What Is a Modem Card?

By Contributing Writer

A modem card is a useful hardware component in a computer, especially for people using a dial-up connection to access the Internet. While modem cards have been around for decades, many computer users may wonder what they actually are.

Concept of Modem

The basic concept of a modem is in the word "modem" itself. The word is a short form of "modulator demodulator", which essentially means that a modem modulates a signal to send it and demodulates a signal it receives.


The modulation of a stream of binary data essentially turns it into an audible signal that can be sent over a phone line. The demodulation of an audible signal into binary data happens at the other side of the communication link. It is similar, conceptually, to a telegraph machine.


This modulation and demodulation of binary data enables a computer to communicate with another computer over a phone line using only sound. This is beneficial to computer users who need to use a dial-up connection for accessing the Internet.


A hardware component that functions as a modem card is actually a PCB (printed circuit board). It plugs into a computer's motherboard, and a standard phone line is plugged into the modem card.


A modem card may also have lights to indicate the active transmission and reception of data. The light labeled "Tx" indicates transmitting and "Rx" indicates receiving. Lights may blink as data is sent and received.

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