Demodulation with variable bitrate?

[Verminox]

I am to understand that modems modulate digital signals using techniques such as shift-keying. When a modem receives a signal it demodulates the modulated signal from the telephone line to the original digital signal again.

Now, the bitrate that is sent/received is often variable. Sometimes the connection is 'slow' or there is line attenuation. In such cases, how does the modem know what the current bitrate is before demodulating? Is there a clock pulse also sent? If the received FSK signal contains a high frequency component for a short duration then does the modem consider it to be '111' or '1111111' or '11111111111111111'? How does it know the bitrate when the modulated signal is continuously the same?

 

[Tom.G]

Now, the bitrate that is sent/received is often variable.

Not usually variable over the short term. When a connection is started, the two modems try to connect to each other at their highest bit rate. If this fails they try again at a lower bit rate. This is frequently called "handshaking." If after several tries they can not speak to each other they give up and disconnect.

If the error rate gets high during use, one of them may start a new handshake process, trying improve the error rate with a lower connection speed.