The best thing about this was having the infrastructure on public show, a bit like having a building's water pipes visible on the outside where everyone can see

The best thing about this was having the infrastructure on public show, a bit like having a building's water pipes visible on the outside where everyone can see them. This exposure, combined with natural curiosity, meant many people had an idea of how it worked. One takeaway from the diagram is how adaptation and channel estimation can increase channel capacity. We start off simple on the left hand side, with the dial tone and DTMF sending only a couple of frequencies at low speeds. As we move to the right each end adapts/learns and more frequencies come into play increasing the information density. Eventually the adaptation has reached some form of optimum and the information density (channel capacity) is maximised with the spectrogram being a solid mass of signals.