What the box does
Fibre brings your internet as pulses of light through a strand of glass. Your router cannot read light, so a small box at the entry point converts it into an ordinary network signal on an Ethernet port. That box is the ONT.
ONT versus ONU
In everyday conversation the two words mean the same thing. Strictly speaking, ONU (Optical Network Unit) is the general term for the device at the customer end of a fibre network, and ONT (Optical Network Terminal) is the term for one that sits inside a single customer's premises. A home or small office normally has an ONT. A large building may have a shared ONU in a comms room serving several tenants.
Reading the lights
Names and behaviour differ between manufacturers, so check the label on yours. The usual pattern is:
- Power: steady means it is on. If it is off, check the power adapter and the socket.
- PON, Link or Sync: steady means it is connected to the fibre network. Blinking means it is still searching or registering. Off means no connection.
- LOS (Loss of Signal): off is good. Red or blinking means no light is arriving, which points to a cut, badly bent, dirty or disconnected fibre.
- LAN or Ethernet: lit or blinking means the cable to your router is connected and active.
First steps before you call
- Check power. The ONT and your router both need electricity, so during load-shedding they need battery or UPS backup. If the whole street is dark, the network equipment outside may be down too.
- Read the lights using the list above and note which are on, off or blinking.
- Look at the fibre lead gently. It should not have sharp bends or be pinched. Do not touch the connector tip, and never look into the end of a fibre or a connector. The light is invisible and can harm your eyes.
- Check the cable from the ONT to your router. Try a different cable or port.
- Restart in order. Power the ONT off and on first, wait two or three minutes, then restart the router.
Most ONTs belong to the fibre network operator. Do not swap, factory reset or unplug it permanently. Tell your provider or us which lights are on or off, especially LOS, and we can usually tell where the fault is.