L-carrier

The Transcontinental Cable System or L-carrier system was developed by AT&T to create a high capacity transmission facility using coaxial cable, a fairly new medium for HF to RF signal transmission invented in 1927 by Lloyd Espenschied of Bell Telephone Laboratories for long distance communications. There were six phases of development of the system, designated by the Bell System as L-1 through L-5E. Later versions, starting in the early 1960s, were hardened against the dangers of the cold war using complete placement of all terminal and repeater equipment in hardened underground vaults.

The initial system developed in the late 1930s had 600 voice channels, far more than could be carried by balanced pair carrier systems, and cheaper per channel for high-usage routes. This version was standardized as "Type L Telephone Carrier System" in 1938, abbreviated at "L-1" after further developments ensued. The first commercial usage of this then-new technology appeared with the construction of the Los Angeles-El Paso "A" cable, which was later upgraded to L-3 in 1956. Each successive version, excepting the 360 channel "L-2," had at least twice as many channels as the previous version, culminating in the L-5E design in 1976. (There was only one L-2 ever deployed, between Washington DC and Baltimore.) AT&T Long Lines built two coast to coast systems of L-3 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. Some were later upgraded to L-4, while others were simply overbuilt with a new L-5 system.

Starting in 1911, telephone networks used frequency-division multiplexing to carry several voice channels on a single physical circuit, beginning with the first Type C carrier in that year, which heterodyned three voice channels stacked on top of one voice circuit. L-carrier systems were loaded by multiplexing and supermultiplexing single sideband channels, using the long-standard 12 channel voice "group" produced by Type A channel banks, occupying a frequency spectrum between 60 and 108 kHz. This basic "group" was the entire line spectrum on previous long haul carrier systems, such as Types J and K. The first Type A-1 channel banks appeared for use on Type J open wire carrier in 1934. It was the work of the aforementioned Espenschied and Herman Affel of Bell Labs who patented their piezoelectric crystal "lattice" filters to provide sharp bandpass cutoff that made all single-sideband carrier work developed at Bell Labs possible. Such "lattice" filters were the heart of all analog multiplex systems using single-sideband/carrier suppressed architecture until active IC-based filtering became available in the mid-1970s.

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