An example of a device that uses PoE *and* runs at gigabits speed?

Discussion in 'Cisco' started by Ramon F Herrera, Jul 25, 2008.

  1. A collegue of mine insists that it is impossible for a port to have
    gigabit speed while simultaneously using 802.3af PoE.

    Can anyone please confirm or deny?

    There's a round of beers running on this one.

    -RFH
    Ramon F Herrera, Jul 25, 2008
    #1
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  2. Ramon F Herrera

    Trendkill Guest

    Re: An example of a device that uses PoE *and* runs at gigabitsspeed?

    On Jul 24, 10:05 pm, "Brian V" <> wrote:
    > "Ramon F Herrera" <> wrote in messagenews:...
    >
    >
    >
    > > A collegue of mine insists that it is impossible for a port to have
    > > gigabit speed while simultaneously using 802.3af PoE.

    >
    > > Can anyone please confirm or deny?

    >
    > > There's a round of beers running on this one.

    >
    > > -RFH

    >
    > Your collegue would be wrong. Done all the time and has been out for a
    > couple years now. In the Cisco world the 7941G-GE and 7971G-GE phones are
    > gigabit, both of them use PoE. Pretty much all switches from the 3560 on up
    > offer a version of Gig PoE, ie 3560G-24PS, 3750G-24PS, etc.


    I think he is saying that the port will then not push gig throughput,
    not that PoE doesn't run on gig ports or something. Can anyone with
    an IP phone run a ftp test to a local server and see the throughput
    they get?
    Trendkill, Jul 25, 2008
    #2
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  3. Ramon F Herrera

    whit3rd Guest

    Re: An example of a device that uses PoE *and* runs at gigabitsspeed?

    On Jul 25, 7:49 am, "Brian V" <> wrote:
    > I believe he is refering to how in one mode of PoE it uses the "unused
    > pairs" but in Gigabit there are no unused pairs so how could it work.


    The third-pair differential signal and the fourth-pair differential
    signal
    are used by GigEthernet. Power is fed to the transformer center
    tap, so is a common-mode DC level on the third-pair, and a different
    common-mode DC level on the fourth pair, and the power adds
    nothing to the differential signal, so does not interfere.

    The Ethernet standard allows up to (?) 500V of DC on the twisted
    pair wiring, I seem to recall.

    Look up 'phantom power'; this was a trick of similar sort played
    with microphone preamplifiers, going back to vacuum tube days.
    whit3rd, Jul 25, 2008
    #3
  4. Ramon F Herrera

    Sam Wilson Guest

    In article <ugnik.274054$1.easynews.com>,
    "Joel Koltner" <> wrote:

    > "whit3rd" <> wrote in message
    > news:...
    > "Look up 'phantom power'; this was a trick of similar sort played
    > wth microphone preamplifiers, going back to vacuum tube days."
    >
    > Phantom power is still very much alive and well in professional audio circles.


    And amateur!

    Sam
    Sam Wilson, Jul 25, 2008
    #4
  5. On Fri, 25 Jul 2008 17:52:09 +0100, in sci.electronics.design Sam
    Wilson <> wrote:

    >In article <ugnik.274054$1.easynews.com>,
    > "Joel Koltner" <> wrote:
    >
    >> "whit3rd" <> wrote in message
    >> news:...
    >> "Look up 'phantom power'; this was a trick of similar sort played
    >> wth microphone preamplifiers, going back to vacuum tube days."
    >>
    >> Phantom power is still very much alive and well in professional audio circles.

    >
    >And amateur!
    >
    >Sam

    In what way?


    martin
    Martin Griffith, Jul 25, 2008
    #5
  6. Ramon F Herrera

    JosephKK Guest

    On Fri, 25 Jul 2008 07:49:50 -0400, "Brian V" <>
    wrote:

    >
    >"Trendkill" <> wrote in message
    >news:...
    >On Jul 24, 10:05 pm, "Brian V" <> wrote:
    >> "Ramon F Herrera" <> wrote in
    >> messagenews:...
    >>
    >>
    >>
    >> > A collegue of mine insists that it is impossible for a port to have
    >> > gigabit speed while simultaneously using 802.3af PoE.

    >>
    >> > Can anyone please confirm or deny?

    >>
    >> > There's a round of beers running on this one.

    >>
    >> > -RFH

    >>
    >> Your collegue would be wrong. Done all the time and has been out for a
    >> couple years now. In the Cisco world the 7941G-GE and 7971G-GE phones are
    >> gigabit, both of them use PoE. Pretty much all switches from the 3560 on
    >> up
    >> offer a version of Gig PoE, ie 3560G-24PS, 3750G-24PS, etc.

    >
    >>I think he is saying that the port will then not push gig throughput,
    >>not that PoE doesn't run on gig ports or something. Can anyone with
    >>an IP phone run a ftp test to a local server and see the throughput
    >>they get?

    >
    >I believe he is refering to how in one mode of PoE it uses the "unused
    >pairs" but in Gigabit there are no unused pairs so how could it work.


    Take a look at the standards doofus.
    JosephKK, Jul 28, 2008
    #6
  7. Ramon F Herrera

    JosephKK Guest

    On Fri, 25 Jul 2008 19:11:01 +0200, Martin Griffith
    <> wrote:

    >On Fri, 25 Jul 2008 17:52:09 +0100, in sci.electronics.design Sam
    >Wilson <> wrote:
    >
    >>In article <ugnik.274054$1.easynews.com>,
    >> "Joel Koltner" <> wrote:
    >>
    >>> "whit3rd" <> wrote in message
    >>> news:...
    >>> "Look up 'phantom power'; this was a trick of similar sort played
    >>> wth microphone preamplifiers, going back to vacuum tube days."
    >>>
    >>> Phantom power is still very much alive and well in professional audio circles.

    >>
    >>And amateur!
    >>
    >>Sam

    >In what way?
    >
    >
    >martin


    It is becoming common in a lot of audio equipment for performers at
    all levels.
    JosephKK, Jul 28, 2008
    #7
  8. Ramon F Herrera

    Sam Wilson Guest

    In article <>,
    JosephKK <> wrote:

    > On Fri, 25 Jul 2008 19:11:01 +0200, Martin Griffith
    > <> wrote:
    >
    > >On Fri, 25 Jul 2008 17:52:09 +0100, in sci.electronics.design Sam
    > >Wilson <> wrote:
    > >
    > >>In article <ugnik.274054$1.easynews.com>,
    > >> "Joel Koltner" <> wrote:
    > >>
    > >>> "whit3rd" <> wrote in message
    > >>> news:...
    > >>> "Look up 'phantom power'; this was a trick of similar sort played
    > >>> wth microphone preamplifiers, going back to vacuum tube days."
    > >>>
    > >>> Phantom power is still very much alive and well in professional audio
    > >>> circles.
    > >>
    > >>And amateur!
    > >>
    > >>Sam

    > >In what way?
    > >
    > >
    > >martin

    >
    > It is becoming common in a lot of audio equipment for performers at
    > all levels.


    Condenser microphones (one of the two main types) require a power
    source. They are either fitted with a battery (which needs to be
    checked and renewed) or are phantom powered. Similarly DI boxes
    (devices which match different signal levels and impedances) are often
    powered and again it's convenient not to require a battery. I think
    I've noticed other things being phantom powered recently but those are
    the traditional things.

    Sam
    Sam Wilson, Jul 28, 2008
    #8
  9. Ramon F Herrera

    Wim Lewis Guest

    In article <>,
    Brian V <> wrote:
    >"Trendkill" <> wrote in message
    >news:...
    >>I think he is saying that the port will then not push gig throughput,
    >>not that PoE doesn't run on gig ports or something. Can anyone with
    >>an IP phone run a ftp test to a local server and see the throughput
    >>they get?

    >
    >I believe he is refering to how in one mode of PoE it uses the "unused
    >pairs" but in Gigabit there are no unused pairs so how could it work.


    Ah, that makes perfect sense as the source of the OP's coworker's
    disagreement: in one mode 802.3af uses the spare pairs, in the other mode
    802.3af uses the data pairs (in common mode, with power being taken off
    the center taps of the powered device's isolation transformer). The
    "spare pairs" arrangement would also work for 1000baseT, as long as the
    same sort of circuitry is used on those no-longer-spare pairs as on the
    data pairs. But a midspan PoE injector probably won't do that unless
    it's explicitly designed for GigE use, so the OP's coworker might
    well have experience with GigE devices that don't work on PoE ports.


    --
    Wim Lewis <>, Seattle, WA, USA. PGP keyID 27F772C1
    Wim Lewis, Jul 31, 2008
    #9
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