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I have a WiFi module until now I supplied through a PoE injector. According to the documentation the WiFi module voltage requirement is the following: 12V Passive PoE (accept up to 22V). I also have the following PoE Plus switch: It complies to IEEE 802.3at PoE standard and is backward compatible to IEEE802.3af. It can power both existing 10/100Base-T network devices and emerging wireless 1000Base-T devices such as Wi- MAX and wireless IEEE 802.11n access points.

Today I plug in the WiFi module to the PoE plus port, but it didn't work. After that I check this standards on the wiki and I found the following table:

Property                         802.3af (802.3at Type 1)           802.3at Type 2
Power available at PD[note 2]   12.95 W                              25.50 W
Maximum power delivered by PSE  15.40 W                             34.20 W
Voltage range (at PSE)           44.0–57.0 V[16]                    50.0–57.0 V[16]
Voltage range (at PD)            37.0–57.0 V[17]                    42.5–57.0 V[17]

For me it's not really clear this PSE and PD. So I can't replace the injector with this switch?

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To me it sounds like your "Wi-Fi module" (as you call it) never did standard 802.3af or 802.3at PoE, but instead it did its own proprietary thing with the power injector that came with it.

Look on your module and the power injector that came with it, and the documentation for them. Do you ever see any mention of 802.3af or 802.3at? If not, then your hunch is probably right, you can never replace that power injector for that device, because that power injector is the only equipment you're likely to find that will be able to do the nonstandard proprietary PoE scheme that your device requires.

In answer to your other questions, "PD" is "Powered Device": It's what you're calling your "Wi-Fi module". PSE is "Power Supply Equipment" -- the device that supplies the power over the Ethernet cable. It could be a PoE-capable switch, or it could be a "midspan PSE" (also known as a "PoE power injector"): something that goes between the switch and the Powered Device and "injects power" into the cable. The reason there's a difference between the power specs at the PSE and the power specs at the PD is because some power is lost in the up-to-100-meter-long Ethernet cable runs. This chart is saying that between ~2.4 to ~8.7 Watts may end up being dissipated by (lost to, consumed by) the cable.

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Thanks, now a bit clear everything. –  Kicsi Mano Mar 5 '12 at 16:59

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