The little white box on your wall is doing something wild.
A laser flashes a strand of glass thinner than a hair. Two hundred milliwatts of infrared light arrive at your wall as your cat video. A photodetector translates it back into voltage. A standard Ethernet cable hands it to your router. The whole thing takes microseconds and the only sound you hear is the fan in your router.
Not a modem. An optical-to-electrical translator.
Most people call any internet appliance a “modem.” The ONT really isn't. A modem takes digital data and shapes it onto an analog carrier — radio waves on coax, audio tones on phone copper. An ONT skips the analog carrier entirely: it just turns flashes of light into pulses of voltage and back.
On the inside there are two small optical parts. The photodetector watches the fiber for downstream light and converts every flash into a tiny current spike. A laser diodesits next to it, pointed back into the fiber, ready to send your upstream data when its turn comes around. Surround that with some chips that handle the protocol housekeeping and Ethernet on the way out, and that's most of an ONT.
- Form factor
- Wall-mount, paperback-sized
- Power draw
- 8–12 W
- WAN connector
- SC/APC fiber
- LAN ports
- 1× 1/2.5/10 GbE
- Standards
- GPON, XGS-PON
- Authentication
- Serial number + optional 802.1X
- Encryption (DS)
- AES-128-CTR
- Management
- OMCI (ITU-T G.988)
- Battery backup
- Optional 8-hr add-on
Some ISPs combine the ONT and router into one unit; treat it as two devices in one box.
- Outside plant fiber
- G.652.D single-mode
- Drop fiber
- G.657.A2 bend-insensitive
- Bend radius
- ≥ 10 mm
- Typical length
- 50–300 ft
- Connector at ONT
- SC/APC (green tip)
- Polish angle
- 8° APC
- Aerial / buried
- Both common
- Lifetime
- 25+ years
Don't touch the green tip. Don't kink it. Don't staple it tight enough to deform the buffer tube.
A glass cable that survives being stapled.
Older fiber would lose signal if you bent it too tight. The drop cables ISPs run into homes today are a different beast — bend-insensitive fiber that keeps the light trapped inside even when wrapped around something the size of a dime.
That's what lets an installer pull a fiber through a wall, around three corners, and behind your TV without measurable loss. The cable itself is rated for 25 years in the ground. The only thing's that's likely to break it is a backhoe.
If your fiber is slow, the answer is almost always clean the connector.
This is where most “slow internet” lives.
You paid for a gigabit. Your speedtest says 180 Mbps over Wi-Fi. Your router is the bottleneck — almost certainly. A few fixes that actually work, in descending order of impact:
- Plug a laptop into the ONT's Ethernet port and test from there. If that is fast, the problem is downstream of the router.
- Replace any router older than Wi-Fi 6. Wi-Fi 5 radios cannot move a gigabit through a typical house.
- Take the router out of the cabinet, out of the basement closet, out of the metal media bay. Radios need air and line-of-sight.
- Add a mesh node for any room more than one wall away.
- Force phones and laptops onto the 5 GHz band; 2.4 GHz is congested by everything from microwaves to garage-door openers.
Inside-your-home questions, answered.
What is an ONT?
ONT stands for Optical Network Terminal. It's the small box on your wall (or in your closet) where the fiber from outside terminates. Inside, a photodetector converts the infrared light pulses arriving on the fiber into electrical signals, and a small laser sends your upstream traffic the other way. The ONT hands you a standard Ethernet port — which is what your router plugs into.
Is an ONT the same as a modem?
No. A modem (modulator-demodulator) modulates a digital signal onto an analog carrier — like RF on coax, or DMT tones on a copper phone pair — and demodulates the return. An ONT does direct optical-to-electrical conversion; there's no analog carrier in between. They serve a similar role at the demarc, but the technology is completely different.
Do I need a special router for fiber?
No. Any modern router with a gigabit (or multi-gigabit) WAN port will work — the ONT just hands you Ethernet. If you've subscribed to a 2+ Gbps tier, you'll want a router with a 2.5GbE or 10GbE WAN port to actually receive those speeds. Wi-Fi 6 or 7 helps if you have many devices, but neither is required to use fiber.
Does fiber internet work in a power outage?
The fiber itself keeps working — there are no powered components between the central office (which has generator backup) and your ONT. But the ONT needs AC power to run, and so does your router. Without a UPS or backup battery for both, your internet goes down with your power. Many ISPs sell an optional 8-hour battery backup that powers the ONT specifically.
What is the fiber drop cable?
The drop is the fiber that runs from the splitter cabinet on your street (or your apartment building's distribution box) to your home's external network interface, where it transitions into the indoor cable that reaches the ONT. Modern drop cable uses bend-insensitive single-mode fiber (G.657.A2) so it can be stapled around corners without losing signal.
What is the connector at the end of the fiber?
Most residential fiber uses an SC/APC connector — a square plastic body with a green-tipped ferrule. The APC (Angled Physical Contact) means the fiber end-face is polished at an 8° angle, which prevents reflected light from bouncing back into the laser and causing noise. Never touch the green tip; oils from your fingers will degrade the signal.
Why is my Wi-Fi slow even though I have fiber?
The fiber link to your ONT is almost never the bottleneck. Slow Wi-Fi at home is usually one of: an old router (replace anything pre-Wi-Fi 6), congested 2.4 GHz channels (force devices to 5 GHz), too few access points for the house (mesh helps), or a router placed inside a media cabinet (radio waves don't love metal). Try a wired speed test from a laptop plugged directly into the ONT — that'll show you the line speed without Wi-Fi noise.
What does the red light on my ONT mean?
It varies by manufacturer, but a steady red light on the LOS (Loss of Signal) indicator almost always means your fiber link is broken — most often a fiber cut somewhere outside the house. A blinking PON light during recovery is normal. Power-cycle the ONT first; if the red light persists, call your ISP and tell them the LOS indicator is solid red — that gets you a truck roll faster.