A modem (Modulator-Demodulator) converts digital data into analog signals (modulation) and turns analog signals back into digital data (demodulation). It was built to carry computer data over analog telephone lines, and it was indispensable in the early days of the internet. The same modulation and demodulation trick is what a fax machine uses, converting scanned page data into signals in the voice frequency band and sending them down a phone line.
Dial-up connections in the 1990s used a modem to reach the internet over the phone line. The distinctive screeching noise while connecting was two modems negotiating their method and speed with each other, a handshake. The ceiling defined by the standard was 56kbps, roughly one eighteen-thousandth of the 1Gbps fiber optic service commonly used in homes as of 2026. And 56kbps was only the ceiling: actual connection speeds came in below it depending on the state of the line.
A dial-up session tied up the phone line, so nobody could make a call while the internet was in use. "Get off the internet!" shouted at a family member was an everyday scene in 1990s households. ADSL broke that constraint by separating the low frequency band used for voice from the higher band used for data, letting a single phone line carry a call and an internet connection at the same time. Strictly speaking, ISDN could also run voice and data in parallel by splitting the line into two channels, but it was the spread of ADSL, which ran over the existing analog phone line and was far faster, that turned always-on connectivity into a household norm. ADSL has since finished its own run: NTT East ended the service in January 2025, and NTT West in January 2026.
As of 2026 the word "modem" is still in use, but the equipment it points to has changed. Devices that convert signals for a particular kind of line, such as the ONU (optical network unit) on a fiber connection or a cable modem on cable television, are still loosely called modems. An ONU converts between light and electricity rather than modulating and demodulating the way a telephone-line modem did, but its role as the box that matches signal formats between the line and your equipment is similar enough that the old name gets reused. When someone tells you to check "the modem," keep in mind that the actual box differs depending on whether your home is on fiber or cable. See the history of telephone equipment to trace how internet connection technology evolved.