Early T-carrier history
Frederick T. Andrews · IEEE Communications Magazine · 2011
In the 1950s AT&T faced the challenge of providing for the growth of traffic in the local telephone plant at lower costs than by simply continuing to expand the number of voice cable pairs between switching offices. Engineers identified pulse code modulation, PCM, as offering a robust solution to the problem and launched a system development effort in 1956. Some difficult technical problems were identified; the solutions were found and incorporated into a new system design. The product was T-Carrier, which was introduced into service in 1962. This new application of PCM was highly successful and the technology was licensed to the independent telephone industry. The race toward an all-digital network of digital transmission and switching was off and running.