Dialup sound

Finnish engineer Oona Räisänen made a clear flowchart with a description of all the sounds that the dialup modem makes when establishing a connection. The horizontal axis corresponds to the time in seconds, the vertical - the frequency of sound in hertz.

The description is superimposed directly on the spectrogram. For each sound, a source is marked (our modem or someone else’s) and an explanation of what this signal means specifically.

For those who have forgotten the sound of a dial-up modem connection, you can listen to it here or here . In the process of “handshaking” when establishing a connection, two modems try to determine the most suitable protocols for data exchange, testing the communication link between themselves. The maximum data transfer rate depends on the protocols selected during the handshake.
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The procedure starts with a dial tone — the same dial tone that we usually hear on the handset before dialing.



After receiving this signal, the modem dials a number using tone or pulse dialing. On the spectrogram is a tone set. Each digit corresponds to a pair of signals at a certain frequency. The modem dials the number in the same way as a regular telephone.

From the fifth second the most interesting begins: the modems begin to actively exchange "replicas", telling each other about their capabilities and limitations. This is the start of phase 1 of the handshake process. The first phase is the longest, it usually ends in the 12th second.



The modems then arrange for a simultaneous data transfer by checking the quality of the line (phase 2) and finally agree on an acceptable connection speed (phase 3).



After the end of the third phase, the dynamics of the modems turn off - and it becomes possible to transfer data over the established connection.

The full size chart can be downloaded on Dropbox , mirror , translated into Russian ( Ocelot ).