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Bellwire Interference

In an effort to improve my connection, which has been steadily deteriorating over the past couple of weeks, I have been experimenting with the wiring to the phone sockets. The connection is equally poor with either a Speedtouch 546 or a Safecom SART2-4115 attached, so not a modem issue. There is documentation that indicates that the bellwire can introduce interference into a line and thus give problems with ADSL.

Using the DMT tool (sadly does not have a version that works with AR7 chips) I can graph the frequencies that my connection uses and analyse where there may be gaps due to interference etc. In the first image we can see a graph of my connection with the bellwire (orange/white stripes connected to terminal 3) still attached:-

Attachment

Note the large area of frequencies above 660kHz that are not used. In the next graph we can see the affects of disconnecting the bellwire:-

Attachment

We can see on this that the missing frequencies are starting to be populated, though with my distance from the exchange these will probably not be as healthy as the main part of the graph. There are some more tweaks that I might try, there are other wires unused that are connected that might be adding to the ‘noise’ in my line.

Since starting the investigation my sync speed has nearly dounled and the connection is much less prone to dropping randomly, I had to raise the target SNR before to try to get a good connection, but it would still drop FNAR.

The bellwire was used on old phones that drew power from the phoneline, it is powered by a 50V battery cell at the exchange so don’t hold the wire as you will get a jolt when the phone rings. New phones tend to have their own power supplies and don’t need this wire. Disconnecting the wire means that older phones will not work, which does give rise to safety concerns during powercuts as the you will no longer be able to use an older phone if the bellwire is disconnected.

Clarification

The bellwire was used on old phones that drew power from the phoneline, it is powered by a 50V battery cell at the exchange so don’t hold the wire as you will get a jolt when the phone rings. New phones tend to have their own power supplies and don’t need this wire. Disconnecting the wire means that older phones will not work, which does give rise to safety concerns during powercuts as the you will no longer be able to use an older phone if the bellwire is disconnected.

Not quite right. The bell wire dates back to days when phones had a dial and a real bell, it is also known as the anti-tinkel wire when then was more than one phone the bell in the other phone would ‘tinkle’ in sympathy with the dialled number the third wire prevented this.

Most phones not supplied by BT have the ‘bell’ ring circuit built in and only require two wires to operate on BT supplied phones without the third wire they would not ring on an incoming call would still be able to make calls (and accept them if you ‘picked’ up the phone when it was ringing).

For ADSL this wire is NOT used the ADSL filters only use the A and B wires the bell ring wire is ‘created’ within the filter for phones that require it.

On a normal phone line the Master socket creates the bell wire (are only two wires to the exchange) which is then linked into each extension socket An ADSL filter appears to a phone to be a master socket.

The reason it can cause problems most noticable when the phone rings is because it carries the AC content on the phone line ( mainly the ring voltage ) which upsets the ballanced phone line and also ‘absorbs’ some of the higher frequency signals ( the faster the connection the higher the frequencys used and the more noticable the effects are).


Last updated by Kieran on Sunday, August 23rd, 2009
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