Line Stats - After Fault

Hi, so recently had a major line fault and openreach had to change lines, can anyone tell me if this line is ok? noise margin seems very low. Openreach tell me its a perfect line and on the fastest port in the cab, not sure i believe the last part. i once had the full speed, but over the years it dropped until it finally gave up. DLM has gone past the 10 days now. Started of as 66mbps

Broadband Link Downstream Upstream
Connection Speed 77184 kbps 18726 kbps
Line Attenuation
D1(12.7 dB) , D2(32.1 dB) , D3(50.1 dB)
U0(4.6 dB) , U1(25.0 dB) , U2(39.2 dB)

Noise Margin 1.9 dB 6.0 dB

Probably set a 3db target because the connection is very good and stable. It's dropped a little since the last sync as it often does due to changes in the noise environment.
All seems good.

Is there a reason their is a massive difference between D0 and D1 SNR and same with U0 and U1?

On my line all 3 are roughly the same give or take 1db

In reply to a post by bobble_bob:

Is there a reason their is a massive difference between D0 and D1 SNR and same with U0 and U1?

On my line all 3 are roughly the same give or take 1db

Per band stats can be meaningless in many cases. What's shown above by the OP is attenuation per band and not SNR per band.

Different bands run at different frequencies. Higher frequencies (higher bands) attenuate more and so the signal is weaker for these on longer lines.

Many lines won't use D3 or U2 at all while others will. These can vary massively between lines.
It also depends on what package you are on as the SNR will likely be higher for anyone only on the 40/10 package vs anyone on 80/20.

SNR per band is not a good indicator of an overall lines performance and isn't a good stat to compare between lines.
In fact it's a stat that I deliberately overlook apart from occasionally looking to see how D3 performs on some mid to long lines.

Attenuation per band (What's shown above) varies massively between modems and is basically meaningless.

Edited by j0hn83 (Sat 02-Sep-23 17:56:30)

Ah yea sorry didn't notice it was Attenuation not SNR

In reply to a post by deathy2010:

Noise margin seems very low.
Noise Margin 1.9 dB 6.0 dB

A higher noise margin generally increases stability, but it isn't necessarily a good sign.

Other telephone lines bundled up alongside your own, other utilities (e.g. power lines close to your line), and/or domestic / commercial electrical appliances can all generate a level of interference. This interference can be observed in the line statistics, as it will cause the noise margin to vary. In extreme cases, interference that is substantial, may cause the noise margin to drop say 10dB. In my experience, very bad interference occurs at random intervals - but it can be more constant also. In this case, with your current noise margin of 1.9dB, the interference at 10dB would cause the noise margin to go negative (1.9 - 10 = -8.1dB). As this isn't possible (the noise margin cannot go negative), what actually would occur is a drop of the connection.

To combat this problem, where interference is present and causing drops, DLM increases the noise margin. If the interference is around 10dB, DLM will probably need to set the margin to 12dB or even 15dB. This means that whenever the interference presents itself, the noise margin will suddenly drop from say 15dB to 5dB. The users line remains connected, without a drop.

Where interference is not present substantially, the noise margin will vary small amounts. Typically, this is +-1dB. In this case, DLM can set the noise margin to 3dB, and the line will not drop, as there is not more than 1dB of interference, so the noise margin may reduce down to approx 2dB, worst case (yours being 1.9dB).

The positive of this, is it provides faster speeds. Hence, the noise margin you have is the lowest it can be, but this is not a problem as such. It simply indicates, the line is not subject to lots of interference and hence is stable even with the low noise margin. You get better speeds, you don't require "interleaving" or it's set very low (which makes pings higher - impacts gaming / responsiveness), and even with the lower noise margin, most of the time it's actually more reliable than a line that's subject to lots of random interference and has the noise margin set high. Largely, as bad interference can vary dramatically, and still kick out a line with a high noise margin if the interference is substantial enough.

Hope this helps.