What Is a Modem?

A modem, short for modulator-demodulator, is a device that turns a computer’s digital data into a signal suited to a particular line or radio link, and turns the signal arriving from the other end back into data. Modulation means varying a carrier signal, its amplitude, frequency or phase, to represent bits. Demodulation reads those changes back.

The first modems sent data as audible tones over telephone lines, which is the hiss and screech dial-up users remember. The last dial-up standard, ITU-T V.92, peaked at 56 kb/s. Modern modems use the same principle at far higher frequencies and with far more complex modulation, carrying hundreds of megabits or more over copper, coax, fiber and radio.

Every internet connection needs one. It is the device that speaks the provider’s line technology, whether that is a phone line, a cable network or a mobile mast.

The modem type follows the connection. Each uses a different line and a different standard.

Strictly, a fiber optical network terminal (ONT) is not a modem, because light is switched on and off rather than modulated onto a carrier in the same way, but it does the same job in the network. Speeds in practice depend on line length, signal strength and the provider’s package.

The two are easy to confuse because most home internet boxes combine them. The modem connects to the provider and produces one internet connection. The router shares that connection between all the devices on the site, gives each a local address, translates addresses with NAT and blocks unwanted traffic with a firewall. A modem alone can serve one device. Add a router, and the whole network can use it.

On industrial networks the two are often separate, so the router can be chosen for its security and VPN features and the modem for the line. In a cellular router the modem is built in, and some routers take a second modem as a backup, which is the idea behind a multi-SIM router.

Industrial computers rarely use an external modem. A cellular modem module fits inside the system and appears to the operating system as a network interface, so the computer connects directly to a 4G or 5G network wherever there is coverage. Because the module sits inside the enclosure, it shares the system’s power supply, cooling and radio approvals, so it is chosen alongside the computer rather than added later as an accessory.

Many of our industrial IoT gateways and in-vehicle computers list 4G or 5G options. The right module depends on five things.

• Bandwidth. LTE Cat 4 suits telemetry and remote access. Video, edge AI uploads and backhaul need LTE Cat 6 or above, or 5G.
• Bands and region. Modules come in regional versions, so check the bands match the networks in the USA, Europe, North America or wherever the system will run.
• Carrier approval. Some networks require approved modules before they accept a device.
• Temperature and power. 5G modules draw more power and run hotter, which matters in a sealed fanless system.
• Antenna placement. A metal enclosure blocks signal, so antennas usually mount outside it or on the vehicle roof.

Our white paper on integrating 4G, LTE and 5G connectivity covers module choice in more depth.

We fit and set up the 4G or 5G module, SIM slots and antennas each system needs as standard, and test the cellular connection before the system ships.