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Fibre Max data rate vs actual data rate

kankali
Grafter
Posts: 92
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Registered: ‎31-10-2011

Fibre Max data rate vs actual data rate

Hi,
Anybody know why the max data rate on fibre is so much higher than the actual? Is it a stuck profile? See the example below.
Regards, David
12 REPLIES
Acassim
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Registered: ‎11-06-2007

Re: Fibre Max data rate vs actual data rate

Hi David B,
Could you drop me a quick DM over with your username please and I'll take a look at this for you to see what's happened, I'm not overly familiar with the stats taken from the HH5 but if I'm reading this correctly you should be seeing closer to 36967.
kankali
Grafter
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Registered: ‎31-10-2011

Re: Fibre Max data rate vs actual data rate

Adam,
Thanks for offering to help, PM sent - let me know if it doesn't arrive.
The stats from the HH5 mean the following (I think). The max data rate is roughly what it could sync at, and the data rate is the actual sync.
I seem to remember reading on this forum that the DLM applies a profile on fibre - and mine seems to be stuck as a resync results in the same gap as per my original message.
Regards, David
Acassim
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Registered: ‎11-06-2007

Re: Fibre Max data rate vs actual data rate

Morning David,
Thanks for getting back to me via PM.
I've taken a look over the details on the connection and so far I have to say that the details shown in the HH5 don't seem to link back to anything I can see on the BT side of the connection. I ran a quick performance fault test (you may have seen your session drop) and this returned banding in the range of: 13.7M-27.4M Downstream, Interleaving Low - 1.8M-3.7M Upstream, Interleaving Off
This means that you aren't hitting the upper limit of the profile that's been applied and seems to line up with the first and highest speed profile we have on record for you which was applied manually and was based on the sync speed you had right after the fibre was activated. Given this I would hazard a guess that there is a possibility that as more people have upgraded to fibre on the same cabinet as you, this has introduced crosstalk which may be why the speed has dropped and settled from the 27Mbps to 22Mbps.
The image attached below of the dslchecker capture shows the estimated speed you should expect on the line and you're actually surpassing the top end of the clean line range which is good. This does leave me a little miffed as to the Maximum Data Rate shown in the router though as I'm not sure what this is based on.
With nothing holding back the sync, you're seeing the correct speed profile which is inclusive of the overheads:
Sync: 23.0Mbps (23000 * 0.967 = 22241)
Profile: 22.0Mbps
This explains the profile speed you're seeing which is currently set to 22Mbps and looks as though everything is set as it should be based on the details we can recover from the line test. I would be interested to see where the additional details are coming from in the HH5 as this is a little puzzling to say the least however I'm not familiar with that particular router so there could well be a valid explanation. I do have a HH4 at home so I'll have a bit of a trawl through the dialogue I can retrieve from it to see if this matches up with what you're seeing.

Update - Attached the screen capture as originally intended  Roll eyes
kankali
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Registered: ‎31-10-2011

Re: Fibre Max data rate vs actual data rate

Adam,
Thank you for a very thorough response.
Yes when I first started on Fibre I would get 27Mb which was in line with the Max speed shown on the HH5. It was only after the HH5 started dropping connections after the firmware update that the DLM seemed to intervene and cap it at 22Mb.
I will reboot the router when I get home to seem if the sync speed changes.
Re the BT speed estimates - these have always been conservative - for ADSL, ADSL2, and now Fibre.
Re the cross talk - interestingly quite the opposite has happened - the max achievable speed has gone up rather than down recently
Regards, David
Acassim
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Registered: ‎11-06-2007

Re: Fibre Max data rate vs actual data rate

Hi again David,
Thanks for confirming those details, I would still be interested on which figures the HH5's are basing the Maximum Stable Rate given the highest speed you've seen along with the estimates. It certainly doesn't match up with anything that we can see within both our system or the suppliers system.
Just to round the whole set of testing off done so far I'll drop a quick phone line check into the queue and see if we get a clean bill of health across all the services. Just out of curiosity, have you noticed any issue with the quality of voice calls at all?
deathtrap
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Registered: ‎23-04-2013

Re: Fibre Max data rate vs actual data rate

Sorry for butting in, the max data rate is what is known on most other modems as Attainable rate, which is basically a calculation based on what the modem thinks it could do if no limits were imposed by hardware ect,.
As for the estimate given by the BTW checker  it is exactly that, an estimate & nothing more and should not be used without other  factors  to determine if a connection is under performing or not
kankali
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Registered: ‎31-10-2011

Re: Fibre Max data rate vs actual data rate

Adam,
And after a reboot we get this - higher actual and lower max rate. I think that is a much more sensible gap between the two.
As a guess I would say that over time the max rate seems to rise due to some error in the way it is calculated.
Regards, David
Community Veteran
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Registered: ‎11-12-2013

Re: Fibre Max data rate vs actual data rate

deathtrap has it right.
basically there is a gap between the max data rate and actual data rate due to FEC correction overheads been applied.  So when a line is interleaved there will always be a gap.
kankali
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Re: Fibre Max data rate vs actual data rate

chrcoluk
Interleaving could be one explanation however there are others e.g. the calc could be based on a 3db SNR margin rather than the 6 db here, also there could be some gap left to allow bit swapping to keep the line stable.
Anyway when the gap was big in my first post it looked very wrong, but the new narrower gap makes a lot more sense
Regards, David
kitz
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Registered: ‎08-06-2007

Re: Fibre Max data rate vs actual data rate

Quote from: chrcoluk

basically there is a gap between the max data rate and actual data rate due to FEC correction overheads been applied.  So when a line is interleaved there will always be a gap.

I second this for a large part.  If the line is interleaved, then overheads for error correction skew the difference by quite a lot.
Quote
the calc could be based on a 3db SNR margin rather than the 6 db here
 
afaik most routers perform the calculation based on a default 6dB.. its some historic g.DMT standard  based on 2 bits per bin to load a tone which needs 6dB of SNR  - same as  BToR use as the default target SNRm.  If your SNRM deviates from 6dB then as you rightly say it will make a difference too, but there are lots of other things too that can make a difference..  Loop loss is an important one.  Another is the no of bits in use.. so if theres been a  lot of bitswap and router marks some tones as unavailable that can also skew things slightly..  and of course Power output.  If theres lots of bit swap going on, then Power output gets adjusted... which affects your DSL signal strength... which is turn affects the max rate Smiley
Community Veteran
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Registered: ‎11-12-2013

Re: Fibre Max data rate vs actual data rate

kitz summed it up well, the gap for bitswapping is the snrm which openreach set to 6db. 
So the gap isnt for that.
kankali
Grafter
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Registered: ‎31-10-2011

Re: Fibre Max data rate vs actual data rate

Kitz,
Thank you for the thorough response. That's my questions answered.
Regards, David