I don't think it will be at the GEA handover at the exchange. More likely at the MSIL that you go through out of WBC into the BT Retail network. Barring network fault conditions, that is a fixed point.
There is a comparable setup on Entanet, where somehow they restrict that throughput for all on that node. Importantly I think they do it top-down, i.e.reduce the highest speed allowed on the node/MSIL.
You can see the Entanet ones here
. (Don't forget, these are the Enta MSILs. Every WBC-using ISP has their own so there is no load correlation between ISPs. It depends entirely on the contracted throughput the ISP has arranged with BT Wholesale for each individual MSIL).
Somewhere in the SINs, IIRC, is a margin for over-use. Let's call it "m". (Unlike Centrals, MSILs are unlikely to fill up. Unless perhaps yours has?)
As I understand it there are 1Gbps and 10Gbps MSILs. Just for example, I'll use 1Gbps. The ISP has contracted for say 600Mbps. If demand rises above that the extra capacity is automatically there, so far as BTW are concerned. It is provided until of course it reached 1Gbps.
However, up to 600+m% the charge is pro-rata. Above 600+m% the charge is much higher. A penal rate.
I see two possibilities. First your MSIL is reaching capacity due to unexpected demand caused by high 80/20 takeup. Or second, the same effect as it somehow gets throttled to 40Mbps as it approaches 100% of contracted. Then lower if necessary, as demand is rising rapidly these days. Queuing effects will also be present, so even at a 40Mbps MSIL cap you won't get anywhere near that. Motorway slowdown syndrome.
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