A system that derives multiple logical channels from a single physical communications path, thereby supporting multiple communications over a single circuit. The first practical analog carrier systems were invented in the 1890s for telegraphy, based on what is now termed frequency division multiplexing (FDM). Early work on analog carrier multiplexing for telephony networks was done in the laboratories at American Bell Telephone Company, the predecessor to AT&T, as early as 1894. The first commercial carrier system was installed between Baltimore, Maryland and Pittsburgh, Pennsylvania in the United States, in 1918.That Type A system provided four full duplex (FDX) carrier channels above the 4-kHz voice band on short haul open wire circuits in the frequency range from 5 kHz to 25 kHz.The last commercial analog carrier system was L5E (1978), which used 22 pairs of coaxial cables to support a total of 132,000 simultaneous voice grade transmissions. The first commercial digital carrier system was T-carrier, specifically T1, which Bell System activated in 1962 in Chicago, Illinois. Digital carrier systems use the technique of time division multiplexing (TDM). See also channel, circuit, FDM, FDX, multiplexer, T1, T-carrier, and TDM.