That’s domestic ADSL for you; however A&A were tarting it up, what they were selling you was BT’s domestic ADSL Max service, and that’s contended at up to 50:1 at the local exchange. Max sucks. Badly. Frankly Max is one reason I’m glad I’m still on cable. I manage a fair few business ADSL connections at work, and the quality of the service since BT began shifting people over (with no consultation or permission) from fixed 2Mbit to “Up to 8Mbit” Max has been abysmal. Connections dropping, synch failures, endless router restarts… Yes, you probably WILL see your speeds drop, and when you complain they’ll tell you “but we advertised UP TO 8Mbit…”.

This oddly makes sense and here is my suggestion as to how.

Your ADSL connection is provided over copper wires, the actual pairs of copper are uniquely allocated to your number so that there is a unique pair from the exchange to the street cabinet, a unique pair from the street cabinet to the telephone pole and possibly but not always, a unique pair from the telephone pole to the building of your home.

In almost all cases the pair entering your house and terminating either at a junction box or a master Line Jack Unit are alone, i.e they are not colocated next to other copper pairs. But from the exchange almost to your house the pair is in a multicore cable carrying up to hundreds of pairs.

This is the reason for your speed drop. One pair will radiate signal electronically to surrounding pairs, this can easily be picked up by those pairs as noise, increase the noise and lower the Signal to Noise Ratio. It is called crosstalk and a BT engineer confirmed to me it is an increasing issue.

Voice over the pair creates far less noise than an DSL signal so the more DSL users in a shared multicore, the more noise and the worse the available speed.

Visit your router they will all quote SNR or SNR Margin, make a note of this figure as well as, if quoted the loop attenuation figure, (the level of the signal on the line or gain).

Add a new neighbour using ADSL and these figures will change and your link speed will probably fall.

In a business context BT will, if you can prove these figures are causing too many HEC errors (ATM errors caused by dodgy signals), change over the pair in the shared section until you get a better SNR and better connection, but my experience is that they will only do this if it really marginal and drops out. Finding the proof of this is really hard, (it took me 3 weeks and I am trained in ATM, have Eclipse as an ISP, who were excellent as always, and finally fixed it with an old hand BT Outreach engineer.

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