It will make a 1000m line possible 1030m long, which means you might lose a little more speed.

The data extension connects to the master socket and runs to the remote location with just a socket for the modem to plug into.

What's the downside of having a data extension exactly?

Just to say, that on the upside...

Some people may not have a power point close to their Master Socket. Using an extension may mean this doesn't become a problem.

I'll be moving to FTTC soon (fibre being installed this month under BDUK) and my Master Socket is in the loft. I don't want to run power up there for a modem as things get pretty hot so I'll be using CAT5 for my install with all my equipment sited in my study as it is now for ADSL. I've tested my current wiring directly in the TEST socket and via my CAT5 extension and get identical stats - so I'm hoping FTTC will work the same for me.

I'll test things with the modem in the Master to see if I'm messing things up though.
You could do the same and see how things are with and without the extension

In reply to a post by MrSaffron:

It will make a 1000m line possible 1030m long, which means you might lose a little more speed.

The data extension connects to the master socket and runs to the remote location with just a socket for the modem to plug into.

That makes sense. Looking at our house, we were thinking possibly running the three Ethernet cables from the window of the room to the front porch where the Hub will be located. It's only about a five meter run and it would enter the same way our phone line currently does. Would you know if this is something the engineer could and would be willing to help us do when he's there, or will we need to contact an electrician? We've got a similar set-up from our living room to a bedroom on the other side of the house with the Sky box and that's worked fine for years.

If you do go with BT Infinity, (I hope you haven't swallowed the Superfast broadband only from BT deception), then their Home Hub 5 has a built-in modem and four gigabit Ethernet ports. You can have Ethernet links to it as long as you like, with no loss of performance. The loss comes between the master socket and the modem.

Engineer will not be interested in running ethernet cables.

Electrician may not be the best option but someone who lists data comms infrastructure wiring.

In reply to a post by RobertoS:

If you do go with BT Infinity, (I hope you haven't swallowed the Superfast broadband only from BT deception), then their Home Hub 5 has a built-in modem and four gigabit Ethernet ports. You can have Ethernet links to it as long as you like, with no loss of performance. The loss comes between the master socket and the modem.

We're going with BT as we were already with them so it's a simple upgrade. The length of Ethernet isn't really a problem, it's more that the room we want connected is upstairs and we're trying to find a solution that doesn't involve trailing cable all the way down.

In reply to a post by MrSaffron:

For FTTC a cabinet like http://www.coolwebhome.co.uk/fibre-cornwall/images/f... but without the stickers

If the manifolds like http://www.coolwebhome.co.uk/fibre-cornwall/images/f... are appearing on the top of poles then get very excited as FTTP is on its way for some people.

Pics from Cornwall http://www.coolwebhome.co.uk/fibre-cornwall/

Very useful Andrew