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Target Downstream SNR

FIXED
Greenbandit
Dabbler
Posts: 11
Fixes: 1
Registered: 14-10-2016

Target Downstream SNR

My connection is stable with the downstream sync rate being 18mbps but the downstream SNR is 9.3db. It is always 9.3db, even before I changed my modem from an excellent (but 2.4GHz only) Billion 8800NL to a Netgear X4S D7800.

I have no noise on the line, no dropouts, the modem is always left switched on, and my SamKnows white box gives readings that are consistent but somethimes lower than expected by the BT Broadband Availability Checker for "VDSL Range A clean".

Is it correct that the target downstrean SNR should be 9.3db and not the 6db that seems to be fairly common?

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27 REPLIES
jafreer
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Re: Target Downstream SNR

The target SNRM (signal to noise ratio margin - not the same as SNR) is 6dB (although they are trialling 3dB right now).

I suspect that DLM has set the SNRM on your line to 9dB to try and stabilise it. What has prompted this is hard to say. It is usually number of errors on the line or disconnections (or a combination).

You say you have no noise on the line, no dropouts, and the modem is always on. In that case, I suspect errors on the line. If you can get some kind of monitoring program (e.g. routerstats or DSL stats), you may be able to monitor errors.

The problem is, the stability you currently have is probably because of the 9dB SNRM. If it was lower, your speed may be higher, but the stability may not be there. DLM has deemed 9dB the best SNRM for your line.

Only other possibility is that the SNRM could be stuck. I don't know much about this or how likely it is, but you would have to take that up with PN. I reckon errors on the line are more likely than a stuck SNRM.

Community Veteran
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Registered: 10-06-2010

Re: Target Downstream SNR

There is no target SNRM higher than 6dB on FTTC. Perhaps the line is banded instead, where the FTTC DLM restricts the speed instead of changing the target SNRM.

jwsg
Grafter
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Registered: 16-08-2013

Re: Target Downstream SNR

Stability is achieved with SNR, a 9dB margin (like I have ended up with now instead of the previous 6dB - and wanting to get rid) may be the consequence of lowering the sync instead of an explicit target SNR but the result is pretty much the same.

Community Veteran
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Re: Target Downstream SNR

Well yes, but like many cases, these things would be much clearer if people would post their full set of DSL stats rather than commenting on individual parts. The banded speed would be readily apparent if it always connects at the same speed. The banding is also shown in the profile information provided by the FTTC diagnostic tests Plusnet can run.

jafreer
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Re: Target Downstream SNR

There is a downstream band that maxes out at 18 Mbps (9 - 18 Mbps) so if the line is banded and the SNRM is up around 9dB, I would expect the connection to max out right at 18 Mbps. So if you can tell us the precise connection speed, that may at least confirm your line is banded.

Only other thing I can think of is if the last time you connected to the cabinet, the SNRM was set at the default 6dB, and between then and now, something dramatically improved with the line that made your SNRM jump to around 9 dB. I think that scenario is pretty unlikely, but if I monitor SNRM and when it is low, reconnect, resetting the SNRM to 6dB, my SNRM can go up to about 7 or 8 as the line conditions change. And of course if that were the case, reconnecting should restore you to 6dB, along with a corresponding speed increase. So yes, does sound like banding.

Greenbandit
Dabbler
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Registered: 14-10-2016

Re: Target Downstream SNR

The D7800 has constantly reported a DSL link connection speed of exactly 18.00Mbps with the SNR varying from 9.20db to 9.40db, so given what jafreer says, it is 100% consistent with banding.

I think I need to find out why my line is banded (i.e. is being capped). As far as I am aware it should not be capped as I pay for a higher rate that 18Mbps!

jafreer
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Re: Target Downstream SNR

It is being capped by DLM. DLM monitors your line and it seems it is detecting conditions (errors, connection drops, glitches, SNR jumps - or any combination of these) and sees fit to banding the line to stabilise it.

So it is not a case of Plusnet capping you and doesn't have anything to do with the fact you pay for a service with a higher rate than 18 Mbps.

So the question now is what are the line conditions that are causing banding. If you can monitor your connection for drops, monitor it for SNRM fluctuations, and it would also be nice to know what interleaving depth you have.

I know it doesn't sound easy, but it may be a route you have to go. I had issues with my connection a while back and ended up buying an OpenReach HG612 modem off ebay for a tenner. I put custom firmware on it which allowed me to monitor in depth line stats (using a program called DSL Stats). I could see I had a very high interleaving depth so monitored for errors and through trial and error (turning things on and off to try and isolate the noise), I got to the bottom of the issue.

The main thing you want to find out is whether the issue is your side of the master socket or the other side.

Greenbandit
Dabbler
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Registered: 14-10-2016

Re: Target Downstream SNR

I do not believe that there is a technical problem. I ran Routerstats for about  9 months when my Billion 8800NL was in use but it does not seem to work with the Netgear X4S D7800. No connection drops, very few errors, no history of glitches & certainly no SNR jumps were ever seen, and the line is quite with no crackles.

Previously over a period of about 11 years while using dial up then using ADSL2 I had an intermittent noisy phone line which caused all the errors that you would expect (as logged by Routerstats, and the line was then subject to ADSL banding. In spite of BT claiming that there were no problems during about 5 engineer visits with the phone line the problems dissapeared one day at about the time the area was being upgraded for FTTC & VDSL None of these problems have ever been heard or seen on my line since several months before it was switched over to VDSL2. And in January before the Billion 8800NL was removed, and when the X4S D7800 was connected, the sync rate was 18.00Mbps with an SNR of about 9.2 to 9.4db.

While the status page of the X4S D7800 is certainly not as good as the Routerstats logs, it gives no indication of dropouts, step SNR changes, sync rate changes or errors being a problem.

I think I need to investigate the banding side of things when I have time to phone Plusnet.

jafreer
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Re: Target Downstream SNR

You say the status page of the X4S D7800 gives no indication of errors being a problem. What information does it give regarding errors?

The things we know for sure...

1) Your line is banded

2) DLM implements banding

3) This means the issue is between your modem and the cabinet

This could be a REIN issue, or some source of interference in your home (e.g. switching power supply), or it could be external to your house. These things are never easy to resolve!

Greenbandit
Dabbler
Posts: 11
Fixes: 1
Registered: 14-10-2016

Re: Target Downstream SNR

 

Back in the old ADSL days I had REIN issues for 4.5 years (next door's washine machine in their garage about 7m from my master socket), & over 11 years I had a line that crackled for a few days after heary rain. Both of these issues were logged by Routerstats when I ran it, but currently I can't run Routerstats or Routerstats Lite because they don't have an X4S D7800 option.

Plusnet online chat confirmed the presence of banding and recommended that I raise a ticket (which I have done) to see if the banding can be removed, I'll see where that goes.

In the meantime I need to get Routerstats (or something else that has yet to be found) working with my D7800.

Community Gaffer
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Re: Target Downstream SNR

I'm just getting the line tested and seeing what needs doing on the fault.

 

 
Test Outcome Pass
Test Outcome Code GTC_FTTC_SERVICE_0001
Description GEA service test completed and no fault found but unable to check for customer equipment connected to modem.
Main Fault Location OK
Sync Status In Sync
Downstream Speed 18.0 Mbps
Upstream Speed 3.1 Mbps
Appointment Required N
Fault Report Advised N
NTE Power Status PowerOn
Voice Line Test Result Voice fault suspected - contact Voice CP to progress copper issue
Bridge Tap Not Detected
Radio Frequency Ingress Not Detected
Repetitive Electrical Impulse Noise Not Detected
Cross Talk Not Detected
Estimated Line Length In Metres 1356.3
Upstream Rate Assessment Reasonable
Downstream Rate Assessment Reasonable
Interference Pattern Not Detected
Service Impact No Impact Observed
Home Wiring Problem Not Detected
Downstream Policing Discard Rate 0.0
Customer Traffic Level Upstream and Downstream Traffic Detected
Profile Name 0.128M-18M Downstream, Error Protection Off - 0.128M-3.2M Upstream, Error Protection Off
Time Stamp 2017-02-01T15:45:00

 

As you can see the line profile is at 18Mb/s and there's a potential fault on the voice line. You'd need to contact your phone provider and ask them to test the line for a fault, it's best to not mention broadband when doing this. Let us know what they come back with, as otherwise the next step from our side is to arrange an engineer but if there's a phone fault it's best to get that resolved first.

If this post resolved your issue please click the 'This fixed my problem' button
 Chris Parr
 Plusnet Staff
Greenbandit
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Registered: 14-10-2016

Re: Target Downstream SNR

Many thanks to the people who have tried to help so far - it is appreciated, especially since the broadband is actually working albeit slower than it used to.

As recommended in this thread I raised a ticket (143895092) & the latest reply back is:-

"As per our conversation,

The fibre connection is within limits and is where we would expect it to be.
The profile does show banded however this is not service impacting as the line is already within expected speeds.

If you have had a higher speed on fibre previously with another provider at this address then it may still be worth opting for an engineer however If this is not the case then I would suggest there is little value in progressing this as a fault."

I accept that the line is within expected speeds but the support agent confirmed in the same phone call that the sync speed used to be 22.6Mbps to 24.4Mbps!!! I really do not believe that there is a fault in the wiring in my house. It is possible to see where the wire comes in to the house and where it goes to the master socket - it hasn't changed since the last time (pre-FTTC) that an engineer came round and didn't find a fault (only because the engineer was sympathetic did I avoid the call out fee that time!).

I understand that the target SNRM cannot be changed by Plusnet, and that VDSL modems are not allowed to have a setting that change the SNRM, but other than there being a fault in the copper line that causes the SNRM to dip (which is not happening) I do not yet see the reason why the SNRM needs to be as high as 9db since the REIN stopped being a problem (next door's washing machine that caused me to lose the 22+Mbps sync speeds).

I have reported a line fault to BT (fault = poor quality) but as expected the automated line checker did not find a problem and it cannot be progressed further without the risk of a £130 fee.

Question:- I read somewhere ( http://www.robertos.me.uk/html/openreach_fttc_dlm.html ) that "to remove banding after a fault fix the engineer has to request that from a Control Centre". Given the evidence that a fault (REIN) that used to be there has now gone, is there a mechanism other than to call out an engineer that can reset the DLM & initiate another 10 day line training period? If anyone can tell me exactly what to ask for I really would appreciate it.

If the result is that it settles down to SNRM=9dB that would be good evidence that there is a fault in the line & I would have to risk a call out fee to progress this.

jafreer
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Re: Target Downstream SNR

I read a lot of forum articles where people are stuck with banded profiles and DLM simply won't remove them (even after faults were fixed). DLM will act (often slowly) to change line conditions where errors are removed etc, but it seems with banded profiles it just gets stuck (which is crazy).

If you can get your copper fault fixed first, then you will probably have to try and convince someone to do a DLM reset. Shameful state of affairs really, and it is all because of DLMs inability to remove banding once whatever was causing it is removed.

brookheather
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Re: Target Downstream SNR


Greenbandit wrote:
I understand that the target SNRM cannot be changed by Plusnet, and that VDSL modems are not allowed to have a setting that change the SNRM

Many third-party routers have settings to allow you to change the SNRM - my ASUS DSL-AC68U allows you to go down to 2db with the latest firmware.