Editing T-carrier (section)

==Transmission System 1==
The T-carrier is a hardware [[Specification (technical standard)|specification]] for carrying multiple [[time-division multiplexing|time-division multiplexed]] (TDM) telecommunications channels over a single four-wire transmission circuit. It was developed by AT&T at [[Bell Labs|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 center]]s in a telephone network, including to private branch exchange (PBX) interconnect points. It uses the same [[twisted pair]] [[Copper wire and cable|copper wire]] that analog trunks used, employing one pair for transmitting, and another pair for receiving. Signal [[repeater]]s may be used for extended distance requirements.

Before the digital T-carrier system, [[Carrier signal|carrier wave]] systems such as [[12-channel carrier system]]s 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).<ref>J.R. Davis, A. K. Reilly, T-Carrier Characterization Program&nbsp;– Overview, Bell System Technical Journal July–August 1981, Vol 60 No 6 Part 1</ref> DS-1 is a [[communications protocol]] for [[multiplexing]] the [[bitstream]]s of up to 24 telephone calls, along with two special [[bit]]s: a ''framing bit'' (for [[frame synchronization]]) and a ''maintenance-signaling bit''. T1's maximum [[data transmission]] rate is 1.544 [[megabit]]s per second.

Outside of the United States, Canada, Japan, and South Korea, the [[E-carrier]] system is used. E-carrier is similar transmission system with higher capacity that is not directly compatible with the T-carrier.