[GLLUG] Broadband signal to noise ratio

Ken Smith kens at kensnet.org
Mon Sep 18 07:40:06 UTC 2017

Chris Bell via GLLUG wrote:
> Hello,
> I have a basic FTTC broadband and have noticed that both up and down
> attenuation figures have remained constant but the signal to noise ratio drops
> significantly when the ground is dry, although it soon recovers when the ground
> is wet. Is this to be expected? The overall speed reaches the specified cap at
> just under 40Mbps, but with many errors logged by the modem, although I do not
> see errors in the final downloads.
> I assume this is due to general radiation such as mobile phone transmission.
> When the original lead covered cable was replaced many years ago the engineer
> managed to find and re-use the rusty remains of the original iron pipe laying
> on the ground and feed the new plastic coated cable through it, but it was
> already too short to reach the house. I believe the first phone was installed
> when the house was built in 1933, and there was no requirement for the cable
> to be laid in a straight line or buried, so it was routed round the small
> front garden.
> The direct route  for a new BT plastic duct from the pavement manhole at the
> correct depth would be under crazy paving for much of the way, although not
> close to any other services.
> Thanks for any information.

I believe that VDSL follows the same/similar scheme as ADSL where there 
are many carriers carrying QAM modulated signals. Looking at my VDSL 
stats (currently running downstream at just under 40 Mbps) its using up 
to BIN (their language for the carrier number)  2600 which works out to 
around 11 MHz. (4.3125×2600).

Mobile phone signals are up around 900 Mhz and above so I doubt there is 
much interference from them.What I have noticed is that dampness does 
affect the capacity of the line to sync the higher speeds and my VDSL 
sync rate does vary with the weather. I have seen ADSL services show 
this behaviour too. Also there is crosstalk between the pairs in the 
cable bundles in the multicore cables that run from the manhole/phone 
pole back to the green street cabinets, and I believe this also affects 
the performance. Is your iron pipe dry inside? The BT cable's 
capacitance will change if its lying in water especially if there are 
any nicks in the plastic sheath allowing moisture in.

I'm astonished xDSL broadband works at all over lines originally 
intended for baseband voice service. Aren't you ex-BBC Chris? I recall 
hearing that TV OB services sometimes used specially equalised telephone 
pairs to carry baseband PAL TV (up to 8Mhz) back to the studio where 
other feeds weren't practical. I was surprised at the time that enough 
bandwidth was available in a telephone pair to carry PAL TV.

:-) Ken

This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.

More information about the GLLUG mailing list