The cables themselves can probably do Terabit speeds in the future, although Openreach may need to change the kit they use at either end. Essentially in an FTTP deployment the cable itself is no longer a constraint like it has been with copper.
I should caveat though that a small number of areas are being built using ECI FTTP because it would be very difficult / expensive to adopt their favoured approach. You can read more about what this means here:
One of the little known gripes against Openreach's on-going UK rollout of "gigabit capable" Fibre-to-the-Premises (FTTP) broadband ISP technology is that some n
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