It was pointed out earlier that 200kbps seems to be the case. Well the affects in reality for me where a dam sight more than only 200kbps when set to 77.4 or a 77.44 BTW profile , it improved when set to 78mbps but still resulted in a lower throughput than what i should of had, I don't really care what part of they system was responsible,
I think you may be saying that there seem to be other throughput losses. I think you may be right, but that isn't the same thing as the hard cap that the Current line speed sets before those kick in.
It could be to do with the buffering involved in the prioritisation system. Given the overall throughput increase due to the increase in the number of customers, plus the radical change in the way people use the net in the last couple of years, it wouldn't be surprising if that side of the system wasn't creaking.
Queuing systems are complex beasties . A small slowdown in one place can cause a huge jam elsewhere, as on motorways.
In the spat between tommy45 and RobertoS, I have to stand on tommy45's side on this one. On some complaints, I find him overly harsh, but this is spot on.
Plusnet's IP profile system does seem to lop 2-3 Mbps off the possible throughput speed on an FTTC line synced at 80 Mbps.
I have experienced this on my current line, this year, where throughputs of 74Mbps+ were possible at first, but (after DLM intervention & de-intervention), the maximum is back down at 72Mbps.
Last year, on my previous line, I did some testing for Dave Tomlinson. When he set the profile to 77.4, I got speeds averaging 72.5. When he changed the profile to 92, I got speeds of 74.7.
I even saw similar issues on my first FTTC line, back when you could only get 40/10 services.
These differences aren't just guesses either. Regular repeated tests with a Samknows box helps produce incontrovertible graphs. An example can be seen from my recent post on the PN forum:
I have made similar posts before. I'm not especially complaining about my line speed, but none of those posts have resulted in changes to the overall throughput, or an explanation why it happens. It means that PN's average speeds show themselves to be lower in Ofcom's reports, for no publicly-explained reason.
The question of whether this happens has been put to bed. The question about why it happens has not.