For those interested in the technical details of digital transmission the following may be of interest.
The T1 digital line was a major component of transmission of the Bell System. It used 24 voice channels, PCM, with a line signal of1,544,000 bipolar pulse positions per second.
The March 1963 BSTJ has a technical article describing the engineering of the T1 for repeaters, which were placed roughly every 6,000 feet. Amplifying signals on a digital line was a different approach than for the older analog lines. This article considers design limits from crosstalk from other T1 lines and noise from the central office, as well as error rates, signal loss, and signal-to-noise ratio.more on T1 (also known as DS1)