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I am trying to buy an Access Point which supports Passive Power over Ethernet, 802.3af.

I don't understand the difference between the Passive Power over Ethernet and 802.3af. If the Access point supports 802.3af, do I need a switch which supports PoE features, or will any switch provide the Access Point with the power supply over ethernet.

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Speaking from my experience, most devices indicating they are being powered by "passive PoE" means that the device does not support standards based PoE (802.3af or 802.3at). While standards based devices "negotiate" power needs, a passive device simply expects the power to be automatically provided.

You may be required to use the vendor's power injectors and/or switch hardware to provide power to those devices. You may or may not be able to use a standards based PoE power source that can be configured to provide power without negotiation.

End devices that are compliant with 802.3af should work with any device that supplies 802.3af or 802.3at power, even if from a different vendor. End devices that are compliant with 802.3at should work with any device that supplies 802.3at and may work with devices that supply 802.3af depending on they need the extra power that 802.3at can provide and/or if they can operate in a lower power mode on 802.3af.

If you have a choice, always go with a devices that supports standards based PoE. It gives you more options due to the interoperability between devices of different vendors.

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If you want to power your WAP with PoE, you need the device to which the WAP connects to support PoE on the port where the WAP connects.

You need to be careful about which PoE version your WAP needs since many 802.11ac WAPs need 802.3at (PoE+).

Wikipedia will provide you with the Passive definition, and has a nice chart to go with it.

Passive

Most passive applications use the pinout of 802.3af mode B - with DC plus on pins 4 and 5 and DC minus on 7 and 8 (see chart below). Data is then on 1-2 and 3-6. This limits operation to 100Mbit/s. Gigabit passive injectors use a transformer on the data pins to allow power and data to share the cable and is typically compatible with 802.3af Mode A. In the common "passive" PoE system, the injector does not communicate with the powered device to negotiate its wattage requirements, but merely supplies power at all times. Passive midspan injectors up to 12 ports simplify installations. Devices needing 5 Volts cannot typically use PoE at 5 V on Ethernet cable beyond short distances (about 15 feet (4.6 m)) as the voltage drop of the cable becomes too significant, so a 24 V or 48 V to 5 V DC-DC converter is required at the remote end.[35] Passive DC-to-DC injectors also exist which convert a 9 V to 36 V DC input power source to a stabilized 24 V 1 A or 48 V 0.5 A PoE feed with '+' on pins 4 & 5 and '−' on pins 7 & 8. These DC-to-DC PoE injectors are used in various telecom applications.[36]

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