Coaxial cable
The L-carrier system was one of a series of carrier systems developed by AT&amp;T for high-capacity transmission for long-distance communications. Over a period from the late 1930s to the 1970s, the system evolved in six significant phases of development, designated by Bell System engineers as L-1 through L-5, and L-5E. Coaxial cable was the principal transmission medium in all stages, initially lending the system another descriptions as the "coaxial system". It was the successor to a series of previous carrier systems, typically identified by capital letters. In the 1960s the system was hardened against the dangers of the cold war using complete placement of all terminal and repeater equipment in hardened underground vaults.