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Line Attenuation (dB) varies considerably

geffers
Grafter
Posts: 49
Thanks: 2
Registered: ‎23-05-2015

Line Attenuation (dB) varies considerably

Folks,

My fibre connection is normally a very stable 32mbps downstream and around 8mbps upstream.  A  few days ago my upload speed plummeted from 8mbps to below 1 and downstream from a stable 32mbps to an erratic 10-25mbps though I seldom actually get what is shown on modem.

The upstream Line Attenuation (dB) is varying wildly from 32 dB to currently 102 dB - strangely it appears to become erratic in the morning and settles, albeit slow, in the evening.

Although not generally connected to main BT socket I have tried it in that socket with all other devices disconnected, wifi turned off and just one ethernet connection.  No improvement to stability of connection or speed.

Plusnet have given the normal warning about a fee involved as an engineer needs to visit the house, I have declined this initially as want to investigate my network before agreeing to any fees.

Any suggestions as to why the Line Attenuation (dB) would vary so wildly?  I think that is similar to SNR margin, which remain stable at around 7-8 dB on both up and downstream.

Geoff

 

 

7 REPLIES 7
jab1
Seasoned Hero
Posts: 7,343
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Registered: ‎24-02-2012

Re: Line Attenuation (dB) varies considerably

@geffers I'm no network engineer, and know little about FTTC other than what I read, but that to me, indicates a major problem in the BT network, resolution of which will be extremely unlikely to end up costing you anything, and is not connected to your local setup.

I would arrange an engineer ASAP if it were me.

John
Baldrick1
Seasoned Hero
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Registered: ‎30-06-2016

Re: Line Attenuation (dB) varies considerably

 

What I would be interested to know is how attenuation is derived. With such a huge drop I would guess that it is estimated in some way, alternatively whether it an actual measured value, if it’s the latter I would assume that it’s a poor connection somewhere. Can any other contributor comment?

Also worth considering is whether this is induced noise that causing the error rate to increase massively but the DLM is not reacting and increasing the SNR.

@geffers  I’m with @jab1 here. If it does it when you are plugged in to the test socket then leave it connected this way and get Openreach out. The only other thing you can do is try a different filter and hub.

jab1
Seasoned Hero
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Registered: ‎24-02-2012

Re: Line Attenuation (dB) varies considerably

@Baldrick1 'What I would be interested to know is how attenuation is derived.' Is it not a function of line length? Or am I barking up the wrong tree?

John
Baldrick1
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Registered: ‎30-06-2016

Re: Line Attenuation (dB) varies considerably

@jab1

Attenuation is the drop in signal level between the sending and receiving end of a line. It's directly comparable in dc systems where only resistance has to be considered but gets a bit more complicated in ac systems, which is what broadband is (the broadband bit means a wide range of freequencies) where attenuation can increase over the same length of line with no change in resistance if reactive elements (line inductance and capacitance) change.

Also if you have a poor connection, which will increase the line resistance, this will increase the attenuation If the break leaves wires close enough for there to be capacitive coupling then this again would mean some signal gets through..

So it's not straight forward.

@geffers Does your telephone work when you have high attenuation? If not it points to a disconnection in one of the two telephone wires to your property.

jab1
Seasoned Hero
Posts: 7,343
Thanks: 2,495
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Registered: ‎24-02-2012

Re: Line Attenuation (dB) varies considerably

Thanks for the further explanation, @Baldrick1 . Just been looking at some of my old records, and over the period covered - about 4 years - my attenuation has only varied by a couple of decimal points.

John
geffers
Grafter
Posts: 49
Thanks: 2
Registered: ‎23-05-2015

Re: Line Attenuation (dB) varies considerably

I did try a different filter but not a different hub.  Might be an option.

 

Geffers

 

geffers
Grafter
Posts: 49
Thanks: 2
Registered: ‎23-05-2015

Re: Line Attenuation (dB) varies considerably

Baldrick,

 

Hardly ever use the phone so not noticed.  I'll check in to it.

 

Geffers