LuxHow a PON network works, from OLT to home
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The path, from OLT to ONT
Most fiber-to-the-home networks are passive optical networks, or PONs. One laser in the operator's building serves dozens of homes over a single fiber, split along the way by glass splitters that need no power. This guide follows the signal from the OLT, through the feeder, splitters and drop, to the ONT in the home, then explains GPON and XGS-PON in plain words.
OLT (optical line terminal). A chassis in a central office, a hut or a street cabinet, with line cards full of PON ports. Each port has a laser and a receiver that serve one PON. The OLT also connects up to the ISP's core network and the internet.
Feeder. The fiber cable from the OLT out to the splitters, with one fiber for each PON plus spares.
Splitter. A passive glass device, in a cabinet (FDH) or a closure, that divides one feeder fiber among 32 or 64 fibers, or does it in two stages such as 1:4 then 1:8.
Distribution. Cables from the splitters along the streets to terminals near the homes.
Drop. The small cable from the terminal to one home, aerial or buried.
ONT (optical network terminal). The box at the home that turns light into Ethernet, often with Wi-Fi built in, or feeding the customer's router.
Everything between the OLT and the ONT is passive: glass, connectors and splices, with no electronics. That passive middle is what fiber engineers design, and it is the part of the network built to last for decades.
How one fiber serves many homes
Light going down from the OLT is split to every home on the PON, so every ONT receives the same signal. Each ONT keeps only the data addressed to it, and the traffic is encrypted so one home cannot read another's.
Going up is harder, because all the ONTs share one fiber back to the OLT. The OLT gives each ONT short time slots to send in, and resizes the slots many times a second to match demand. That is why a PON's capacity is shared: every home on a port shares its speed, and the operator plans the split so each home's share stays generous at busy hours.
Downstream and upstream travel on different wavelengths of light in the same fiber, so they do not collide.
GPON and XGS-PON in plain words
GPON and XGS-PON are ITU-T standards for how the OLT and the ONTs talk. The fiber, splitters and closures are the same for both.
GPON (ITU-T G.984) sends about 2.5 Gbit/s down and 1.25 Gbit/s up, shared by every home on the port. It sends down on 1490 nm and up on 1310 nm.
XGS-PON (ITU-T G.9807.1) sends about 10 Gbit/s down and 10 Gbit/s up, shared the same way. It uses 1577 nm down and 1270 nm up.
Because the wavelengths differ, GPON and XGS-PON can run on the same fiber at the same time. Operators use this to upgrade: they add XGS-PON ports at the OLT, combine the two signals onto the feeder, and move homes to XGS-PON ONTs one at a time, without touching the outside plant.
The OLT and the ONTs are managed from the operator's OSS/BSS: the systems that activate a customer, assign an ONT to an account and bill for the service. When an ONT goes dark, those systems and the network record are how the operator finds the splitter, the fiber and the port it depends on.
Worked example, key terms and questions
Worked example: does the light reach?
Every PON has an optical budget: the most loss the light can take between the OLT and the ONT. A common GPON optics class, B+, allows 28 dB. Add up a typical link, using the upstream wavelength, 1310 nm, where fiber loss is higher:
A 1:32 splitter: about 17 dB.
10 km of fiber at about 0.35 dB per km: about 3.5 dB.
6 splices at about 0.1 dB and 4 connectors at about 0.3 dB: about 1.8 dB.
The total is about 22.3 dB, which leaves about 5.7 dB of the 28 dB budget for aging and repair splices. A 1:64 split would add about 3 dB and leave less. A real design uses the values on each part's datasheet.
Key terms
PON: Passive optical network: one OLT port serving many homes over shared fiber and splitters.
OLT: Optical line terminal: the equipment in the operator's building that drives each PON.
ONT: Optical network terminal: the box at the home that ends the fiber and hands off Ethernet.
ODN: Optical distribution network: the passive fiber, splitters and closures between the OLT and the ONT.
Optical budget: The most light loss a PON link can take between the OLT and the ONT.
GPON: The ITU-T G.984 PON standard: about 2.5 Gbit/s down and 1.25 Gbit/s up, shared.
XGS-PON: The ITU-T G.9807.1 PON standard: about 10 Gbit/s each way, shared.
Common questions
What is the difference between an ONT and a router?
The ONT is the end of the fiber: it turns light into an Ethernet signal. A router shares that connection across the devices in the home and usually runs the Wi-Fi. Many ISPs supply one box that does both.
How many homes can one PON port serve?
Usually 32 or 64, set by the split ratio. The standards allow more, but every home on a port shares its capacity and every extra split costs light, so most designs stay at 1:32 or 1:64.
Do we need new fiber to move from GPON to XGS-PON?
No. XGS-PON runs on the same fiber, splitters and closures as GPON, on different wavelengths. The upgrade is in the OLT ports and the ONTs, plus a check that the optical budget still holds for the XGS-PON optics class you choose.
How Lux does this work
Lux designs the passive network from an area you select, with the splitters, closures and cable to every home, and checks the optical budget along with your other rules. Try the design demo.
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