I am looking to purchase some PoE cameras. The camera description specifically states that they require power to be delivered on the data pairs (1,2,3,6), which indicates PoE standard 802.3af Alternative A. (see https://en.wikipedia.org/wiki/Power_over_Ethernet for description of A vs. B)

Shopping for PoE injectors, most of them state "802.3af compliant". However, very few state whether power is provided on the unused pairs or on the data pairs.

Is it safe to assume that if the device is 802.3af compliant, it will support either Alternative?

Otherwise, how can I be sure an injector will work with these devices unless I specifically ask the mfgr?

For example, this injector specifically states that data are on 1,2,3,6 and power are on 4,5,7,8, so I assume it will NOT work.

But I've found none which state that power is sent on 1,2,3,6, or which can do both.

So should I assume if it's 802.3af compliant, it will "just work"?


1 Answer 1


Th spec requires all powered devices (PD) to be able to be powered from both alternatives.

This way, the power sourcing equipment (PSE) can freely choose which way it wants to provide power. The reason is that, indeed, simple PoE injectors (called mid-span) can then provide power on the unused pairs to simplify their design. And full-blown ethernet switches that act as PSE can provide power on the signal pairs, to eventually reduce wiring requirements.

So you're fine, as long as the camera claims 802.3af compliance. However, if it doesn't claim full 802.3af compliance and explicitly asks power to be delivered on the data pairs, you may want a clarification from the manufacturer.

  • \$\begingroup\$ Per the camera mfgr note in product description:" Note: Both Data and Power transmit from Pin, does not support Pin for power supply " \$\endgroup\$ Jan 9, 2017 at 15:21
  • \$\begingroup\$ So the mfgr is not following 802.3af spec? How can I find an injector for this? \$\endgroup\$ Jan 9, 2017 at 15:22
  • \$\begingroup\$ It seems, indeed, that it is not fully compliant with 802.3af. It is the first time I see this. In that case, buy a PoE switch. Not a PoE injector. Most PoE switches I know of provide power on the signal pairs (you may double-check with the specs of the switch you choose, though). \$\endgroup\$
    – dim
    Jan 9, 2017 at 15:24

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