Question about modem (analog/digital)
A very simple explanation is as follows:


Most digital systems use "logic level" signals to signal a binary 0 or 1. This is typically 0 V for '0' and 5 V for a '1'.
The system may have to transmit long strings of zeros or ones. This is, in effect, a steady DC signal.
The telephone system is designed to transmit voice signals and has a bandwidth of about 300 Hz to 3000 Hz. DC can not be transmitted through it.




Figure 1. A modem connection between two computers. Source: MyStudentSite.


To work around the problem a simple two-tone system (think of two musical notes with a reasonable interval between them but not harmonically related) is used. To transmit a 0 we can send a low tone. To transmit a 1 we then use the high tone.
The transmitting modem MOdulates the digital data into analog audio. The receiving modem DEMmodulates the audio tones back into digital signals. MO+DEM = MODEM.