Friday, August 26, 2005


More on phone wiring

Or should that be Moron phone wiring :-)

After the success with taking the ring wire off and improving the SNR I was blessed with a free speed upgrade to 2M by my ISP Demon which was facilitated by ADSLguide getting my entry changed in BT's line prequalification number checker.

This speed upgrade took the downstream SNR margin down to 25 dB, still healthy but I was pleased I did the ring wire thing to improve it. Last night I hooked the ring wire up again and the ADSL modem lost sync, it re-trained to an SNR of 12 dB and ran OK but clocked up errors. The reason I put it back on was to look at the frequency effects of the interference, using a Sagem F@st 800 modem's diagnostic mode.

The first plot is with the ring wire disconnected, showing a wide range of useable frequencies. The second plot shows the effect of reconnecting the ring wire - it loses virtually everything above frequency band 140 with a consequent loss of ADSL data carrying capacity. The lower frequencies are a lot more ragged too. It would still operate at 2Mbits/s but was clearly less capable than before.

With the higher frequencies rendered unuseable by the ring wire interference, the modem is forced to cram more data into the lower frequency bands. This in turn makes it more sensitive to interference as a higher SNR is required in order to use more bits per channel.

So at least we can see why the ring wire impacts upon the SNR, it is adding interference in the 600 kHz - 1 MHz range. This is AM radio territory, so perhaps this should not be a suprise - for all I know the ring wire may be a nice wideband AM radio antenna, built into my house :-)

For my final experiment I hooked up a filtered socket from ADSL Nation in place of the master. This provides a filtered and unfiltered socket on the front, and an IDC terminal block for filtered extensions on the back. So I used it to provide me with a filtered ring wire that could supply ringing voltage to all the extensions without compromising the ADSL. This works well and maintained the 25 dB SNR and over 10 hours my router only clocked 3 CRC errors.

LocalSNRMargin = 25.0 dB
LocalLineAttn = 32.5 dB
LocalTxPower = 10.5 dB
LocalFastChannelRxRate = 2272000
LocalFastChannelTxRate = 288000
LocalFastChannelFEC = 0
LocalFastChannelCRC = 3
LocalFastChannelHEC = 0
RemoteLineAttn = 22.0 dB
RemoteSNRMargin = 29 dB

The only point in doing the filtered ringwire bit, as opposed to simply disconnecting the ring wire, is to leave the extension wiring fully functional, such that if you aren't using ADSL any more or leave the property the phone sockets will behave as expected. One wonders if at some point BT will abandon the ring (3rd) wire, it may have made sense 20 years ago for electromechanical bells but its a real nuisance with ADSL broadband.

The images below show the "before" bin plot with the original ringwire connected, the filtered ringwire connection and the "after" bin plot using the filtered ringwire.

Finally, just a reminder that the above refers to star or spur wired systems where an NTE5 master socket can't be fitted. The NTE5 filtered faceplate is the preferred solution where it can be used, it incorporates filtered extension wiring and, if needed, an unfiltered extension connection for a modem distant from the NTE5 linebox / master socket. Having said that, one ADSLguide user has reported improved SNR through removing the ring wire from his filtered faceplate - so if you are trying to squeeze the last few kbits/s of speed or are trying to make a dead or unreliable line work it may be worth a try.

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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