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Understanding the SNR profile

NorthPole
Aspiring Pro
Posts: 255
Thanks: 39
Fixes: 6
Registered: 05-07-2013

Understanding the SNR profile

Having recently transferred to Plusnet from an LLU ISP (Be), I am trying to understand the process whereby the SNR profile is set up on the BT network infrastructure, and how fixed its value is after the 10-day 'line training' period has ended. I have read the 'sticky' about this, but am still not entirely clear.
When I was first connected to Plusnet, my downstream connect speed was around 8.1 Mbps, almost identical to what I was getting with Be. IIRC, the SNR value at that time was around 6.5dB. However, I had to reset my router two or three times (I'm using my own) on the first day while sorting out the authentication setup; and unfortunately my BT voice line developed a fault early on during the training period. Immediately after this was rectified (another router reset!), the downstream speed fell to 7.9 Mbps, with the SNR varying between 6.8 and 7.0 dB.
I realise that minor variations like this are quite normal for ADSL connections. My concern is that this activity may have permanently lowered my stable downstream speed by a small amount. Does the BT system begin to lower the SNR value again after a lengthy period of line stability even after 'line training' has ended, thereby raising the speed; or do I have to ask for my profile to be reset, something I have seen mentioned on this Forum?
Just looking for clarification at this stage.
8 REPLIES
Plusnet Alumni (retired) orbrey
Plusnet Alumni (retired)
Posts: 10,540
Registered: 18-07-2007

Re: Understanding the SNR profile

Hi NorthPole,
Quote from: NorthPole
Having recently transferred to Plusnet from an LLU ISP (Be), I am trying to understand the process whereby the SNR profile is set up on the BT network infrastructure, and how fixed its value is after the 10-day 'line training' period has ended. I have read the 'sticky' about this, but am still not entirely clear.

Welcome to the club Smiley We're still learning about it. Basically it'll adjust your line speed (and SNR) according to the drops and/or errors it sees on your line. There are thresholds but I'm afraid we're not totally aware of what they are though.
Quote from: NorthPole
Does the BT system begin to lower the SNR value again after a lengthy period of line stability even after 'line training' has ended, thereby raising the speed; or do I have to ask for my profile to be reset, something I have seen mentioned on this Forum?

Yes, it does begin to lower SNR (in theory) once it sees the line more stable, though as far as we know it needs to record a few resyncs at decent speeds to help this along. Often though it is quicker to request a reset, which fortunately we are able to do on 20 and 21CN lines (but not Fibre as yet).

Hope that helps explain.
Community Veteran
Posts: 4,979
Thanks: 376
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Registered: 10-06-2010

Re: Understanding the SNR profile

The target downstream SNR margin is set in steps of 3, giving 3,6,9,12, and 15 - 6 is the default. To see what the target is, you have to check the router stats immediately after a resync, or have a router that records the margin in the log at the time.
NorthPole
Aspiring Pro
Posts: 255
Thanks: 39
Fixes: 6
Registered: 05-07-2013

Re: Understanding the SNR profile

Thanks for the feedback, chaps. Interestingly, although I am well out of the 10-day line training period, my connection, which seemed to have settled at above 9dB downstream, briefly dropped out earlier this afternoon; and when it reconnected, I was on 6.8dB downstream! So assuming it stays around 6dB (which is what it was when I first switched from Be to Plusnet), it looks like my line has been automatically re-profiled from a target 9dB to a much better 6dB downstream without any intervention.
I'm now wondering what might happen on Monday, when my voice line is switched from BT to Plusnet - does this entail any physical work at the exchange?
Plusnet Alumni (retired) orbrey
Plusnet Alumni (retired)
Posts: 10,540
Registered: 18-07-2007

Re: Understanding the SNR profile

I don't believe so, as I understand it everything continues to work as is except the call data is forwarded to us for billing purposes. I wouldn't expect there to be any physical work needed so your connection should stay as is - certainly hope that is the case.
NorthPole
Aspiring Pro
Posts: 255
Thanks: 39
Fixes: 6
Registered: 05-07-2013

Re: Understanding the SNR profile

Well, something seems to have happened again over the weekend, as my downstream SNR is currently at 9.9 dB, while the upstream is 5.9dB (I did have the upstream speed uncapped during last week) - no disconnections have been made on my part. Does my profile need re-setting? - I'm so used to being on a consistent 6dB downstream with my old ISP.
Plusnet Alumni (retired) orbrey
Plusnet Alumni (retired)
Posts: 10,540
Registered: 18-07-2007

Re: Understanding the SNR profile

Hi there,
Your downstream SNR target is still set to 6dB so it's not that, I suspect things will settle. Not sure why the SNR on your side has jumped though :/
NorthPole
Aspiring Pro
Posts: 255
Thanks: 39
Fixes: 6
Registered: 05-07-2013

Re: Understanding the SNR profile

Matt, thanks for checking that - I'll just have to be patient I suppose, and accept that the BT system seems more volatile than I am used to with LLU ADSL. That said, I'm currently on 10.6 dB downstream - perhaps because it's raining hard just now!
Although I realise Plusnet (like almost all ISP's) doesn't currently support Seamless Rate Adaptation (SRA), it seems to me there might be a case for it, as that is what the BT DLM process is in effect trying to do but in a less immediate way?
Plusnet Alumni (retired) orbrey
Plusnet Alumni (retired)
Posts: 10,540
Registered: 18-07-2007

Re: Understanding the SNR profile

Heh. It'd be great, but it'd also involve (I imagine) a reworking of the software that runs on the various bits of kit across the UK... and possibly the hardware as well. This is all kit that's owned by BT as well rather than ourselves.
I'll happily pass on your request but wouldn't expect to hear anything on it for quite some time, if at all :/ We'll do what we can though.