Finnish engineer Oona Räisänen has compiled a clear block diagram describing all the sounds that a dial-up modem makes when a connection is established.

 

Dialup sound

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Finnish engineer Oona Räisänen has compiled a clear block diagram describing all the sounds that a dial-up modem makes when a connection is established. The horizontal axis corresponds to time in seconds, the vertical axis corresponds to the frequency of sound in hertz.

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

For those who have forgotten the dialup modem connection sound, you can listen to it here or here . During the "handshake" process when a connection is established, two modems try to determine the most appropriate protocols for communicating by testing the communication link between them. The protocols selected during the handshake determine the maximum data transfer rate.

The procedure begins with a dial tone, the same tone we usually hear on the handset before dialing.

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

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

The modems then set up a simulcast session, checking the line quality (phase 2) and finally agreeing on an acceptable connection speed (phase 3).

After the end of the third phase, the modem speakers are turned off - and data transmission over the established connection becomes possible.

Full size chart can be downloaded on Dropbox , mirror , Russian translation ( Ocelot ).

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