L-CARRIER

System	Year	Frequency	Coax per cable	Distance between repeaters	Capacity per coax
L-1	1941	3 MHz	4	8 miles	600 voice circuits
L-2	1942	840 kHz	4	16 miles	360 voice circuits
L-3	1953	8 MHz	8	4 miles	5,580 voice circuits
L-4	1967	17 MHz	20	2 miles	32,400 voice circuits
L-5	1972	57 MHz	22	2 miles	132,000 voice circuits

The 'Transcontinental Cable System' or 'L-carrier system', was developed by AT&T to create a hardened telecom network using coaxial cable for long distance communications, including civil defense related communications during the cold war. There were five phases of development of the system, designated by the Bell System as L-1 through L-5.
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, 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 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.
L-carrier also carried the first television network connections, though the later microwave radio relay system soon became more important for this purpose.

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 generators, blast doors and accommodations for staff for a two-week post-attack period.
In the late 1970s and early 1980s the system was determined to be redundant with the advance of satellite and fiber-optic communication. Most stations were never upgraded past L-4 due to advancement of technology.
One use of the system was for nuclear early warning systems and blast detection.