Are These Figures To Be Expected and Noise Margins

[Paul608085]

Historically I had very poor broadband service, until about a year ago Openreach agreed to move us to a much closer cabinet with a brand new dedicated cable, (long story, related elsewhere!). Because of the ongoing issues I kept a daily spreadsheet of my hub statistics, and for interest still do now.

We are now almost exactly 50 metres from the fibre cabinet and the copper cable runs underground in a straight line from my house to it. There is just one other customer on this cable - they are also with BT Broadband.

It is the Maximum Data Rates and Noise Margins that I am asking about:

When we were first cab shifted we were the only ones on the new cable and I was recording Max Data figures of around 38 / 104Mbps and noise margins of 17.8 / 11.6dB. On the day that my neighbour was also shifted to the new cab I saw these figures drop to about 31 / 87Mbps and 13.5 / 7.3dB noise margin, (I know about crosstalk in cables and expected this).

Over the past year, I have observed a very steady, but noticeable, decline in these figures. Today I am seeing Max Data of 29 / 86 and noise margin of 12.0 / 6.8:

My line is clearly very stable - it has been up for almost 34 days now - and I know that the 20 / 80 Mbps sync is the maximum that the product allows. What has intrigued me though is seeing reports on these forums of people a couple of hundred metres from their cabinet who are still syncing at the full 20/80, so to me it looks as if my Max Data Rate and  Noise Margin should be a bit higher at only 50 metres from my cab?

Also, at what noise margin does the speed start to be reduced to maintain stability? I have seen 6dB mentioned as the norm, but also seen reference to 3dB?

I know that there is not currently a problem with my line - these questions are purely for interest  😀

[licquorice]

The initial drop was, as you say, probably due to crosstalk. The subsequent very slight drop is possibly purely due to temperature fluctuations which will affect the impedance of the cable. As far as speeds at a given distance is concerned, bear in mind the gauge of the cable will also be a key factor. Your cable may be a very thin gauge cable compared to others where a higher rate is seen at a further distance. Target Snrm has historically been set at 6dB, but Openreach have realised that this is rather conservative and are now setting the target at 3dB. Not all cabinets are able to set this target however, and remain at 6dB target. 

As your speed is above the maximum threshold, your Snrm is effectively irrelevant as it is as low as possible to give a speed of 80M

[pompey1898]

Hi @Paul608085 

Table of FTTC speed against distance from the street cabinet
Distance from the street cabinet (km)	FTTC fibre broadband connection speed (Mbps)
0	80
0.1	80
0.2	80
0.3	76
0.4	69
0.5	60
0.6	51
0.7	43
0.8	35
0.9	31
1	28
1.1	26
1.2	24.5
1.3	23
1.4	22
1.5	21
1.6	19.8
1.7	19
1.8	18
1.9	17
2	16.5
2.1	15.5
2.2	14.8
2.3	14
2.4	13.3
2.5	12.5
2.6	11.7
2.7	10.8
2.8	10.2
2.9	9.4
3	8.7

[Paul608085]

@licquorice  @pompey1898 

Thanks both for your replies.

I am aware that because I am syncing at the maximum and the line is very stable there is currently no issue. As I said, these questions were posed purely out of interest because I am recording a decline, (albeit very slow), in the noise margins and was wondering what would happen if it drops below 6.0dB.

Is there any way of telling if my cabinet is set to 6 or 3dB noise? I am on Bradwell Abbey exchange (Milton Keynes), Cab 50.

[licquorice]

That is an Huawei cabinet so should be set for 3dB margin. Hence your speed should remain at 80M until Snrm drops below 3dB

[Paul608085]

@licquorice 

Great! - Thanks for checking.

A long way to go until it gets to 3dB, (I hope!).