Retraining Line Drops

If a retrain occurs on a line, say because the SNR Margin drops below target, I'm presuming the line would drop??

I believe my Target SNR has been raised to 15db. I'm currently sync'ing at 8128 with a Margin of 15db, but the line does occasionaly drop when it gets to around 12db and I then resync at 7628 or 7313kbps.

Interleaving is on (Damn thunderstorms during training period) and I see <500 FEC errors on the line.

Anyone want to take a stab at whats happening here please?

Thanks

If interleaving is on and you are on a Max connection you shouldn't be getting an 8128Kbps sync, but rather 7616Kbps.

If a target of 15dB is set, the line does not retrain if it drops below 15, but the modem should hold the line until it cannot hold it anymore, i.e. somewhere between 0 and 6dB.

The higher target margin gives you more leeway

Andrew

I've seen quite a few >7616 syncs with interleaving on, one was even the full 8128. My own router will go over 7616 occasionally and its interleaved. It may be down to particular chipsets or combinations of DSLAM and modem chipset, but it does happen.

Okay - always those exceptions.

Andrew

yep, I didn't believe them at first either

Ok - given what you have just said then....what would cause the line to drop AND resync at 7618, when it's happily running at 8128 with 7db Attn (down) and an SNR margin that never drops below 10db (hovers between 15db and 12db normally - i have a perl script which checks these vital's every 5 secs).

Interleaving is definitely on.

Any other clues to the cause of this behaviour?

Thanks

If there is a sharp drop in the noise margin, the modem may resync without you seeing the actual figures change, and 5 seconds is a long time in terms of impulse noise

Andrew

whats impulse noise then? its something that only lasts for very short time?

Some may last for a very short time, e.g. noise from the sparks trains can make, through to the effect of AM radio on untwisted pairs

Andrew

ok so it doesnt have to last for short time then?

I am still trying to decide what I should be calling whats going on with my line, currently the SFI have assigned a Electro-magnetic compatability consultant to look at my line with no appointment set yet and so far I have been defining it as noise bursts since a&a have been calling it that, the behaviour has slightly changed, before the noise bursts were coming and going at an instant now they seem to be variating if I keep refreshing the stats I see it hovering all over the place during a burst, with normal snr been about 13-14 and during a noise burst it bottoms out around 4 but hovering between 4 and 7 moving up and down. The length it lasts for recently has been shorter only usually minutes rather then hours but they can last for hours.