Any explanation for this?

Hey all,

So last night I checked my sync and max attainable, to find that my SNR margin had increased to 6.7 from 3.4. However strangely my max attainable is now 80000 from 68000, and upload has remained the same at 13Mbps. However there's been no increase in errors since the modem was rebooted about a week ago.

My sync currently is 67340, but kind of worried about losing this if I resync at this higher SNR margin.

Potentially a cessation of a crosstalker? I know my neighbour is due to move out in a matter of days so this might be the cause, but a SNR and around 13000 in sync increase seems a bit big to me?

Cheers!

What was the change in sync speed? You said the max attainable had changed, but did not say what the sync speeds are/were.

Pedantic to note but 6dB to 3 is a decrease.

In reply to a post by MrSaffron:

What was the change in sync speed? You said the max attainable had changed, but did not say what the sync speeds are/were.

There has been no change in sync speed so far as I haven't rebooted the modem. Currently sitting at 67380 Sync and 80000 attainable. Previous attainable was 68000. I'm just curious as to how an increase in the SNR has increased the attainable so much, I always thought that the closer to 3 it was (as it was previously) was a perfect line. Might be wrong though!

In reply to a post by Zarjaz:

Pedantic to note but 6dB to 3 is a decrease.

Surely if it goes from 3dB to 6dB that's an increase?

Okay so your increase suggests that if the modem was to resync that it might sync at a higher speed.

Most likely a source of noise (such as another VDSL2 line) has gone away and as modems only resync when they cannot maintain the current sync speed it will sit happily synced at the speed.

If it is another VDSL2 line that has turned off (so less crosstalk noise) then if it was to come back you'd probably lose the extra speed you might gain from a resync.

SNR - signal to noise ratio - is measured at the receiver, and says how much stronger the received signal is than the underlying background noise ("noise floor"). This is limited by the amount of transmitted power, the losses on the line, how much noise is picked up on the line (crosstalk, thermal noise etc), and importantly, how fast they transmit. The faster they try to cram data bits into the same spectrum, the harder it is to detect from the noise, and hence the lower SNR.

The modems agree on what speed they will talk at, and the speed they choose will affect the SNR. Conversely, if they have decided to aim for a particular target SNR, they will adjust their speed to meet this.

For a given line, the noise floor means there is a theoretical maximum rate which can be achieved (Shannon limit) - the lower the SNR, and hence the faster you are sending, the closer you are to that limit.

A higher SNR is more "conservative": by choosing a lower data rate for the line, the signal is easier to separate from the noise at the receiver, so more reliable.

With a 6dB SNR target, the modems choose a line speed so the receiver sees the signal as 6dB "louder" than the noise. With a 3dB SNR target, they send more data in the same spectrum so the signal is only 3dB "louder" than the noise - making it harder to decode, and more susceptible to errors. Any additional noise, such as impulse noise (e.g. fridge turning off and on) is more likely to cause data corruption.

Please excuse the very woolly description, but it gives a rough idea

That's the symptom of a crosstalker turning their modem off.

If they ceased their service you would keep the higher sync. You may find it reverts back though.

Edited by j0hn83 (Sun 19-May-19 18:08:59)

In reply to a post by MrSaffron:

Okay so your increase suggests that if the modem was to resync that it might sync at a higher speed.

Most likely a source of noise (such as another VDSL2 line) has gone away and as modems only resync when they cannot maintain the current sync speed it will sit happily synced at the speed.

If it is another VDSL2 line that has turned off (so less crosstalk noise) then if it was to come back you'd probably lose the extra speed you might gain from a resync.

I've been tempted to reboot to see - but have a fear of DLM kicking in if it gets unstable.
A while back I was seeing a 77mbps sync at a 3dB margin which was pretty impressive, so the elimination of a crosstalker I think might be a decent reason for it.

In reply to a post by candlerb:

SNR - signal to noise ratio - is measured at the receiver, and says how much stronger the received signal is than the underlying background noise ("noise floor"). This is limited by the amount of transmitted power, the losses on the line, how much noise is picked up on the line (crosstalk, thermal noise etc), and importantly, how fast they transmit. The faster they try to cram data bits into the same spectrum, the harder it is to detect from the noise, and hence the lower SNR.

The modems agree on what speed they will talk at, and the speed they choose will affect the SNR. Conversely, if they have decided to aim for a particular target SNR, they will adjust their speed to meet this.

For a given line, the noise floor means there is a theoretical maximum rate which can be achieved (Shannon limit) - the lower the SNR, and hence the faster you are sending, the closer you are to that limit.

A higher SNR is more "conservative": by choosing a lower data rate for the line, the signal is easier to separate from the noise at the receiver, so more reliable.

With a 6dB SNR target, the modems choose a line speed so the receiver sees the signal as 6dB "louder" than the noise. With a 3dB SNR target, they send more data in the same spectrum so the signal is only 3dB "louder" than the noise - making it harder to decode, and more susceptible to errors. Any additional noise, such as impulse noise (e.g. fridge turning off and on) is more likely to cause data corruption.

Please excuse the very woolly description, but it gives a rough idea

Thank you, this was really helpful! At the moment i'm seeing a 6.7dB.

In reply to a post by j0hn83:

That's the symptom of a crosstalker turning their modem off.

If they ceased their service you would keep the higher sync. You may find it reverts back though.

I think this could be my immediate neighbour - which would make sense. Would crosstalk be eliminated even if someone was on ADSL instead of FTTC?

You describe fairly well the way sync speed is set to attain a target SNRM. However the SNRM then varies according to the way the noise level changes.

A huge increase in noise can drop the margin to an unacceptable level, causing a re-sync at a lower speed to reset back to the target. However, a decrease in noise levels will cause the margin to rise as has happened to the OP. This does not cause a re-sync. Merely a rise in Attainable.

More detail here. Although the page could do with some updates, the principles have not changed.

My understanding was that at 3dB that it was basically the clearest line possible, but what i'm getting by your suggestion (feel free to correct me if i'm wrong!) is that if the line is virtually clear it doesn't need to be at 3dB in order to maintain a stable line, so has switched to 6dB thus increasing max attainable and potentially sync?

EDIT:

Okay, so I bit the bullet and forced a resync. Huge increase in sync!

Before: https://ibb.co/9wQYy0d
After: https://ibb.co/C8f7gHR

Not too shabby considering i'm on a 800M line, but i'm certainly not expecting it to last forever!

Edited by Nitro93 (Sun 19-May-19 19:40:37)

Great re the higher speed, particularly at 800m. I was getting around 70Mbps sync on 3dB, and mine was a pretty clean line.

Re your "understanding", you start right but then get confused. It hadn't switched to 6dB. The noise had fallen so the margin rose. If the noise creator had just turned their modem (or modem/router) off and gone on holiday that would cause it, but when they came back and turned it on again it would revert to roughly what you had before.

On FTTC the sync-time margin is set by DLM and applied at the DSLAM. Your modem basically accepts that then they negotiate the speed that can be maintained. The noise varies continuously however, so the margin your modem reports will fluctuate in the opposite direction to the noise change. The Attainable is just the modem telling you that "Hey - we could go faster, (or slower), than we are doing because the noise level has changed".

The theory is that you lose connection when the margin hits zero, and get a re-sync. In practice it depends on the quality and condition of the actual hardware. Some can lose it at 1dB, others can stay connected on -1dB or even -2dB.

Error rates are likely to be high however when close to losing connection, and correcting those or retransmitting the packets can reduce the actual throughput to well below what you would get with a re-sync at a lower speed.

The base point is only ever set at the time the connection/sync is made. (On Openreach FTTC and BT Wholesale ADSLx). The whole point of the margin, as explained in the page I linked to, is to stop you losing connection when the noise goes up.

ADSL is a lot lower frequencies than VDSL2 so yes dropping to ADSL would remove a crosstalk source

Minor point: the SNR margin isn't how far above the noise floor the signal is, it's how far above the level needed to provide that data rate the signal is.

A tone set to deliver 4096 QAM, 12 bits per symbol, needs better SNR than a tone delivering 256 QAM, 8 bits per symbol. Margin is the level above the needed SNR to deliver almost zero bit error rate for the negotiated modulation order the actual SNR is at.

EDIT: As a rough guideline 3dB more SNR = 1 more bit per symbol. 64 QAM / 6 bits per symbol needs about 28dB, 128 QAM / 7 bits per symbol 31dB, etc. These numbers are without coding gain, which means error correction, so FEC can and does make a difference, as does G.inp.

Edited by deleted (Sun 19-May-19 23:44:05)

SNR margin is the ratio between the current signal level, and the signal level at which it would just become impossible to distinguish signal from noise.

Thus, the higher the SNRM, the clearer the line is; this is made a bit more complicated as other posters have said because higher sync speeds mean that the signal is harder to distinguish from noise.

So, assuming you hold sync speed constant, higher SNRM is better - a line with a 9dB SNRM and a sync of 80,000 kbps is "clearer" than a line with a 3dB SNRM and same sync speed. If you hold SNRM constant, then clearer lines have higher sync speeds - a line that gets 80,000 kbps sync at 3dB SNRM is clearer than a line that gets 68,000 kbps sync at 3dB SNRM.

In reply to a post by farnz:

SNR margin is the ratio between the current signal level, and the signal level at which it would just become impossible to distinguish signal from noise.

Thus, the higher the SNRM, the clearer the line is; this is made a bit more complicated as other posters have said because higher sync speeds mean that the signal is harder to distinguish from noise.

So, assuming you hold sync speed constant, higher SNRM is better - a line with a 9dB SNRM and a sync of 80,000 kbps is "clearer" than a line with a 3dB SNRM and same sync speed. If you hold SNRM constant, then clearer lines have higher sync speeds -

Only for any particular line and attenuation. Attenuation is the prime variable in most cases. What you say does not apply in the general case.

For instance, "a line that gets 80,000 kbps sync at 3dB SNRM is clearer than a line that gets 68,000 kbps sync at 3dB SNRM" is not a valid statement. Unless more information is available the assumption would be the 80Mbps one is shorter.

Edit: SNRM is not a ratio anyway. SNR is. SNRM is the arithmetic difference between the current SNR and the SNR at which connection would probably be lost. A little difficult to explain simply, given the fact that SNR is a logarithmic scale.

Edited by RobertoS (Mon 20-May-19 16:10:30)