These are digital signals coming from the computer to which you are connecting being modulated into audible sounds. I and a lower-pitched tone to represent the digit 0. At the other end of your modem connection, the computer attached to its modem reverses this process. The receiving modem demodulates the various tones into digital signals and sends them to the receiving computer. Actually, the process is a bit more complicated than sending and receiving signals in one direction and then another.
Modems simultaneously send and receive signals in small chunks.
The modems can tell incoming from outgoing data signals by the type of standard tones they use. Another part of the translation process involves transmission integrity. The modems exchange an added mathematical code along the way. This special code, called a checksum , lets both computers know if the data segments are coming through properly. If the mathematical sums do not match, the modems communicate with each other by resending the missing segments of data.