Converting Digital Information to Analog Information

You can't simply transmit bits directly across telephone lines. It first needs to be converted into sound waves.

If you've ever picked up your telephone while you were using your modem, you've no doubt heard a constant loud, screeching noise. This noise is the sound of digital information after it's been converted to analog sound waves.

Modems constantly transmit a carrier signal. The carrier signal's function is, basically, to "carry" information from one place to another.

Modem transmit information by varying, or modulating, this carrier signal.