Sunday, August 14, 2005


ADSL tweaking.

Most broadband in the UK is ADSL supplied over phone lines. This involves a cunning plan to send frequencies of 100 kHz up to 1 MHz (AM through Short Wave radio) along the line superimposed on the low frequency voice signal. This allows speeds of several MBits/s to be transmitted over twisted pair copper cables only ever intended for analogue voice, quite neat really.

Nowadays ADSL is supplied "wires only" ie BT put the signal on the line and the user gets to make it work. When the twisted pair phone wire arrives in the user's house it is converted by a "master socket" to a 3-wire system where the 3rd wire provides a ring circuit, originally designed to power clunky bells in old steam phones, see this explanation. This 3rd wire is a "bit of an issue" as it makes the nicely symmetrical balanced twisted pair into the equivalent of a 3-legged ballerina.

In my case, the ring wire runs around my house as part of the feed to at least 6 extension phone points, in doing so it acts as a nice big aerial (antenna) to collect any radio transmissions and random electrical noise that may be passing through. It then feeds this mush back into one side of the twisted pair through the capacitor that is there to provide the ring signal and consequently degrades the ADSL signal.

This "degradation" appears as a reduction in the signal to noise ratio (SNR) and as CRC errors counted up by the modem and in some cases causes disconnections or prevents the end user getting the speed their line should be capable of. As we move to faster speeds and to "rate adaption" where the ADSL goes as fast as the line allows it, the dear old ring wire will slow down our connections.

So, what to do with the 3rd wire. The Rolls Royce solution comes in the form of a filtered faceplate (photo on right) which fits onto modern BT NTE5 lineboxes and provides ADSL and voice on separate sockets with filtered extension wiring connectors at the back. This keeps mush from the extension wiring, including the ring wire, out of the ADSL signal. It also avoids needing to use microfilters on each telephone appliance, which is A Good Thing™. Also available from

However, many of us don't have a nice NTE5 terminating our incoming phone line.

My phone line comes into a junction box in the roofspace from which the wires run in a "star" or spur configuration, the "master" socket is on one leg of the spur system and is identified by having a ring capacitor and a surge arrestor fitted.

As I have no NTE5 I can't use a filtered faceplate without a re-wire, so a bit of innovation is called for.

The photo on the right shows the back of my master socket with the ring wire disconnected from terminal 3, the other half of the orange/white pair is also disconnected from terminal 4. A higher resolution photo is at the bottom of this article.

By disconnecting the ring wire from the master we remove the 3rd leg and restore the balance / symmetry to the line.

Well, so much for the theory, what difference does disconnecting the ring wire actually make ? The stats from my router below show the results :-

Before, with ring wire (1 hour stats)

LocalSNRMargin = 20.5 dB
LocalLineAttn = 32.5 dB
RemoteLineAttn = 23.5 dB
RemoteSNRMargin = 25 dB
LocalFastChannelCRC = 22
LocalFastChannelHEC = 6

After, ring wire disconnected (20 hours)

LocalSNRMargin = 32.0 dB
LocalLineAttn = 32.5 dB
RemoteLineAttn = 23.5 dB
RemoteSNRMargin = 29 dB
LocalFastChannelCRC = 2
LocalFastChannelHEC = 0

So we see an increase in local SNR margin of 11.5 dB, meaning the router sees 11.5 dB more of the useable signal from the exchange after subtracting the noise. Going the other way the exchange gets to see 4 dB more of useable signal from the router. These figures after rebooting the router so it could adapt to the new line conditions.

20 hours after disconecting the error counter hadn't shifted from its initial value of 2 errors, compared to 28 CRC & HEC errors in just one hour with it connected. Result !!!

The phones still ring because they are using micro-filters and each of these has its own ring capacitor on the phone outlet side of the filter. If you disconnect the ring wire it would be prudent to re-connect it if you move out of the property, so the next occupier's phones work as expected.

Enjoy !


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