L-carrier

[1][2] A distance of 3,800 miles (6,100 km) was simulated by repeatedly remodulating signals and looping them twenty times between the endpoints.

The first production installation of the L-1 carrier system went into service between Stevens Point, Wisconsin and Minneapolis, Minnesota in 1941 over a distance of almost 200 miles (320 km).

A small-scale L-type carrier system between Baltimore, Maryland and Washington, D.C. was intended for short-distance low-volume traffic.

[4] With the anticipation of the end of war-time responsibilities, AT&T announced in December 1944 a development plan for nationwide build-out of the coaxial carrier network for support of not only long-distance telephone service, but also for television transmissions.

L-carrier also carried the first television network connections, though the later microwave radio relay system soon became more important for this purpose.

[6] A variant of the 1950s L-3 system was designed in the early 1960s to provide for land line connections between key military command and control facilities in the United States.

The main stations had emergency power systems, 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.