How two computers held a conversation in tones
A dial-up modem could not send a stream of computer bits directly through an ordinary telephone connection. The phone network was built to carry voices, so the modem translated digital data into patterns of sound that could travel through the line. At the other end, another modem listened for those patterns and translated them back into bits.
The first burst of noise was the devices establishing that they could hear one another. Each sent recognizable tones; the receiving modem used what came back to gauge how the connection behaved. A phone line might distort some frequencies or carry interference, so the pair needed to find a signaling method the line could support.
The handshake set the terms
Once they had a usable signal, the modems exchanged more precise patterns to settle on compatible settings. They had to agree on how to encode data and how to detect or recover from errors. The best possible speed depended not just on the modems’ capabilities but on the quality of that particular call: a noisy or imperfect line could force them to use a slower, more reliable mode.
That exchange is called a handshake. It was not speech or random noise, but a sequence of signals with specific meanings to the machines. To a person picking up a handset, however, the tones blended into a strange screech because the negotiation happened in the same audible frequency range as a phone call.
Why the noise stopped
When the modems had settled on settings, they could begin carrying data. The line had not become a dedicated computer cable; it was still a phone connection carrying carefully shaped audio. That is why hanging up ended the session—and why the opening screech was the brief, audible moment when two computers adapted their conversation to the limits of one copper phone line.
