[Gllug] ADSL SNR

[Daniel P. Berrange]

On Wed, Jun 07, 2006 at 05:57:13PM +0100, - Tethys wrote:
> >I didn't really notice the difference.  Most downloads still max out
> >at around the 2 Mbps mark.
> 
> I noticed the difference. When I first switched from 2Mb/s to 8Mb/s,
> my downlaod speeds dropped by 75%, and the reliability of the line got
> noticably worse. Coincidentally (or perhaps not), this change happened
> at the same time that my ISP was bought out by Pipex. Fortunately,
> things have settled down somewhat now, and I get decent (~4Mb/s)
> downloads and good reliability.

This actually correlates back to the question at the start of the
thread - the max speed you can get over ADSL is directly related to
the S/N ratio & line attenuation. Say, for sake of discussion[1], the
thresholds are

   1mb  8db
   2mb  10db
   8mb  12db

Then if your line S/N is curently 10db & they re-grade you to an 8mb
line, you'll clearly suffer from unreliability. I had this problem at
my last place in London - we had dodgy internal wiring & so although
BT tested the line & declared it good for 2mb service, the internal
wiring was so crappy it would only cope with 512kbs service. Needless
to say, when our ISP "upgraded" us (without telling us they were planning
to do so) reliability went through the floor.

Regards,
Dan.

[1] The BT engineer did tell me what the actual thresholds are, but I
    can't remmber now - but looking over the engeering's shoulder at
    his laptop I could see the line into my place was comfortably  
    within the threashold to get 8 mb service. Damn the internal wiring!
-- 
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|=-   berrange at redhat.com  -  Daniel Berrange  -  dan at berrange.com    -=|
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