How a dial-up modem handshake works

dial-up-handshake-infographic

If you’re old enough to have used a dial-up modem we’d bet you can do an imitation of the sounds it made while connecting. Those not-so-beautiful sounds heralded the dawning of a technological era. But few actually know what each of those distinct sounds were doing. Now’s your chance to learn. This post explains each step in the dail-up handshake process.

This may be the most useful infographic we’ve ever seen. Normally we just seem them as gimmicks, but [Oona Räisänen] really put together something special with this one. Her blog post includes an audio clip so that you can play back the full handshake sounds. The main box on the graphic shows the audio spectrum from that clip, with an explanation below it. But you’ll also want to read through her full write-up for a more narrative description.

The part we found the most interesting is that these modems needed to disable the echo suppression used by the telephone system in order to operate at full-duplex. Apparently land lines disabled the speaker while you were talking so that you didn’t hear your own voice. This was a problem if the modem was trying to send and receive at the same time.

[via Reddit]

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