eli5: The “handshake” between dial-up modems and why it made those sounds
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The phone lines could only transmit up to about 4Khz (human voice muffled). The dial up modems would try to establish a connection to the other side. Part of the process was to find out how good or bad was the phone line link. In this process tones of different frequencies are sent and received, and if the channel is muffling a particular part, then the modem would learn to increase the volume in that particular part (frequency). Later the modem would start trying to transmit data using sound and try to determine how many errors were detected on the other side. The modem would try to go to the highest data rate it could with limited errors, and once it found it, it would then start sending data, which to us sounded like white noise.
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