We've been lucky to generally get an 80Mb down rate, and BTHH5 showing a max data rate of around 100Mb. e.g. image 1. What I've noticed recently is that the noise margin is getting reduced from around 10 to 3 or so, and then the max reported data rate drops to below 70. I get that the exchange automatically adjusts the noise margin, but if a noise margin around 10 is giving rates that comfortably allow 80Mb to be delivered, why is the margin dropping and with it the max data rate? Shouldn't the exchange be leaving the noise margin well alone?
Don't think there's a problem with the Hub.
Firstly, the exchange isn't doing anything, you're getting mixed up with ADSL and VDSL. On VDSL, DLM is intergated into the DSLAM in the cabinet, whereas on ADSL it's in the exchange. DLM isn't changing the noise margin, because the drop occurs without a resync. It's simply a noise source turning on and off. That noise source could be within your property, but it may not be. Do you know of anything that turns on when the NM drops and off when it increases? It could also be a crosstalker (another connection to the cabinet) that is connecting/disconnecting.