usweefive
join:2008-04-27
East Hampton, CT
| reply to NormanS
Re: [help] DSL Stats. Normal?
What I meant to say is that in traditional magnetic theory that two parallel conductors in close proximity to the other will have about -45 dB crosstalk between them.
This implies that the actual desired signal be above this -45 dB 'noise floor' to be seen as something usable. Manchester encoding is used to ensure valid data streams in DSL data transfers to mak sure packets do not have missing or dropped bits.
Electrically a signal that has 12 dB of signal to noise margin shows that the actual useable signal is only 4 times bigger in amplitude than the noise. ( 6 db = x 2 difference in amplitude ). The signal attenuation of -51dB means very low signal amplitude overall. |
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NormanS
Premium,MVM
join:2001-02-14
San Jose, CA
 ·Pacific Bell - SBC
| said by usweefive  :What I meant to say is that in traditional magnetic theory that two parallel conductors in close proximity to the other will have about -45 dB crosstalk between them. I am reasonably certain that the numbers reported by the modem have no direct relationship with that figure.
This implies that the actual desired signal be above this -45 dB 'noise floor' to be seen as something usable. Manchester encoding is used to ensure valid data streams in DSL data transfers to mak sure packets do not have missing or dropped bits.
Electrically a signal that has 12 dB of signal to noise margin shows that the actual useable signal is only 4 times bigger in amplitude than the noise. ( 6 db = x 2 difference in amplitude ). The signal attenuation of -51dB means very low signal amplitude overall.
In my experience (watching a line deteriorate due to problems in the physical plant) ADSL will be usable down to 9 db noise margin, reported by the modem. Others have reported usability at 6 db; but I have my doubts. Clearly getting too close to the crosstalk interference at that level. I do know that my SS4100 kicked over to a "noise profile" (Interleaved path) at 6 db S/N. But at 3 db it fell apart (constant loss of synch). BTW, phone repair handled that problem; I had lost dial tone due to an open on the "Ring" side of the loop.
--
Norman
~Oh Lord, why have you come
~To Konnyu, with the Lion and the Drum |
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usweefive
join:2008-04-27
East Hampton, CT
| There you go ! The attenuation was low due to a bad connection. There is a reason why you had partial success: the differential input of the DSL modem. Twisted pair systems use common-mode rejection similar to an Op Amp where common noise on a pair of wires is canceled out. The DSL modem's phone line input was operating single-input instead of dual and there was no common-mode rejection possible... ergo the very low noise figure incoming. |
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NormanS
Premium,MVM
join:2001-02-14
San Jose, CA
·Pacific Bell - SBC
edit:
May 1st, @02:31PM
| said by usweefive :The attenuation was low due to a bad connection. I would expect an open to raise the attenuation, not lower it. An effective impedance of infinite ohms should provide perfect attenuation. It was my S/N ratio which was low.
I believe the reason the DSL worked into an open circuit is that it operates at an RF freqency, so an extremely short gap, while looking "open" to DC current, still looks like a usable path to AC.
--
Norman
~Oh Lord, why have you come
~To Konnyu, with the Lion and the Drum |
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