Just what can an ISP test on a faulty ADSL connection

I've just had a problem on my ADSL link with my data rate being less than 50% of what it should be. I contacted the support desk (John Lewis/Plusnet) who did various tests and reached the conclusion that there was no fault. They kept telling me that the connection speed was Ok. Presumably they meant the sync speed which I know was OK. But I couldn't seem to get through to them that sync speed was not the same as the data rate.

So, my question is just how much of the connection can an ISP test ? I'm guessing that they can't actually tell what the data rate really is at my end.

FWIW: after 3 days of this slow speed, 30 minutes after they run their tests with NFF, it magically started running at full speed again.

Edited by deleted (Tue 19-Mar-19 14:28:48)

Well, they have no control over the Net beyond their own network & can't see it from your viewpoint anyway.

Three days sounds like DLM sorting itself out.

Also if it is ADSL as in old IPStream there is the length of time the IP Profile takes to change