eli5: The “handshake” between dial-up modems and why it made those sounds
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Modems (MOdulate/DEModulate) are a way of sending digital signals (a stepped system, e.g. ones and zeros) over an analog transmission carrier (continually varying power and/or frequency levels). Phone lines were originally designed to carry voice, a continually varying signal, so they required modems to send newer digital signals over them. The beeping noises that are the “handshake” are very simple, slow digital signals, that changes the frequency in steps, resulting in a sound at a specific frequency, so it sounds like a tone or note. The receiving side can detect these tones, and convert them back into ones and zeros. The sending side tells the receiving side what speeds it can do, and the receiving side picks a speed it is happy with, and tells the sending side what speed it’s picked. They then transmit at this speed, which is what sounds like static after the initial beeps, but it’s actually a similar sort of changing of frequency, just happening much faster.
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