Slowly dropping noise margin

I upgraded from ADSL to FTTC three weeks ago, on BT's Infinity 2 80/20 service, and was provided with a HH5 and no Openreach modem. I'm about 125m away from the cabinet, and the hub is reporting the maximum data rate as being well above the data rate I'm configured to receive.

The strange thing is that over the three week period, the noise margin (and so the maximum data rate) has been slowly dropping. The downstream margin started at 17.6 on the day of go-live (max. data rate 121370), and drops by about 0.2 a day -- no major drops, but little drops day by day.

24 Feb - 17.6 (max attainable 112613)
27 Feb - 15.1
03 Mar - 15.0
06 Mar - 14.4
08 Mar - 13.9
13 Mar - 12.9 (max attainable 105583)

It seems strange that the noise margin is dropping over time - is this normal? I'd think the noise of the line would stay fairly consistent over time.

Were you one of the first on the cabinet? As connections are added, crosstalk between lines can come into play. Sometimes dramatically.

Crosstalk is one reason... The cabinet may have reduced the transmit power as not needed to maintain your 80/20Mbps sync therefore the margin will be less but the sync is the same so doesn't matter.

As others have pointed out, if you are a relatively early adopter then you will, over time, see a drop-off in the noise margin. That's due to increased cross-talk. In your case, it's nothing to worry about as, at 175m from the cabinet, you'll almost certainly be able to achieve up to 80Mbps, even if all the lines are enabled. To give you an idea, the normal target minimum noise margin is 6db, and you are a long way above that. Indeed there may even be more so spare as the line card power output might be scaled back on shorter lines. If that can be boosted, there may be more potential.

For those, like me, who are further from the cabinet (700m line length), this also happens, but when it does it has a real affect in that it reduces the line sync speed (I went from 57mbps to 50mbps at 6db noise margin).

In the medium/long term, this situation may improve as there are trials going on with something called "vectoring". This involves a sort of co-ordination across multiple lines with the aim of cancelling out much of the cross-talk. If this works (and can be retrofitted), then it will improve things.

nb. ADSL was subject to the same effect and, eventually, the situation stabilises once a large proportion of lines are enabled.