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.


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