from plesiochronous to synchronous

Up until the 1960s, telephone calls were carried as analog signals. There was a great deal of frequency division multiplexing onto open wire, and sometimes coaxial cables, but in the post-war period buried multipair telephone cables began to dominate. While the network was surprisingly functional—long distance calls from coast to coast were still quite intelligible—there were fundamental limitations to analog carriers. The quality of the call would degrade with each step, a problem that was much more acute for the sensitive digital connections that were already the apparent future of the telephone system. These were digital signals modulated over the top of telephone lines: the telephone system itself still handled them as analog calls, just ones that required special care for quality. During the Second World War, it became apparent that many of the long distance network's challenges could be greatly simplified by using digital signaling as the basis of the entire long distance…