1.544 Mbit/s) data rates, such as T2 (6.312 Mbit/s) with 96 channels, T3 (44.736 Mbit/s) with 672 channels, and others. Although a T2 was defined as part of AT&T's T-carrier system, which defined five levels, T1 through T5, only the T1 and T3 were commonly in use. The T-carrier is a hardware specification for carrying multiple time-division multiplexed (TDM) telecommunications channels over a single four-wire transmission circuit. It was developed by AT&T at Bell Laboratories ca. 1957 and first employed by 1962 for long-haul pulse-code modulation (PCM) digital voice transmission with the D1 channel bank. The T-carriers are commonly used for trunking between switching centers in a telephone network, including to private branch exchange (PBX) interconnect points. It uses the same twisted pair copper wire that analog trunks used, employing one pair for transmitting, and another pair for receiving. Signal repeaters may be used for extended distance requirements. Before the digital T-carrier system, carrier wave systems such as 12-channel carrier systems worked by frequency-division multiplexing; each call was an analog signal. A T1 trunk could transmit 24 telephone calls at a time, because it used a digital carrier signal called Digital Signal 1 (DS-1). DS-1 is a communications protocol for multiplexing the bitstreams of up to 24 telephone calls, along with two special bits: a framing bit (for frame synchronization) and a maintenance-signaling bit. T1's maximum data transmission rate is 1.544 megabits per second. Outside of the United States, Canada, Japan, and South Korea, the E-carrier system is used.