Why does BT Openreach clobber upload speed?

I have FTTC with AAISP and I am about 1.5Km from my Huawei cabinet. My line does 21Mbit download and 1.6Mbit/s upload, but BT's DLM profile seems to want to restrict my upload speed to 0.8M/s, which is really annoying, as it is killing 50% of my upload speed capability.

To try and fix my upload speed, AAISP arranged a DLM Reset for me and during the 10 day training period afterwards, my line held sync perfectly, not dropping the line even once, with upload running at 1.6Mbit/s. I'm using the TP Link W9970 which I understand has a Broadcom chipset and supports G.INP in both download and upload directions.

My modem reported that I had about 12db of noise margin on upload and my profile before the DLM reset looked like this - Note the 0.8M profile limit applied to upload:

BT Test E2E Access Test/DCN:Inconclusive OR test pass. Unable to find the fault.
Down:18.5 (19.8/20.9/21.6) Up:0.8 (0.8/0.8/0.8)
2016-05-05T09:30:00 0.128M-30M Downstream, Retransmission High - 0.128M-0.8M Upstream, Error Protection Off
In Sync Pass:GTC_FTTC_SERVICE_0001 GEA service test completed and no fault found but unable to check for customer equipment connected to modem

Immediately after the DLM reset, my line went back to running at 6db SNR margin and my upload speed was restored to 1.6Mbit/s, where it should be. The line was very stable and according to the DSL stats program, I am seeing only about 8 errored seconds per hour on upload. The line was completely stable throughout the 10 day DLM training period with no resets at all. The profile looked like this:

BT Test E2E Access Test/DCN:Inconclusive OR test pass. Unable to find the fault.
Down:21.4 (18.0/19.8/21.4) Up:1.6 (1.6/1.6/1.6)
2016-05-24T12:15:00 0.128M-40M Downstream, Interleaving Low - 0.128M-10M Upstream, Error Protection Off
In Sync Pass:GTC_FTTC_SERVICE_0001 GEA service test completed and no fault found but unable to check for customer equipment connected to modem.

Immediately after the end of the 10 day period, I see that DLM has decided that I am to be rate limited back to 0.8M on upload again. Why??? Why has it not tried enabling G.INP on upload, which is supported by my modem. I've not yet re-synched my line and therefore it is still running at 1.6M, but already I can already see that the latest upload profile is going to limit me to 0.8M again - See here:

BT Test E2E Access Test/DCN:Inconclusive OR test pass. Unable to find the fault.
Down:21.4 (17.9/20.3/21.4) Up:1.6 (0.8/1.6/1.6)
2016-05-24T00:15:00 0.128M-30M Downstream, Retransmission High - 0.128M-0.8M Upstream, Error Protection Off
In Sync Pass:GTC_FTTC_SERVICE_0001 GEA service test completed and no fault found but unable to check for customer equipment connected to modem.

Can anyone suggest why BT Openreach is deliberately crippling my upload speed (from 1.6Mbit/s to 0.8Mbit/s when my line has been stable?

Here's my line data from DSL Stats:

Tone usage
Connection speed
Noise Margin
Errors
Line Stats

Because the second band of upload is right on the cusp of not being available to you due to line attenuation it looks like. So hit and miss and as the DLM tends to favour stability rather than speed its kicking in

Which of the three DLM regimes does AAISP say the line is configured with?

BTW there is no 10 day period on VDSL2, if the DLM decides lines need slowing to reduce errors they will do so in day 9 or day 333 and if things are stable they do relent eventually

Day 9 or 333?

Surely long before day 9. It can apply banding on the night following day 2 can't it?

I wonder if it is to do with the downstream being lowered to "0.128M-30M Downstream, Retransmission High" from "0.128M-40M Downstream, Interleaving Low"?

Re G.INP it isn't enabled for upstream at the DSLAM for your line, so setting it in your router won't have any effect unless that changes.

In reply to a post by MrSaffron:

Because the second band of upload is right on the cusp of not being available to you due to line attenuation it looks like. So hit and miss and as the DLM tends to favour stability rather than speed its kicking in

But the line has been completely stable with upload running at 1.6M, so DLM didn't need to fiddle and make it so that I cannot sync my upload more than 0.8M. In any case, I thought DLM should enable G.INP on upload first? G.INP has been working very well on the download side - When it was enabled a few days after the DLM reset, I gained 3Mbit/s of download speed and the errors dropped considerably. Does anyone have G.INP enabled on upload?

These DSL stats show what happens to connection speed when G.INP is added / removed on my line:

This is the speed change when BT enabled G.INP on download
When my line had a DLM reset, I lost 3M on download (removal of G.INP) and my upload jumped from 0.8M to 1.6M

In reply to a post by MrSaffron:

Which of the three DLM regimes does AAISP say the line is configured with?

Not sure - Nothing on the AAISP control screens says and I have not instructed them to doing anything different with my line, but I read elsewhere that AAISP asks BT to prioritise speed over stability on VDSL2 lines.

In reply to a post by MrSaffron:

BTW there is no 10 day period on VDSL2, if the DLM decides lines need slowing to reduce errors they will do so in day 9 or day 333 and if things are stable they do relent eventually

I waited for a month for DLM to relent, but it never did. Having had a DLM reset, it looks like it is clobbering my line again, even though it has been very stable. Is it just that BT don't care much about maintaining speed on uploads? Given that I am on a BDUK funded cabinet and my line is below the superfast speed level, I can understand why they have put everything into boosting download speeds, but they don't seem to care much about upload.

In reply to a post by RobertoS:

I wonder if it is to do with the downstream being lowered to "0.128M-30M Downstream, Retransmission High" from "0.128M-40M Downstream, Interleaving Low"?

Re G.INP it isn't enabled for upstream at the DSLAM for your line, so setting it in your router won't have any effect unless that changes.

The question is why is BT not enabling G.INP for upload on my line? I believe my modem supports it (TP Link W9970) and the cabinet is Huawei, so it presumably supports it too.

I had a line fault recently and I found that G.INP was only enabled on the upstream when there were huge swings on snrm, often below zero.

In reply to a post by RobertoS:

I wonder if it is to do with the downstream being lowered to "0.128M-30M Downstream, Retransmission High" from "0.128M-40M Downstream, Interleaving Low"?

That's an interesting theory... Does anyone know if there is any relationship between the downstream and upstream profiles?

Just picking random days that are not the 10 mythical figure

I don't know about clobbering upstream, sync/throughput, But they won't sell anything near symmetrical for whatever reason they have a 80/20 why not 80/30 or 40 product? same goes for FTTP services why only a 30mbps upstream on a 330 downstream service? 30mbps is laughable there's no real reason that they could not offer at least 150mbps
With their dodgy claims that upload speed isn't as important as downstream is, that maybe true for those that only want to browse stream and get e-mails but as a result the rest are stuffed thanks BT

Edited by tommy45 (Sun 05-Jun-16 22:00:36)