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.
The T-carrier is a hardware specification for carrying multiple time-division multiplexed (TDM) telecommunications channels over a single four-wire transmission circuit.
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.
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https://en.wikipedia.org/wiki/T-carrier
