3db SNR equals lower speed

[#1]

Hi,
I'm wondering the reason behind the 3db SNR? I don't quite understand it but it seems the lower my SNR the lower my speed and max attainable. Router seems determined to get down to 3 when I would get a higher speed at a higher SNR? Being on a long line of about 1km doesn't help either 😂

Thanks

[#2]

The convention is that the lower the SNR the higher the speed BUT dependent on how constant the noise level is. The more random the noise applitude is, the greater the number of errors. Errors require data to be repeatedly transmitted and hence slow the achievable speed. The purpose of DLM is to minimise the SNR whilst keeping the errors down to an acceptable level. After a DLM reset by Openreach this is set to 6dB and then left to the DLM to find the best level. Maybe yours needs a reset. The minimum DLM setting is 3dB. I have a reasonable line and if the noise level increases my connection will quite happily work at 2dB without resetting back to 3dB.

It's all part of the bodge that is required to maximise digital data speed over old analogue telephone lines that were never designed for this purpos.

[#3]

Hi,

Thanks for the explaination, that helps understand it. 

From what I understand it our house used to be an exchange only line (before we moved here) from FENCEHOUSE exchange as we are only 250m from it as the crow flies. The introduction of VDSL meant that OR needed to use a cabinet (No. 32) so put one on the road outside of the exchange and followed the roads to run new cabling increasing the distance of the line to around 1km.

The router shows around 19mb with a max of 19.5mb on a 3db SNR. if the SNR increases the max follows suit so in our case a lower SNR doesn't seem the best answer although it seems to aim for 3db. I've not been with Plusnet long (1 month maybe) so not sure a DLM reset is needed as we started at 17mb following a swap from TT and its slowly increased to 19mb. DSL checker shows the below. When your used to 50mb from a previous home gaining the max from a slower connection helps! 

 VDSL Range A (Clean) Max 23.3   Min 15
VDSL Range B (Impacted) Max 22.6  Min 12.4

Sorry for the long explaination!! 

Thanks

[#4]

Thanks for your post @AlienatedCargo

A lower SNRM would generally yield a faster line speed, I've tested your line and I can see your DLM profile is: "Downstream: 0.128M-40M 3dB with Retransmission (High). Upstream: 0.128M-10M with no error protection"

I agree that a DLM reset isn't needed as there's no artificial restrictions affecting your current line speed of 19mbps and as you've mentioned you're a very long away from the cabinet at about 1407 metres (almost 1 mile). 

If your SNRM goes up from 3dB I'd expect your sync speed to go down from this. 

Let me know if I've misunderstood what you've said.

[#5]

Hi Gandalf,
Yeah I understand that, especially since DLM must have stepped in again overnight and slightly upped the speed to 19.35mb with a max of 19.6 and SNR holding around 3db. Yesterday with SNR 3 it was reporting max of the most recent re-train so looks like 3db does seem better after all. I imagine given a long enough wait with no unexpected downtime it may get close to the max quoted by openreach. One question though, what does retransmission (high) mean?
Thanks for your help!

[#6]

Hi @AlienatedCargo Retransmission is a form of error correction similar to interleaving. It's possible that this is keeping errors at bay to prevent your sync speed from reducing. 

Fingers crossed your speeds improve a bit more with time. Let us know how it goes and if there's anything else you'd need help with. 

[#7]

Will do thanks!