With the arrival of digital systems in the 60s, improving the performance of the old analog communication networks became a real possibility. These networks were based on a frequency modulation scheme for transmitting voice channels, but this approach was very rigid and degraded the channel quality, due to successive analog modulations and demodulations, which introduced noise in the transmitted signal.

The first digital communications system was set up in 1962 by Bell Labs in the USA, and consisted of a system of 24 digital voice channels running at what is known today as T1, that is, 1544 kbit/s. This technology was not completely adopted until the mid 70s, however, due to the large amount of analog systems already in place and the high cost of digital systems, as semiconductors were very expensive.