Where's my Uplink Speed gone?

Oblivion

I'm on Virgin ADSL2+ and being quite a distance from the exchange and on ancient phone lines I can't get much more than 5mbps.

Last month my connection suddenly started dropping every minute or two and watching the router's management screen I could see the SNR margin was dropping well below the optimum 6db, often falling to 0db or even a minus figure. A this point the Max Rate would leap from the usual 6Mbit to something crazy like 18 or 20Mbit and then the connection would be dropped.

I phoned Virgin who overnight set my SNR to 6db and all has remained well until yesterday when it all started happening again.
Last night I got a call from an engineer who said he thought the people in charge of the cabling had set everybody to 3db and this was affecting anyone like me who was a good distance from the exchange. He manually set it back to 6db and within a few minutes my connection was stable again.

This morning it appears they've tinkered with it a bit more because it has now been set to 9db. Oh, and the DSL Modulation Type is no longer showing ADSL2+ but has changed to ADSL_G.dmt whatever that is. My router currently shows ...

Current Rate Kbps Upstream 800 Downstream 5824
Max Rate Kbps Upstream 996 Downstream 6016
SNR Margin dB Upstream 8 Downstream 9
Line attenuation dB Upstream 22 Downstream 43
Error packets None

Running Speedtest.net I'm getting around 5Mbps Download but it shows my Upload as only 0.09Mbps which is pathetic.

What on earth could be causing this disparity?

kwikbreaks

What happens is that a target noise margin gets set for the initial sync. After that if noise rises then your SNR falls. This can continue to the point where sync can no longer be held and the modem will resync - almost certainly slower than before unless the noise has cleared and you will again show a noise margin of whatever the target was set to. Target setting is automated on any BT based system and it's only LLU connections where this gets done manually. I though non-cable VM only used BT connections but the upstream sync rate suggests it must be LLU.

ADSL_G.dmt is 8Mbps maximum. If you weren't getting over that on ADSL2+ then this may prove a better option for you.

Nothing in your stats suggests an upstream issue - are you sure you measured it correctly?

Oblivion

Thanks kwikbreaks. Yes I definitely interpreted the speed tests correctly. Very similar results on both Ookla speedtest.net and BT Wholesale speed test.

Yes I believe it is LLU and was administered by Cable & Wireless but I think it may have changed hands, not sure.

Actually, just moments after posting the OP I had a phone call from a Virgin techie to enquire if my connection was now stable, which it is. I told him about the disparity between upstream and downstream tests and he reckons there's definitely something wrong with my line. So he's leaving the ticket open, continuing to monitor the line, and will be having a chat with the BT engineers about the issue.