Modem
Modulator-demodulator
The device that connects your home to the ISP's network by converting their signal into Ethernet.
A modem (modulator-demodulator) converts the signal from your ISP's network — coaxial cable, fibre, or phone line — into an Ethernet connection your router can use. Without a modem, your router has no path to the internet.
What a modem does
The word "modem" is short for modulator-demodulator, which describes its core function: it takes digital data from your router, modulates it onto the carrier medium your ISP uses (radio frequencies on coax, light pulses on fibre, or audio-range signals on phone lines), and sends it toward the ISP's network. In the other direction, it receives the modulated signal from the ISP, demodulates it back into digital data, and passes it out via Ethernet. Everything happening on the internet side of the modem is the ISP's infrastructure; everything on the Ethernet side is your home network.
Modem types by technology
- Cable modem (DOCSIS) — connects to the coaxial cable from the wall; used with Xfinity, Spectrum, Cox, Optimum, and other HFC cable ISPs; DOCSIS 3.1 is the current standard for plans above 400 Mbps
- DSL modem (ADSL/VDSL) — connects to the phone line; used with legacy AT&T DSL, CenturyLink DSL, and similar; ADSL tops out around 24 Mbps download; VDSL2 can reach 100 Mbps within short distances of the DSLAM
- Fibre ONT (Optical Network Terminal) — converts the fibre light signal to Ethernet; ISP-supplied and ISP-maintained; not user-replaceable; used with AT&T Fiber, Verizon Fios, Google Fiber, and most FTTH providers
- 4G/5G cellular modem/gateway — contains a cellular radio that connects to a mobile network tower; used for fixed wireless access (T-Mobile Home Internet, Verizon Home Internet, rural LTE services); typically a self-contained gateway with built-in router and Wi-Fi
Modem vs router vs gateway
These three terms are often confused because ISPs bundle the functions together:
- Modem — bridges the ISP medium to Ethernet; has one WAN input and one Ethernet output; performs no routing or Wi-Fi
- Router — manages the home network; assigns private IP addresses via DHCP; performs NAT; provides Wi-Fi (in most consumer models)
- Gateway — a combined modem and router in a single device; what most ISPs provide by default; convenient but limits your control over firmware, QoS, and security settings
Running a separate modem and router — rather than an ISP gateway — gives you full control over your router's firmware and lets you upgrade each component independently.
Owning vs renting: cost over 2 years
| ISP rental | Own modem (DOCSIS 3.1) | |
|---|---|---|
| Monthly cost | $10–15/month | $0/month after purchase |
| Purchase cost | $0 | $80–150 (one-time) |
| Total over 24 months | $240–360 | $80–150 |
| Savings over 2 years | — | $90–210 |
Always verify the modem is on your ISP's approved compatibility list before purchasing. ISPs occasionally de-certify older modems, and an uncertified modem may be provisioned at a lower speed tier.
Modem signal quality indicators
Cable modems expose a diagnostics page (commonly at 192.168.100.1) showing the following metrics — useful for diagnosing physical plant problems:
- Downstream power level — healthy range: −7 to +7 dBmV. Outside this range indicates signal level problems from the cable plant or too many splitters
- Downstream SNR (signal-to-noise ratio) — healthy: above 33 dB for QAM256, above 38 dB for QAM1024. Low SNR causes high error rates
- Upstream power level — healthy range: 38–48 dBmV. Values above 50 dBmV mean the modem is struggling to push signal upstream, often due to corroded connectors or signal ingress
- Uncorrectable codeword errors — should be zero or near zero. Any significant count indicates a physical layer problem that warrants an ISP technician visit
When to replace your modem
Replace your cable modem if: it is DOCSIS 3.0 and you have a plan above 400 Mbps; it is more than 6–7 years old and showing performance degradation; the diagnostics page shows consistently high uncorrectable errors; or the ISP has removed it from their approved list. For fibre ONTs, replacement is the ISP's responsibility — report performance issues to them directly.
Frequently Asked Questions
Can a bad modem slow down my internet?
Yes. An aging cable modem (DOCSIS 3.0 on a gigabit plan) physically cannot achieve plan speed. A modem with a failing power supply can cause packet loss and random disconnections. If speed tests were good last year and have degraded without any plan change, the modem is a likely culprit.
How long does a cable modem last?
5–7 years is typical. Cable modems degrade as their components age and as ISP firmware updates push them past their original design specs. If your modem is over 5 years old and you are seeing performance issues, replace it.