How fiber internet actually works.
Fiber internet works by sending data as pulses of light through hair-thin glass strands. A laser flashes on and off billions of times per second; a receiver at the other end translates those flashes back into the 1s and 0s your devices use. Because light barely loses energy moving through glass, fiber delivers symmetric 1–10 Gbps speeds with single-digit-millisecond latency over the same wire for decades.
A strand of glass thinner than a human hair, carrying light that won't let go of it.
The cable that brings fiber to your house is mostly empty space. The part doing the work is a strand of ultra-pure silica glass — about 9 micrometers across at the center, one-tenth the width of a human hair.
Around it is another layer of slightly different glass. The two materials bend light differently, and that mismatch is what keeps the light trapped inside the strand. Hit the boundary at a shallow enough angle and the light bounces back — every time, forever — even when the fiber wraps around a corner.
Physicists call this total internal reflection. It's the same trick that makes water look silvery when you look at it from underneath, and it's the only reason any of this works.
- Standard
- ITU-T G.652.D
- Core diameter
- 9 µm
- Cladding diameter
- 125 µm
- Refractive index
- ≈ 1.467
- Attenuation @ 1310 nm
- 0.35 dB/km
- Attenuation @ 1550 nm
- 0.20 dB/km
- Numerical aperture
- ≈ 0.14
- Bend radius (G.657.A2)
- ≥ 10 mm
The glass is purer than drinking water. A 1 km length transmitted side-on would still let you see a flashlight.
- GPON downstream
- 1490 nm
- GPON upstream
- 1310 nm
- GPON max rate
- 2.488 / 1.244 Gbps
- XGS-PON downstream
- 1577 nm
- XGS-PON upstream
- 1270 nm
- XGS-PON max rate
- 10 / 10 Gbps
- RF video overlay
- 1550 nm (optional)
- Modulation
- NRZ-OOK
GPON and XGS-PON live on the same glass — the wavelengths don't fight.
Lasers flashing billions of times a second.
Light from a tiny semiconductor laser shoots into the glass at a wavelength your eyes can't see — deep infrared. The laser blinks on and off in a pattern that encodes your data. A flash means 1; the gap means 0. Billions of those flashes go down the fiber every second.
Different colors of light don't interfere with each other, so the network can run downstream traffic on one color (1490 nanometers, for GPON) and your upstream traffic on another (1310 nm) over the same single strand. Newer fiber service — XGS-PON — uses different colors still (1577 / 1270 nm), so it can coexist on the same glass without disturbing the older signal.
One fiber from the central office, shared by 32 homes — with zero powered hardware in between.
Your ISP doesn't run a separate fiber from their building to every house on your street. That would be insanely expensive. Instead, one fiber arrives at a cabinet on your block and gets split into 32 (or 64) identical copies of the same light, using a small piece of fused glass shaped just so.
The splitter has no power, no fans, no moving parts. It could sit in that cabinet for thirty years without anyone touching it. Every house on the branch receives the same downstream signal at the same instant — and a tiny key on each box is the only thing that lets your box decrypt only your data.
Going the other direction is harder. Thirty-two homes can't all transmit on the same fiber at once or their signals would smash into each other. So the central office hands out precise time slots — you transmit during your slot, your neighbor during theirs, microseconds apart, never overlapping.
- Typical split
- 1:32
- Power budget (Class B+)
- 28 dB
- Insertion loss @ 1:32
- ≈ 17 dB
- Fiber loss budget left
- ≈ 11 dB
- Logical reach
- 20 km
- Extended reach (Class C+)
- 60 km
- Downstream model
- Broadcast → AES-128
- Upstream model
- TDMA, OLT-scheduled
One fiber out of the CO, up to 64 ONTs hanging off it, zero amplifiers between.
The optical distribution network is passive — there are no active elements between the OLT and the ONUs. This is the whole reason the fiber outside plant is essentially indestructible.
The light hits a small white box. It becomes Ethernet.
We pulled the ONT apart in more detail on Inside your home.
How fiber works — questions people actually search.
How does fiber internet work in simple terms?
Fiber internet works by sending data as pulses of light through ultra-thin glass strands. A laser flashes on and off billions of times per second; a photodetector at the other end translates those flashes back into the 1s and 0s your devices use. Because light travels through fiber at roughly two-thirds the speed of light in vacuum — and barely loses signal over distance — fiber delivers symmetric multi-gigabit speeds with single-digit-millisecond latency.
What is fiber optic cable made of?
A telecom fiber strand has two parts of nearly pure silica glass: a central core about 9 micrometers across (single-mode fiber) and a 125 µm cladding around it with a slightly lower refractive index. Around that is a protective coating, a buffer tube, strength members like aramid yarn, and an outer jacket. The glass itself is the part that carries the light.
How fast does light travel through fiber?
About 200,000 km/s — roughly two-thirds the speed of light in vacuum (299,792 km/s). The slowdown comes from light interacting with the glass; engineers call the ratio the refractive index, and for single-mode silica it's about 1.467. The fiber itself is the fastest part of the whole internet — everything else is queuing and routing.
What is total internal reflection?
When light hits the boundary between two materials at a shallow enough angle and the second material has a lower refractive index, all of the light bounces back into the first material instead of escaping. Fiber's core has a slightly higher index than its cladding, so light injected at the right angle bounces down the core and never leaks out — even around bends.
What is a PON (passive optical network)?
A passive optical network is a fiber architecture where everything between the ISP's central office and your home runs on glass with zero powered components. A single fiber from the central office reaches a passive splitter — a piece of fused glass that broadcasts the light to up to 32 or 64 homes — and each home gets its own ONT to terminate the connection.
What is GPON vs XGS-PON?
Both are passive-optical-network standards from the ITU-T. GPON (G.984) delivers 2.488 Gbps downstream and 1.244 Gbps upstream, shared across a tree of up to 32 or 64 homes. XGS-PON (G.9807.1) is the modern replacement: 10 Gbps symmetric, using different wavelengths so it can ride the same fiber and splitters as legacy GPON.
Why is fiber immune to electromagnetic interference?
Fiber carries light, not electricity. It doesn't act as an antenna, can't pick up RF noise from nearby wires or AM transmitters, and is dielectric — so it doesn't conduct surges from lightning. That's why fiber outside plant is far more reliable than coax in storms, near power lines, or anywhere with serious EMI.
How long can a single fiber run before the signal weakens?
Modern single-mode fiber loses about 0.20 dB of signal per kilometer at 1550 nm — astonishingly little. Without amplification, a passive PON tree comfortably reaches 20 km logical distance, and extended-reach designs hit 60 km. Long-haul backbone fiber uses erbium-doped amplifiers (EDFAs) every 60–100 km to keep light strong enough to cross oceans.