>> rightly so, as the ISP wont be paying for bandwidth use, just pipe size <<That doesn't quite make sense to me. Regardless if its CBC, WBC or WMBC as the more bandwidth the EU uses, then the more capacity the SP has to purchase in order to prevent congestion. "Pipe size" requirements are based upon EU usage requirements.
With CBC the ISP had to ensure they bought sufficient centrals to cover peak demand. So if for example their customers were downloading 100GB of data at peak, then they need to ensure they purchased sufficient pipes to meet 100GB*. Although CBC didn't actually charge for metered bandwidth, the necessity was there to purchase more pipes to meet the heavier downloaders.
If an ISP had EU's that didnt download very much then they could allow more users to connect to that central without it being hot... and they could perhaps get away with just purchasing a few 622Mb pipes.
More than any other product, CBC was _the_ product that made ISPs care about how much bandwidth their customers used. It could make the difference between an ISP having to buy say 4x 622Mb pipe or 6x 622Mbp pipes per 100k* customers. So although BTw didn't charge per monitored bandwidth, the ISP was actively forced to purchase more centrals based on customers [peak] usage. When each 622 central pipe cost circa ~£1.75 million per month, then although BTw weren't monitoring bandwith**, the ISPs sure as hell had to.
With 21CN/WBC the ISP purchases a '
MSIL' that gives the SP access to the 21CN backhaul. Cost to the SP depends upon how much bandwidth they rent. More info about the MSILs and APs on the page I just linked to.
The ISP still has to ensure that they have sufficient capacity, but what has made the big difference these days is the use of
Wave Division Multiplexing (WDM) technology used in the 21CN platform has substantially increased bandwidth availability & reduced the cost of bandwidth on the back-hauls.
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*made up figures.
** not strictly true as BTw policed the centrals to ensure pipes routinely weren't at near 100% capacity.