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)