How Do Fax Modems Work?by Quinten Plummer
Fax Modulator/demodulators (modems) convert analog signals that travel over telephone lines into digital signals. The conversion from analog to digital gives your computer the ability to interpret data sent over analog lines; the modem acts as an interpreter for the digital-based computer and the analog-based phone lines. Fax modems are equipped with phone outlets for a connection to the telephone line.
The fax function of the modem allows you to use your modem as a fax machine--sending data from one fax machine or fax modem to another. Data sent from the computer is converted at the fax modem into a digital signal into a series of analog pulses, or "tonal signals," that represent the data. The data passes along telephone lines and through Public Switched Telephone Networks (PSTN) until eventually getting the faxed information to the recipient.
Internal v. External
Internal fax modems are installed directly into the computer's motherboard via a AGP or PCI card slot. External modems use USB cables to connect to the computer. There are generally no advantages as for speed between internal and external modems; however, internal modems save space.