OK, well since I've already hijacked this thread, I was wondering if you could help me figure out at what level the problems are occurring with my BT Infinity connection.
So what I've figured out so far - firstly, whatever the issue is it's somewhere between my exchange (Denburn) and Edinburgh. That leg reaches saturation pretty much all day from 2pm to 2am now (reminiscent of VM issues I used to get). Download speeds drop from line rate (70Mbps+) to less than 10Mbps at peak times. Latency and packet loss pretty much exactly match your example of "bad 21CN congestion" earlier:
Now according to AAISP, Denburn should be fixed. Now I'm not sure what is fixed, and what other variables could perhaps be in play that cause issues specifically for BT, but not AAISP. Nonetheless BT Retail is seeing very low speeds and heavy packet loss, and I'm guessing AAISP is not. Both are BT Wholesale based.
What I also know is the physical pipe between Denburn and Edinburgh is not at saturation, as there are other links (e.g. the ptm.301 "backdoor" network on the Openreach modem) where I can ping some Juniper device somewhere around Edinburgh/Glasgow and receive consistent, loss-free and jitter-free responses throughout the day that do not get worse at peak times. Pinging the PPPoE gateway though, again in Edinburgh, does see increased packet loss and latency. So it's only the PPPoE based "Internet" connection that is affected by the congestion.
The physical pipe to the nearest core node or WBC interconnect (21CN backhaul?), isn't saturated, but whatever logical pipe BT Retail uses for Infinity is... What could it be? Is it likely to be the SVLAN again, or something else?
And more importantly, is it something BT can easily fix?
[Edit] Just noticed AAISP say Denburn is fixed, but Plusnet say is it not
http://community.plus.net/exchange-information/