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?