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What is an ONT or CTU?

Written by KCOM Support on 22/03/2022. Posted in Broadband.

There are two types of boxes that we attach to your wall during a Lightstream install - one is installed inside your home (called an ONT) and the other is installed outside (called a CTU).

What's an ONT?

An ONT (Optical Network Terminal) is a white or black box that we attach to the wall during your Lightstream install.

The fibre cable that we feed into your home connects to the ONT through the wall, which in turn connects to your router. Your Lightstream router will be connected to this via an Ethernet cable and is normally plugged into the LAN 1 port.

You can tell what is happening with your connection by looking at the lights on your ONT - we have a guide for this here.

What does it look like?

The ONT looks similar to the images below and will be attached to your wall:

White ONT

Black ONT

If you need to know the dimensions of the ONT for placement, they're listed below (White ONT measurements only):

Width:19.5cm (7.75'')
Height:15.5cm (6.25'')
Depth:5cm (2'')

What's a CTU?

A CTU (Customer Termination Unit) is the grey, black, or brown box that we install on the outside of your home before your Lightstream install.

The fibre cable that we connect to your property feeds into the CTU from underground, or from a telegraph pole before going through the wall to your ONT.


If you need to know the dimensions of the CTU, they're listed below:

Black and Grey CTU:

Width:15cm (6'')
Height:20cm (8'')
Depth:5cm (2'')

Brown CTU:

Width:21cm (8.5'')
Height:10cm (4'')
Depth:5cm (2'')

What happens during your Lightstream install?

On the day of your Lightstream install, one of our engineers will drill a small hole through an outside wall of your home - this is so that we can feed the fibre cable from the CTU into the ONT.

The ONT will normally be fitted in the same place (but on the inside wall) so that you don't see the fibre cable. If this isn't possible, or you want the ONT placing somewhere else, we'll run a fibre cable through your home to the ONT.

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