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Sky, Talk Talk and a number of others use their own backhaul network though beyond the exchange. Where as BT and PlusNet, amongst others, use BT Wholesale's backhaul network.
This CAN make a difference depending on how much capacity each ISP has, or has "bought" for exchange areas. I usually only affects peak speeds but almost every ISP will suffer "hot VP's" in different areas from time to time.
Nice simple description of the situation.
The problem for a customer is knowing what the situation is with a specific ISP in THEIR area ! Whereas monthly contracts were available on ADSL, for FTTC one has to sign up for 12 months so its not so easy to try an ISP out.
I have excellent speeds during the evening but often poor speeds during the day but have no idea whether switching ISPs is going to help.