In the 1920s, 3 and 12 channel frequency multiplexing on twisted pair was introduced. It made most single-channel long links obsolete within about a decade. I

In the 1920s, 3 and 12 channel frequency multiplexing on twisted pair was introduced. It made most single-channel long links obsolete within about a decade. In the early 1930s, both time and frequency multiplexing were introduced on coaxial cable, carrying up to 600 channels per conductor, it rendered the above twisted pair multiplexed lines obsolete within about a decade. c. 1950 microwave relay is introduced (the AT&T long lines in question here) and undergoes explosive growth, with the cross-continental system linking most major cities built by 1960, and reaching essentially every part of the country by 1970. Such links could carry hundreds of megabit/s worth of data (or multiple 6 MHz TV analogue channels). Much cheaper than coaxial, it largely halted new coax installations. It would remain dominant into the 1980s, so more like 20 years. Fibre optic communication was introduced in the mid-1970s. Immune to atmospheric problems, less path loss, potentially capable of carrying far more data than microwave. It made most of the microwave systems obsolete by the 1990s.