I thought I would give an update on this as things have changed recently.

To cut a long story short, at the beginning of the year I was told that nothing could be done and I would have to put up with speeds of 12Mb down 0.5 Mb up so I took no more action.

A couple of weeks ago I rang BT just to check when my contract ended so that I could look at alternative suppliers. That really set things moving and I was passed to an extremely helpful lady, who was horrified to see the speed I was getting and said that she would do something about it. A couple of the usual obligatory missed BT engineer appointments then followed, interspersed with one engineer who said there was a fault where the line went underground but someone else would have to fix it. Another engineer then came and said there was no fault but he would try and find a different pair of wires to use. When he did this the speed immediately shot up significantly and after a reset it is now 23mb download (was 12mb) and 2mb (was 500kb) upload.The noise margins are both really steady at 6db whereas before the upstream margin was constantly fluctuating. Speedtest.net gives 21.67 down, 1.98 up.The engineer warned that the speed might drop again during the 10 day settling in period but after a day and a half the speeds are still holding. If the connection is sound and the noise margins are steady, is there any reason why the system should want to reduce the speed now? Full marks to the lady at BT who phoned me regularly with updates and was determined to get something done.

In reply to a post by philflyer:

If the connection is sound and the noise margins are steady, is there any reason why the system should want to reduce the speed now?

DLM will cap your sync speed to raise your noise margin if it sees a high number of disconnections.

The engineer warned that the speed might drop again during the 10 day settling in period

It's a bit sad when even an Openreach engineer believes this .

Re my comment just now to BatBoy, there is no ten-day period on FTTC, despite what BT Wholesale say. Even on ADSL2+ the ten days did not do what they believe, and that was under their control.

On FTTC the line speed is controlled by the Openreach DLM running in the FTTC cabinet, not the BT Wholesale one running in the exchange which merely jumps in and adds the IP Profile at the point where Openreach hand your connection over to them at the exchange.

There is a document SIN 498. Here is the relevant extract:-

2.2 User Network Interface - General

2.2.1 Dynamic Line Management

Dynamic Line Management (DLM) is employed in GEA-FTTC. DLM constantly manages lines to maintain a target link quality (speed and stability). It does this for as long as the product exists.

At provision, the line is put on �wide open� VDSL2 line profiles allowing the upstream and downstream line speeds to run at the upper limit of the product option selected.

On the first day of operation, DLM will intervene if severe instability is detected. Otherwise, DLM will wait until the day after provision before deciding if it must intervene, provided that the line has been trained up for at least 15 minutes during the preceding day.

In essence/practice that means if anything is going to change a lot it will normally be some time during the second night since installation and use.

As with BT Wholesale based ADSL2+ the DLM does run for ever. The most common intervention is to add interleaving if there are bursts of noise. Whereas on ADSL2+ speed wasn't really affected by interleaving, on FTTC it is quite dramatic. The initial intervention normally loses you about 10Mbps and the latency increases by 8ms.

The good news is that if the noise was something unusual such as a thunderstorm, after a fair while it will be automatically removed again. That was very rare, if at all, on ADSLx.

Edited by RobertoS (Wed 22-Oct-14 20:24:34)

Thank-you both for your interesting and informative comments. I have left my PC running continuously with Routerstats. At 2.30 am this morning it reconnected even though the noise margin had remained steady at 6.0. This resulted in a a small speed drop to 22.9 / 1.8. So all appears to be well and I just hope it stays there.

Looks good.

The engineer will have requested a DLM reset when he did the job, so you should be OK now. From your earlier post I gather Wed night was the second since the pair change so this would be DLM altering something. Quite what is hard to know, as that looks like a drop in sync simply due to night-time connection.

Have you got a separate modem, or just the HomeHub?

Yes - Wed night was the second night and so now we have gone through the third night which had no reconnect even though the margin seemed to drop fractionally below 6.0. To answer your question I have the homehub 5A.
So after 11 months of half speed I wonder if BT might offer me any special price to tempt me to stay with them?

They may offer you a tenner off a month, but isn't the lure of BT Sport enough?

I think the lure of a considerably cheaper phone and broadband package from Plusnet is much greater - unless they get Formula 1 on BT sport which is highly unlikely.