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Groups > comp.os.linux.networking > #2840

Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender

Cross-posted to 3 groups.

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John Hasler <jhasler@newsguy.com> wrote:
>mechanic wrote:
>> As to this twisted pair thing, I've no idea what the specs are in
>> particular cables but no-one seems to have mentioned the purpose of
>> such a feature (if indeed it exists) - presumably to reduce crosstalk
>> between pairs, but is that important in Ethernet applications?
>
>Think about two twisted pairs running close together for a long
>distance.  The field is not confined between the wires in a pair: a bit
>leaks out into the surrounding space.  If there is another pair close by
>the field couples a bit of the signal into it.

Up to this point that is correct.

>If the pairs have
>exactly the same twist rate this coupled interference will always be in
>the same phase and so will add up over a long run and cause problems.

The twist rate will not change the phase of the signal, either
in the wire or induced into an adjacent wire, to any significant
degree.

>If the twist rates are slightly different the interference will vary in
>phase by 180 degrees over a length proportional to the difference in
>twist rate and so will average to zero.  The problem gets worse at
>higher frequency so Ethernet is more sensitive to it.

While that change in phase rate will happen, it will not average to zero
at any point in time at the cable termination.  It will average to zero
over a period of time, which is not useful

>This effect was observed (and dealt with) in long 600 ohm open wire
>lines more than 100 years ago.

That's not the significance though.  Phase makes exactly zero
difference in regard common mode rejection of induced noise.  If
the signals have a given phase relationship it will remain
exactly the same as long as the *length* of each pair is the
same.

The problem with having two pairs with the same twist is that
they can and will end up physically aligned in such a way that
each tip, for example, will have the same proximity to the other
tip over an extended distance that will not be on an average
equal to the proximity to the other ring. Thus unequal voltages
will be induced between the tip and ring of each pair.

Because each pair has a differential signal, resulting in common
mode rejection of induced voltages, the twist is there to ensure
that exposure to both tip and ring is of equal intensity for any
given external field.  If a field, for example, extends along
one inch of the pair, and the pair has 50 twists per inch, the
exposure to the tip will be very close to exactly the same as to
the ring.  Without the twist the field would be closer to one
wire than to the other, and the induced voltages would be
different.

If the source of a field is another pair though, having the same
twist would allow the wires to have the same unequal exposure
tip to tip over an extended distance because a twist in one pair
would match the twist, and distance, to the other pair.  By
using a different twist in each pair there will be a varying
exposure over a significant distance, and that will cancel out
the unequal exposure over any short distance.

It is purely the strength of the signal induced, which is a
function of distance.  Other variations could affect it, but the
cable is of course manufactured to minimize the significance of
any other variation.  Thickness and composition of insulation
are examples where uniformity is important.

-- 
Floyd L. Davidson                         http://www.apaflo.com/
Ukpeagvik (Barrow, Alaska)                      floyd@apaflo.com

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How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Liam O'Connor <liamoconnor@example.com> - 2014-03-08 00:53 -0800
  Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Dan Purgert <dan@djph.net> - 2014-03-08 07:46 -0500
    Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Liam O'Connor <liamoconnor@example.com> - 2014-03-08 10:56 -0800
    Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Liam O'Connor <liamoconnor@example.com> - 2014-03-08 10:58 -0800
      Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Liam O'Connor <liamoconnor@example.com> - 2014-03-08 11:51 -0800
      Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Dan Purgert <dan@djph.net> - 2014-03-08 15:25 -0500
      Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Scott Hemphill <hemphill@hemphills.net> - 2014-03-08 21:07 -0500
        Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Scott Hemphill <hemphill@hemphills.net> - 2014-03-08 21:18 -0500
        Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-08 20:49 -0800
        Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Mark Lloyd <mark@this.is.not.email.invalid> - 2014-03-09 10:50 -0500
      Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Bit Twister <BitTwister@mouse-potato.com> - 2014-03-09 02:49 +0000
        Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-08 20:51 -0800
          Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-08 23:56 -0900
            Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-09 09:39 -0700
              Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-09 11:07 -0800
        Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-08 23:20 -0900
          Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Liam O'Connor <liamoconnor@example.com> - 2014-03-09 07:40 -0700
            Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Dan Purgert <dan@djph.net> - 2014-03-09 21:19 -0400
          Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-09 09:21 -0700
            Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-09 10:57 -0800
              Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-09 16:07 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-09 17:05 -0800
            Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Liam O'Connor <liamoconnor@example.com> - 2014-03-10 10:40 -0700
              Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-10 12:06 -0800
              Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-10 20:07 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-11 02:36 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-11 19:30 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-11 23:13 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-12 21:30 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-13 04:44 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-13 20:20 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-13 20:30 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-13 22:59 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender mechanic <mechanic@example.net> - 2014-03-14 11:31 +0000
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-14 21:50 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-14 04:31 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender mechanic <mechanic@example.net> - 2014-03-14 13:45 +0000
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-14 07:26 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender John Hasler <jhasler@newsguy.com> - 2014-03-14 11:07 -0500
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-14 10:23 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-14 22:02 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-14 21:53 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-15 09:49 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender mechanic <mechanic@example.net> - 2014-03-15 10:45 +0000
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-15 09:57 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-15 12:28 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-15 14:02 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-15 14:22 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-15 21:55 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-15 21:25 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-14 21:54 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-14 21:58 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-11 19:33 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-11 23:36 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender ceed <ceed@is.indeed.invalid> - 2014-03-12 17:05 +0000
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-12 21:48 -0700
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-13 04:46 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Eef Hartman <E.J.M.Hartman@tudelft.nl> - 2014-03-12 19:27 +0000
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender floyd@apaflo.com (Floyd L. Davidson) - 2014-03-12 13:32 -0800
                Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-13 19:18 -0700
    Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Liam O'Connor <liamoconnor@example.com> - 2014-03-08 10:59 -0800
    Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Liam O'Connor <liamoconnor@example.com> - 2014-03-08 11:01 -0800
      Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Dan Purgert <dan@djph.net> - 2014-03-08 15:36 -0500
        Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Liam O'Connor <liamoconnor@example.com> - 2014-03-08 19:06 -0800
          Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender DecadentLinuxUserNumeroUno <DLU1@DecadentLinuxUser.org> - 2014-03-08 20:54 -0800
          Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Dan Purgert <dan@djph.net> - 2014-03-09 08:03 -0400
            Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Liam O'Connor <liamoconnor@example.com> - 2014-03-09 07:56 -0700
  Re: How to set up a spare Linksys WRT54Gv2 as a wired access point network extender Big Bad Bob <BigBadBob-at-mrp3-dot-com@testing.local> - 2014-03-08 17:42 -0800

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