[Gllug] ADSL on BT phone wires

[James Roberts]

Chris Bell wrote:

>    I am very suspicious about further information given to me, so can anyone
> please confirm or rubbish the following:
>    I have been told that ADSL is phase sensitive, and should also arrive at
> the customer's termination phase correct relative to the normal phone, and
> that occasionally the ADSL is not connected correctly at the exchange,
> resulting in the wrong phasing. This is sometimes sorted at the local
> termination rather than correcting the exchange fault.


This is an interesting question, because it could be true. Certainly 
wire phase was an issue with old modems.

I don't know the answer but I suspect that this *is* true for ADSL 
modems. The receiver and transmitter are still analogue but FFT's are 
applied to the data in its transition to the digital domain together 
with many error correction strategies. FFT's are indeed sensitive to 
*relative* phase, I don't think there is sensitivity to *absolute* phase 
(though the output would be inverted).

I found quite a nice intro to the whole thing at:

http://www.commsdesign.com/csdmag/sections/feature_article/OEG20010221S0082

and

http://www.commsdesign.com/design_corner/showArticle.jhtml?articleID=16502772

if you are at all electronic-al.

Certainly ADSL would appear to be sensitive to phase shift 'cos it's 
using QAM which implies phase sensitivity. However that phase shift 
would be within each 4kHz bin.

There's more basic stuff (for the electronic-al) at

http://www.commsdesign.com/design_corner/showArticle.jhtml?articleID=12801877

I also found some discussion that may relate (on QAM) at

http://www.dsprelated.com/showmessage/31751/1.php

(concerning MIMO - but WiFI uses similar techniques to ADSL at higher 
frequencies) which suggest the absolute phase (or pilot symbols) would 
be used as a reference for the decoding, that it would be possible to 
detect absolute phase error and introduce an inversion to fix it, but 
obviously not saying if this is actually done in real ADSL/SDSL 
transceivers (which don't AFAIUI use pilot symbols).

So I suspect that the answer is 'yes crossed wires would matter'.

Anyway sorry to ramble: I still don't *know* the answer - does anyone? I 
want to know too!

MeJ

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