Though your old dial-up modem (hopefully) now sits in a junk yard somewhere, its beeps and whistles surely remain etched into your brain. With this massive image, Oona Räisänen decided to explain every single note your modem used to make—and it's fascinating.
If you click the image above, you'll be able to explore the visualization in all its (massive!) glory. Räisänen explains:
When humans talk, only one of them is usually talking while the other one listens. The telephone network exploits this fact and temporarily silences the return channel to suppress any confusing echoes of the talker's own voice.
Modems don't like this at all, as they can very well talk at the same time (it's called full-duplex). The answering modem now puts on a special answer tone that will disable any echo suppression circuits on the line. The tone also has periodic "snaps" (180° phase transitions) that aim to disable yet another type of circuit called echo canceller.
Now the modems will list their supported modulation modes and try to find one that both know. They also probe the line with test tones to see how it responds to tones of different frequencies, and how much it attenuates the signal. They exchange their test results and decide a speed that is suitable for the line.