cancel
Showing results for 
Search instead for 
Did you mean: 

BT Business Hub Strange SNR behaviour

wisty
Aspiring Pro
Posts: 434
Thanks: 53
Fixes: 3
Registered: 30-07-2007

BT Business Hub Strange SNR behaviour

I am using a BT Business Hub 5A on PlusNet. Since I started using it at the beginning of July SNR has hovered between 7.7 & 8.0. I am on the 40/2 package so the speed is governed by that rather than the SNR.

Over the weeks since, the router has disconnected a few times of its own volition and reconnected. After the disconnections, the router usually reconnects with a sync speed of either 38170 or 39950, although download speed tests don't show any significant difference between the two.

I have always assumed this was DLM trying to optimise the line in the face of some sort of external noise burst.

However this morning at 05:16 the router was running at 39950 (7.9 SNR) when it disconnected for  3 minutes and reconnected - still at 39950 but now with a SNR of 16.5-16.7 (see attached plot from Routerstats). The Hub 5 log shows nothing significant beyond the disconnection and restart.

I am inclined to think that it's a "normal" DLM reset and the Hub has got something crossed up - a manual restart would probably get me back to 7.5 or 7.6 SNR. But I just thought I would ask if anyone else has seen similar behaviour. Have BT rolled out something new? Could DLM switching on/off interleaving cause that sort of change in SNR?

The HUB 5A doesn't give much information,. Could someone from Plusnet have a look at my line and see if anything obvious has changed?

Before this morning the Max data rate reported by the router was 44K-45K, now it is saying 66K - so that would make it worth upgrading to the 70/20 product.

 

 

 

 

 

 

12 REPLIES
Browni
Seasoned Champion
Posts: 1,521
Thanks: 430
Fixes: 35
Registered: 02-03-2016

Re: BT Business Hub Strange SNR behaviour

What are the maximum attainable speeds before and after?

Missed them at the end, perhaps G.INP has been applied to the line?

I must have been really bad in a previous life. This is my 3rd ISP in a row that uses lithium.
Community Veteran
Posts: 4,918
Thanks: 341
Fixes: 16
Registered: 10-06-2010

Re: BT Business Hub Strange SNR behaviour

Not sure if G.INP could account for such a large difference.

Possibly the cabinet has had been enabled for vectoring, which cancels out all the crosstalk noise from other lines.

wisty
Aspiring Pro
Posts: 434
Thanks: 53
Fixes: 3
Registered: 30-07-2007

Re: BT Business Hub Strange SNR behaviour

I guess that vectoring implementation could be a possibility. When I first got fibre ( one of the first in the village) I saw a SNR of 10+. It dropped over the last two years to 7.7-7.8 which I presume was a consequence of crosstalk. If vectoring eliminates that cross talk and the crosstalk from ADSL lines, then that might explain the jump.

Any idea how I can find out if they have implemented vectoring on our cabinet?

At 66M down  I would definitely be up for a switch to the 80/20 product.

Community Veteran
Posts: 4,918
Thanks: 341
Fixes: 16
Registered: 10-06-2010

Re: BT Business Hub Strange SNR behaviour

I think it's not possible to check the vectoring status with a BT Business Hub 5. You'd need to use a modem which provides the information.

Vectoring only cancels the crosstalk from the other (also vectored) VDSL2 lines, it can't do anything about interference from ADSL lines, but ADSL lines would only affect a small part of the VDSL2 frequency range, so they have a relatively minor impact.

Another possibility is that the high SNRM is only temporary, and due to one or more other VDSL2 lines switching off their modems or having some fault which has temporarily disconnected their service.

Browni
Seasoned Champion
Posts: 1,521
Thanks: 430
Fixes: 35
Registered: 02-03-2016

Re: BT Business Hub Strange SNR behaviour

 Which one?


ejs wrote:

Not sure if G.INP could account for such a large difference.

Possibly the cabinet has had been enabled for vectoring, which cancels out all the crosstalk noise from other lines.



ejs wrote:

Vectoring only cancels the crosstalk from the other (also vectored) VDSL2 lines, it can't do anything about interference from ADSL lines, but ADSL lines would only affect a small part of the VDSL2 frequency range, so they have a relatively minor impact.

I must have been really bad in a previous life. This is my 3rd ISP in a row that uses lithium.
Community Veteran
Posts: 4,918
Thanks: 341
Fixes: 16
Registered: 10-06-2010

Re: BT Business Hub Strange SNR behaviour

Which one what? Is right? Is wrong? Isn't clear? Doesn't contain enough Openreach bashing?

wisty
Aspiring Pro
Posts: 434
Thanks: 53
Fixes: 3
Registered: 30-07-2007

Re: BT Business Hub Strange SNR behaviour

10 days down the line the router is still showing 16.4-16.7 SNR. I power cycled the router, and it reconnected at the same level, and is reporting  Maximum data rate: 11139 / 64267.

 

A quick check of the BT Wholesale availability checker gives

                                     Downstream Line Rate(Mbps)  Upstream Line Rate(Mbps)
                                          High    Low                           High    Low
VDSL Range A (Clean)      49.2    35                             11.5    7.5
VDSL Range B (Impacted)  40      25                             10      5.2

But the Last Test Date is given as 08-03-2017.

 

So something significant has changed, but the availability system has not re-run the tests. Perhaps I will give the 70/20 product a go and see how much of that new speed I can get.

 

 

wisty
Aspiring Pro
Posts: 434
Thanks: 53
Fixes: 3
Registered: 30-07-2007

Re: BT Business Hub Strange SNR behaviour

Final update. I upgraded (Thanks Gandalf) to the 80/20 product for the last month of my contract - just to see what was now possible.

Router reset this morning 13/9 at 03:20 with 58.5/11.1 and a downstream SNR of 6.1 which has since risen to 7+. I will give it a few days to allow DLM to settle things down and see if I can reset it when the SNR is at it's best and see what I can squeeze out.

So despite Openreach saying my line max is 49.2, I am happily sitting at almost 60.

I wonder how often they re-run the tests?

 

 

 

 

wisty
Aspiring Pro
Posts: 434
Thanks: 53
Fixes: 3
Registered: 30-07-2007

Re: BT Business Hub Strange SNR behaviour

DLM has been playing again. Overnight  it dropped the line twice (05:20 and 07:59), reducing the SNR figure each time - see attached.

I am now synced at 62783/11193 which is way faster downstream than the DSL checkers most optimistic 49.2 estimate. It seems accurate upstream. I suspect the BT Hub 5A only has vectoring capability downstream.  

Community Veteran
Posts: 4,918
Thanks: 341
Fixes: 16
Registered: 10-06-2010

Re: BT Business Hub Strange SNR behaviour

I think it's more likely that the DSLAM has only enabled vectoring for the downstream. The current LR-VDSL trial documents say they're only doing vectoring for the downstream.

wisty
Aspiring Pro
Posts: 434
Thanks: 53
Fixes: 3
Registered: 30-07-2007

Re: BT Business Hub Strange SNR behaviour

Does the fact that this is potentially a trial suggest that It might go away again?

 

Community Veteran
Posts: 4,918
Thanks: 341
Fixes: 16
Registered: 10-06-2010

Re: BT Business Hub Strange SNR behaviour

Not really, what's going on at your cabinet may have nothing to do with LR-VDSL, it may just have vectoring to improve speeds generally or to meet BDUK or local targets for the number of lines receiving "superfast" speeds. Vectoring is an established technology, and once installed in a cabinet, it would make little sense to switch it off.