Modem

What It Means

A modem (modulator-demodulator) is the device that converts the physical signal from your ISP (coaxial cable radio frequency, DSL analog voice-band, or fiber optical light pulses) into digital ethernet that your router can use, and vice versa for outbound traffic. For cable internet, the modem implements the DOCSIS protocol and must be certified for your ISP's network, DOCSIS 3.0 modems support up to about 600 to 800 Mbps delivered, DOCSIS 3.1 modems handle multi-gigabit speeds (1.2 to 2.5 Gbps typical), and early DOCSIS 4.0 modems are shipping in 2024 to 2025. Cable ISPs typically rent modems for $10 to $15 per month, a good-quality Arris Surfboard SB8200 or Motorola MB8611 modem costs $120 to $200 to purchase and pays for itself in under 18 months while typically outperforming rental equipment. Not all purchased modems are compatible with all ISPs: Comcast, Charter, and Cox publish lists of approved modem makes and models. For fiber internet, the modem equivalent is an Optical Network Terminal (ONT), which the ISP always provides at no monthly fee because the ONT must be matched to the specific GPON or XGS-PON optical protocol the ISP uses. For DSL internet, DSL modems are nearly always bundled with the ISP service. For fixed wireless (T-Mobile, Verizon 5G Home), the ISP provides a gateway device (modem + router combined) that cannot be replaced with customer equipment. An outdated or marginal modem is one of the most common causes of slow cable speeds, every 5 to 7 years, upgrading the modem is worthwhile even if the old one still works.

Frequently Asked Questions