A digital transmission service from a common carrier. Although developed in the 1960s and used internally, AT&T introduced it as a communications product to the public in 1983. Initially used for voice, its use for data grew steadily, and T1 and T3 lines were and still are widely used to create point-to-point private data networks. T-carrier lines use four wire cables. One pair is used to transmit; the other to receive.The cost of the lines is generally based on the length of the circuit. Thus, it is the customer's responsibility to utilize the lines efficiently. Multiple lower-speed channels can be multiplexed onto a T-carrier line and demultiplexed (split back out) at the other end. Some multiplexors can analyze the traffic load and vary channel speeds for optimum transmission. See T1, T2, T3, DS, DSU/CSU and inverse multiplexor.
The United States Bell System activated the first commercial digital carrier system in 1962 in Chicago, Illinois, where electrical noise from high-tension lines and automotive ignitions interfered with analog systems. The system was designated T1, with the T standing for Terrestrial to distinguish the land transmission from satellite transmission. (Bell Laboratories also launched Telstar I, the first communications satellite, in 1962.) T-carrier was designed for a four-wire twisted-pair circuit, although the DSX-1 interface is medium-independent, i.e., can be provisioned over any of the transmission media, at least at the T1 rate of 1.544 Mbps. At the T3 rate of 44.736 Mbps, twisted pair is unsuitable over distances greater than 50 feet due to issues of signal attenuation. As the first digital carrier system,T-carrier set the standards for digital transmission and switching, including the use of pulse code modulation (PCM) for digitizing analog voice signals. (Note: T-carrier uses the µ-law (mu-law) companding technique for PCM.) T-carrier not only set the basis for the North American digital hierarchy, but also led to the development of E-carrier in Europe and J-carrier in Japan.The fundamental building block of T-carrier is a 64-kbps channel, referred to as DS-0 (Digital Signal level Zero). Through time-division multiplexing (TDM), T-carrier interleaves DS-0 channels at various signaling rates to create the services that comprise the North American digital hierarchy, as detailed in Table T-1. See also analog, carrier, digital, DS-0, E-carrier, fractional T1, J-carrier, µ-law, signaling rate, T1, T1C, T2, T3, T4, T5, TDM, and transmission rate. See also digital signal hierarchy for a side-by-side comparison of the North American, European, and Japanese digital hierarchies.