L-carrier

From Wikipedia, the free encyclopedia

L-carrier was the Bell System designation for analog Long distance frequency-division multiplexing transmission systems using co-axial cable. The initial L1 systems in the 1930s had 600 voice channels. Each successive version had twice or more channels than the previous version, culminating in the L5 design in the 1970s. AT&T Long Lines built two coast to coast systems as well as shorter ones connecting major cities, especially the big cities of the eastern United States, as a supplement to the mainstay microwave radio relay systems. L-carrier systems were loaded by multiplexing and supermultiplexing Single sideband channels.

In the middle 20th Century, telephone networks used FDM to carry several voice channels on a single physical circuit. In Single-sideband modulation schemes, 12 voice channels would be modulated onto carriers spaced 4 kHz apart. The composite signal, occupying the frequency range 60 – 108 kHz, was known as a group. In turn, five groups could themselves be multiplexed by a similar method into a supergroup, containing 60 voice channels. One 48 KHz Group band circuit was sometimes used for a single high speed data link rather than for voice circuits.

In long distance systems, supergroups were multiplexed into mastergroups of 300 voice channels (Europe) or 600 (AT&T Long Lines L-Carrier) for transmission by coaxial cable or microwave.

There were even higher levels of multiplexing, and it became possible to send thousands of voice channels down a single circuit. The accompanying diagrams are of the process of a Bell System A Type Channel Bank forming a mastergroup in three stages.