The digital network is based on pulse code modulation (PCM). The general design
of a PCM system was invented by Reeve, an ITT engineer from Standard Telephone
Laboratories (STL), in 1937, while visiting a French ITT subsidiary. It did not become
a reality until Shockley’s (Bell Telephone Laboratories) invention of the transistor. Field
trials of PCM systems were evident as early as 1952 in North America