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Why is my internet so slow – How to understand ADSL Routers on BT telephone lines


The three most important aspects of your telephone line are, Attenuation, Noise Margin and Sync.

Attenuation (The lower the better!):

This figure in an indication of your lines length. Attenuation is all to do with quality of the signal passing through a conductor – copper or aluminium. The higher the lines attenuation the worse the sync speed will be.

Attenuation is measured in db’s (decibels)

NB. The maximum reported attenuation figure with the majority of routers is 63 or 63.5db.

As you can see – my router reports this maximum attenuation figure – which is VERY BAD.

SNR router
If your attenuation is displayed as 63/63.5db then it may in fact actually be higher.

You unfortunately have a very long line and/or poor copper/aluminium between you and the exchange.

BT can’t move your house… nearer the BT exchange.  So there’s nothing that can be done if you’re 6 or 7 miles from the exchange, UNTIL Fibre Optics are installed at your specific telephone exchange.  Copper is low quality compared to Fibre Optics (Optics means glass cables… the light bounces around the inside of the cable, and it can travel long distances without signal loss).

With luck, your BT Exchange can run “BT Infinity”, which is their posh term for Fibre Optic cables.  Upgrade if you are a long distance from the BT exchange – you’ll be delighted with the improvement.

BT Infinity Post Code Checker – Check with OpenReach about Fibre Optics at the Exchange

If not, the roll out schedule (by Month) for those “Coming Soon” 2013 Exchange upgrades

SNR – Noise Margin (The target is 6db)

Noise Margin or Signal to Noise Ratio (SNR) initially has a ‘target’ of 6db. i.e. The BT kit will attempt to give you your best connection speed (sync) using 6db of SNR.

BT have achieved 6db of SNR on my line.. so I can’t complain.

Sync (The higher the better!)

To find out what your line is connecting at, you need to look at your Router or Modem Statistics.

One Comment
  1. I’d also add latency to the mix, as that’s sometimes the biggest factor. You’ve got at least three potential bottlenecks: your home router, the switching box thingy at the end of your street and the carrier routing system.

    All of these are basically electronic circuits, and as such, there’s a limit to how fast they can process and re-encapsulate TCP/IP packets, translate between addresses and do whatever packet inspection stuff, or convert between photon pulses and their electrical equivalent. Often it’s because people are using standard issue budget routers at home, and those poor routers are trying to re-encapsulate a massive volume of packets in real-time.

    Where does the latency exist? You could always use ping and traceroute to get an idea, but you already knew that.


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