If you've ever used a phone line to connect to the Internet or sent a fax, you're familiar the racket that precedes the actual data transmission. Not exactly a Beethoven symphony. Even if you are very familiar with the sounds and know it's "handshaking" with the other modem in order to create a comms channel, you probably don't know precisely what is going on with all that warbling and hissing.
Oona Räisänen (windytan) has put together a very nice annotated sonogram and explanation on her blog. That the whole thing sounds more like bursting artillery shells than dueling banjos is partly a result of the protocol trying to defeat sophisticated circuitry for noise cancellation and echo suppression on the telephone network.
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