Modem

Traditional Meaning

A modem (short for modulator–demodulator) is a device that converts signals so your computer or router can communicate with your Internet Service Provider (ISP).

• Modem = Modulator–Demodulator.
• Originally used in dial‑up Internet (1980s–1990s), converting digital signals ↔ analog telephone signals.
• Later expanded to DSL modems (telephone lines) and Cable modems (TV coaxial lines).
• In all cases, the modem’s role was to translate signals between your local network and the ISP.

Fiber Era: ONT

• With fiber‑optic Internet (FTTH, Fiber to the Home), the device provided by the ISP is called an ONT (Optical Network Terminal).
• ONT’s role:
  • Converts light signals from fiber into electrical Ethernet signals.
  • Provides a port for your router or computer.
• Technically, the ONT is not a “modem” in the old sense, but it performs the same essential function: translating ISP signals into usable digital form.

Core Functions

• Modulation: Converts digital data into analog or carrier signals for transmission.
• Demodulation: Converts received signals back into digital data.
• Physical link: Provides the actual connection between your home/office network and the ISP.

Types of Modems

• DSL Modem: Uses telephone lines (Digital Subscriber Line).
• Cable Modem: Uses coaxial cable TV lines.
• Fiber Modem (ONT): Optical Network Terminal for fiber connections.
• Dial‑up Modem: Legacy technology using analog phone lines (very slow).
• Cellular Modem: Uses mobile networks (3G/4G/5G) for Internet access.

Modem vs Router vs Switch