Maximum Rate verus SNR

Last February my FTTC is maximum rate of 113136k / 38700 with snr of 15.1 / 23.2
Current now my FTTC is maximum rate of 93260k / 33562k with snr of 9.7 / 15.0

However sync rate is not affect at the moment still getting 79999k / 19999k

Question are what is the maximum rate if the downstream snr went down to around 6? Will snr carry on reduced down around 3 or will stop at 6?

P.S. I guess could be around 82000k with snr of 6 something like that.

max.

the snr is an actual figure.

the attainable is an estimated figure, usually based on the excessive snr (if any).

The estimate is based on a target margin of 6db and no FEC overheads.

so if your snrm drops to 6db, the attainable will drop to around the same as your sync speed which is 79999

Your snrm can go below 6 in which case your attainable will be lower. Meaning if you then resync in that situation the new sync will be a lower speed.

Edited by Chrysalis (Thu 09-Oct-14 22:38:16)

What those stats show is not the SNR, but the SNR margin. It must be one of the most common misunderstandings posted to boards.

The actual SNR on your line (which is not being reported in the stats) is what ultimately defines how fast your line can pass data. However, if there was any attempt to actually run at that rate, the line would constantly lose sync due to variations in the electromagnetic noise in the environment. So what operators do is set a margin of safety, which can absorb some changes in the noise environment without losing sync. Typically this is set to a minimum 6db, but if a line is unstable it might be set to 9db or even 12db. (And some ADSL operators would even use 3db on very stable lines). If you see a high margin on a line achieving maximum, or near maximum speeds, that's good as it means there's an even larger margin before sync drops and is a characteristic of a good quality, short line.

What you are seeing is that margin is being eroded, very probably due to more cross-talk as market penetration increases. If the margin drops towards 6db, you will start seeing a loss of sync speed. You've clearly got a lot to spare on upstream, with less on downstream. Don't expect to see the margin drop below 6db as Openreach do not (as far as I know) allow modems to negotiate anything less.

The good news is that the rate of erosion in performance seems to drop off so there's a fair chance that you won't get hit. Secondly, should vectoring be enabled, then expect to see those margins increase a lot.

In reply to a post by TheEulerID:

Don't expect to see the margin drop below 6db as Openreach do not (as far as I know) allow modems to negotiate anything less.

The modem may not negotiate less, but it will quite happily drop well below 6dB and continue at the higher sync rate for some time (possibly weeks) until such time as a resync is forced, either by DLM or the modem being switched off.

Ok - it might drop a little below 6db once negotiated, but all the experience I've had is if it fluctuates significantly below that figure, you get lots of drop-out re-syncs. For instance, I haven't seen mine go below 5.8db. I tend to find VDSL is much more prone to re-syncs than ADSL. I used to maintain sync with ADSL for months. The longest I've had with VDSL is 6 days.

Not my experience at all, I remained below 3dB for several weeks and have seen others below 1dB. I have also maintained sync for weeks on end, unlike on ADSL.

Ok, if the FTTC downstream snr fall below 3dB and become unstable, then will the DLM will try to push snr up until it remain stable or does DLM doesn't care how lower the snr can go below 3dB and surely the ISP can ask BT Openreach to reset DLM if that happen.

In reply to a post by adslmax:

Ok, if the FTTC downstream snr fall below 3dB and become unstable, then will the DLM will try to push snr up until it remain stable or does DLM doesn't care how lower the snr can go below 3dB and surely the ISP can ask BT Openreach to reset DLM if that happen.

I suggest you re-read this post from earlier in this thread

Eventually the modem will resync lower and SNR margin will go back to around 6dB, whether that be manual or through DLM. Resetting the DLM doesn't enter into it.