Power Over Ethernet

Discussion in 'Computing and Networks' started by mtcw, Oct 30, 2007.

  1. mtcw

    Thread Starter New Member

    Apr 26, 2007
    Is this stuff not yet ready for prime time?

    I want to order the Zyxel G-470 client device.
    I'd like to be able to utilize the POE feature.
    The device is POE standard 802.3af ready.
    This is not for not for a multi-unit WISP installation.

    I belive the following two products are tailored for just such an installation.
    One is an 802.3af supply/injector
    ..the other is a "non 802.3af passive injector"

    Microcom does not offer any tech support at all so they could offer no explanation. Will either of these work? What is the difference?

    I've been to many internet pages of explanation regarding the 802.3af standard and there seems to be no simple instructions for the home user.

    Xyxel sent me an email full of jargon about "midspan endspan mode-a mode-b legacy compatibility, power switching, etc" without ever getting to the point about which of these two units would work with the G-470. Zyxel offers no instructions at all, on their site or in the downloadable operators manuals for their products, on utilizing the POE capability of their products.

    Is this a case of "if you have to ask you probably sholdn't know"? Rather like particle physics?

    This is a little frustrating.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    The POE standard may still be in draft status. There is an article that discusses it in Electronic Design for 7/5/07. You may be able to get 13 watts @ 48 volts if you have the right router.
  3. GS3

    Senior Member

    Sep 21, 2007
    How does POE work? I am assuming it just uses a couple of the unused wires. Ethernet has 8 conductors but only uses 4 so I am assuming it just uses two for power. Right?

    Is there some control of this power supply by the host? Or is it just there regardless. Because it would seem quite simple to just connect 48 V to the appropriate wires.
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    Wish I could be more informative. The routers were by Cisco, and the unused pairs were simply shown to be connected by a DP switch to a rectifier, filter & regulator. As of July, the power limit was 13 watts. The new standard may also raise it to 15.

    I suspect the lack of info on the net means that the standard is still pending.
  5. mtcw

    Thread Starter New Member

    Apr 26, 2007
    Well, cisco only adds another complication to this. When I did find "idiot" level information about the 802.3af standard cisco is a frequently asterisked footnote cautioning about their own standard of POE that was separate from the industry prior to 802.3af standardization.

    The pin assignments are, as you suggested, straightforward. But the new standard allows for use of data carriers as well as unused pairs in order to get more voltage to the end unit. In most cases this is not a compatibility (legacy) problem. There is also signal sensing circuitry in the 802.3af power supplies that switched dc on/off when the end unit is actually in use.

    So for average voltage/amperage draws (any wifi client device currently on the market) the 802.3af standard powersupply and the "passive" supply are utilizing the same unused pairs to send the necessary voltage to the end unit. The passive supply merely lacks the circuits necessary to use data pairs to carry extra voltage/amperage if necessary for higher draws such as motorized security cameras, etc. Passive supplies also cannot auto sense the voltage on/off - so they are always powering up the end unit (no big deal I suppose. The cost of keeping a router warm is negligible and probably better for it in the long run than cycling on/off I would guess.

    And you're correct. "injecting" 48vdc into an ethernet cable couldn't be simpler. But you add up the cost and trouble of tracking down a 48vdc SMPS, two RJ-45 jacks (unless you want to just hard wire it all) and the very small, but considerable, possibility of mistaking the wrong pairs of wire and frying something expensive, and when you see a product like the passive 48vdc SMPS with jacks, ready to go, (albeit at .40 amps) for 15usd then it's hard to get motivated to plug in the soldering iron.

    I've answered my own question, no? Well I know none of the above beyond doubt. It's what I've guessed to be the case after reading quite a bit about the standards, but I can find no souce available on the web that puts it even a little bit as succinctly as I've attempted to do above.

    Still noone here can clear any of this up for me?

    Thanks so much.