|System||Year||Frequency||Coax per cable||Distance between repeaters||Voice circuits per coax|
|L-1||1941||3 MHz||4||8 miles||600|
|L-2||1942||840 kHz||4||16 miles||360|
|L-3||1953||8 MHz||8||4 miles||5,580|
|L-4||1967||17 MHz||20||2 miles||32,400|
|L-5||1972||57 MHz||22||2 miles||132,000|
The initial L1 systems in the 1930s had 600 voice channels, far more than could be carried by balanced pair carrier systems, and cheaper per channel for high-usage routes. Each successive version had at least twice as many channels as the previous version, culminating in the L5 design in the 1970s, which used the then-novel error-control method of feed-forward. AT&T Long Lines built two coast to coast systems of L3 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. Many were later upgraded to L4. L-carrier systems were loaded by multiplexing and supermultiplexing Single sideband channels.
In the middle 20th Century, telephone networks used Frequency-division multiplexing 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 channel 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.
The system was designed to provide for land line connections between key command and control facilities inside the United States. Starting with L-4 the system was upgraded to withstand a nuclear attack. The system consisted of over 100 "Main Stations" and 1000 individual repeater vaults. The "Main Stations" had emergency generators, blast doors and accommodations for staff for a two-week post-attack period. Nuclear early warning systems, blast detection and other emergency services were generally provided by redundant underground and microwave circuits in case one failed.
In the late 1970s and early 1980s the system was determined to be redundant with the advance of satellite and fiber-optic communication. A few cables were upgraded to T-4 and T-5 instead of L-5, but most were never upgraded past L-4 due to advancement of technology.
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