BT Infinity 1 speed drop

I have a long line to the exchange and used to get 1.25Mbps on ADSL2+. FTTC finally arrived and BT forecast 27Mbps even though I am over 1km from the cabinet. I ordered Infinity 1 and the day of installation came and the BT engineer showed me that the speed was in fact 27Mbps as forecast. Later that day I did a test on Speedtest.net and the download speed was around 26Mbps, and upload was around 1.2Mbps.

A day after the installation the download speed had dropped to 15Mbps and 9 days later it is still around 15Mbps and the upload speed has fallen to 0.7Mbps, slower than when on ADSL2+. No changes have been made to any wiring or sockets.

I realise there is a settling in period but can anyone offer any advice re:

Is it normal for the speed to drop so much after the first day?
Why is the upload speed so slow?
FTTC has only just become available in our area so a lot of people are being connected. Could this have an effect?
Is the target noise margin 6db as with standard ADSL?
Should I take this up with BT?

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Stats as follows

3. Firmware version: Software version 4.7.5.1.83.8.173.1.4 (Type A) Last updated Unknown
4. Board version: BT Hub 5A
5. VDSL uptime: 0 days, 03:12:33
6. Data rate: 797 / 15217
7. Maximum data rate: 1092 / 15808
8. Noise margin: 6.4 / 6.2
9. Line attenuation: 0.0 / 27.5
10. Signal attenuation: 0.0 / 22.1
11. Data sent/received: 3.1 GB / 2.1 GB
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Thanks in anticipation.

Is it normal for the speed to drop so much after the first day?
No
Why is the upload speed so slow?
Because the upstream and downstream are weaved together so upstream drops off over distance more than it used to with ADSL2+ and the DLM will have eaten a chunk.
FTTC has only just become available in our area so a lot of people are being connected. Could this have an effect?
Yes
Is the target noise margin 6db as with standard ADSL?
The DLM works differently, tends to cap/band speeds rather than work to target margins.
Should I take this up with BT?
Maybe, but if you think line length is closer to 1.5km, then your speeds are what I'd expect once cross talk kicked in.

The fact the maximum rate is close to the data rate suggests you were the first in your bundle, and its the next couple of people in your cable bundle that will have killed your speed I suspect.

Thank-you so much for your prompt and informative answer. I think my main problem is that it started out at 27mbps. If it had started at 15Mbps and stayed there, I would have been happy. Should I assume that at 27Mbps it would have been unstable and therefore I'm better off where I am? Think I'll still contact BT to see what they say. Thank-you.

Near impossible to say for sure, but the cross talk theory is that it can cause this sort of drop and combine this with a DLM that aims for an error free line then feasible.

If it was me, and I knew for sure line was 1km long I'd moan more at ISP, if I knew line was likely to be over 1250m then I'd just take it as the way VDSL2 works...i.e. first user goes great for a while.

So best way to avoid wasting hours on the phone getting no where, is to check the distance using a walking route via Google Maps.

Google maps indicates 1500m along the road and the line distance cannot be significantly shorter than this, assuming there's not a cabinet somewhere that I don't know about.

In which case you look to have settled down at the speed expected with some cross talk involved

This is very worrying. Presumably this also applies to superfast broadband from other providers. The poor upload performance probably makes cloud storage and computing unviable.

In this case, the Noise Margin is above the target indicating the presence of noise which may have reduced the sync speed.
This indicates there is possibly something wrong with the install.

This has now got a lot worse. The speed gradually got worse until it was around 10mb. BT came out at the end of December and said it was impossible to synch at 27mb ( as it had done on day 1) and I would have to settle for 14 / 15 mb and they would not respond to any further complaints at this speed. Bearing in mind the advice received here I was happy to accept this. They said the reason for it being 10mb was because I had switched off the router once, for 3 days when I was away over Christmas and that it would recover to 15mb.

Router stats at that time were as below although I noticed that the maximum data rate varied a lot from day to day.

4.Board version: BT Hub 5A
5. VDSL uptime: 1 days, 18:30:45
6. Data rate: 579 / 11741
7. Maximum data rate: 661 / 12871
8. Noise margin: 8.4 / 6.7
9. Line attenuation: 0.0 / 29.0
10. Signal attenuation: 0.0 / 22.7

As of today thing have not improved, although the maximum data rate has halved.

5. VDSL uptime: 7 days, 01:01:49
6. Data rate: 579 / 11741
7. Maximum data rate: 622 / 6440
8. Noise margin: 7.2 / 7.1
9. Line attenuation: 0.0 / 29.0
10. Signal attenuation: 0.0 / 22.7

I am in contact with someone from with BT Broadband Digital Care who are has been helpful and is looking into this but I wondered if anyone here would care to comment. Also could anyone explain the difference between Data Rate and Maximum data rate and how the maximum can be less than the Data rate?

Thanks for all your advice and comments so far.

Who ever you spoke to has mis informed you, because for one thing switching off your router/modem for several days even hrs or 10-15 minutes would not normally cause DLM to intervene and lower your IP profile But if you repeated the switching it off and on in quick succession it would cause DLM to think there was an issue,

What is your IP profile? if you don't know go to
Bt performance tester

Run the speed test and then it will give you the option to do further checks, enter you number in the dialog box, and run it will eventually display details of your ip profile and what throughput speeds you achived , if you can copy &paste those results here it may help us help you further ,