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Aspiring Contributor
Message 1 of 4

Two Lines - Both have same attenuation two vastly different Noise Margins



First some background:


I moved into a house in 2001 which had a line which I will call A. For work I needed another line and Broadband - so I ordered a BT Broadband package. The engineer came to my study, replaced the original extension socket which serviced line A with a Master socket (2 connectors - if you open it up it has 2 1cm long yellow 'resistors'). From this master socket came the new line which I will call B upon which the Broadband service would run. So I now had a new Master Socket in my study with Line A & Line B sockets. Beneath the Master plug runs a cable for Line B which is about 6 feet long and runs to an ADSL Filter Socket.


Line B is connected to a router and the Attenuation has always been about 30 downstream & 18 Upstream. The Noise Margin varies between 27.5 and 29 Downstrean and always seems to be abouyt 29 Upstream. Worked great and downstream was 2200kbps upstream 288kbps. Just as it says on the router data rate info.


Recently I decided to add BT Broadband to Line A in order to seperate my Business and Home usage. This new Home service on the original Line A is rated at 7488kbps downstream & 448kbps upstream. I expected about 3500kbps based on near neighbours BT service. I now get:


  • 1800kbps downstream on line B - the original broadband line - Down 400Kbps. Upstream remains the same.
  • 1800kbps downstream on line A - The new Broadband line - Upstream is good at 400kbps.

So, my problem is line A. The original line connected to the house. Which has a 30/18.5 attenuation and a 5.9 Noise Margin!!!!! Measured at Master socket near the front door (although it is the same on any of the extension sockets in the house). There is no noise on te 'noise test' to Line A or B.


My question relates back to the original changes the BT Engineer made in my study. I unscrewed the Master Socket he installed in place of the original extension line. There is a set of blue thin wires (amongst many different coloured wires) which have been taken from the bundle of coloured strands in the original cable and connected to various points in the back of the master socket.


So, does this mean my house originally had two physical lines (as no extra physical lines were installed to the house - he only worked in the study)? Or is line B somehow 'multiplexed' off the original physical line and this line given favour when it comes to ADSL signal noise margin?


I guess if it is two physical lines then one has a fault (my guess)?


If it is multiplexed (not sure if that is possible - but only one blue wire bunch is used from the original cable coming into the Master socket in my study - I would have expected 2 colours for 2 lines!!!!) then I guess it is a local problem in teh way BT originally installed it?


I need to figure this out before I contact BT as the better informed I am the easier it will be for BT to get it fixed.


Any help much appreciated, Nick.




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Distinguished Sage
Message 2 of 4

Re: Two Lines - Both have same attenuation two vastly different Noise Margins

it will most like be a two pair cable coming into your house one pair for line a the other is for line b
it is possible for one pair to have a fault
you would need openreach to investigated
Aspiring Contributor
Message 3 of 4

Re: Two Lines - Both have same attenuation two vastly different Noise Margins

Many thanks John.

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Distinguished Sage
Message 4 of 4

Re: Two Lines - Both have same attenuation two vastly different Noise Margins

Glad to have been of help
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