802.3af PoE injector difference in Gigabit vs 10/100 ?

the1337moderate · January 11, 2016

I have an EnGenius EAP350 Access Point that supports 802.3af PoE. After digging through the specs sheets online, I believe that the AP requires 48V@.375A when using POE. Currently I have the AP running directly on a power adapter, however I'd like to move it to a ceiling mount in the middle of the house. So far I haven't been able to find any mention of supported PoE injector models for this AP on the EnGenius website. When I called their technical support, I asked the person about whether or not this AP supports mesh and which types of POE injectors were supported. The support person just stammered on about how their new products had so many more better features and told me the only way to use PoE for the AP was to use a PoE compatible switch.

I hung up the phone.

So here I am asking you all if anyone knows the difference between PoE injectors that are 802.3af compliant, but some say Gigabit and others say only 10/100. According to the Wikipedia page on PoE, the power pinouts for 10/100 and 10/100/1000 appear to be the same; Using pins 3,4,7, and 8 for power. I know that the physical difference for 10/100 and 10/100/1000 is that the for gigabit, all eight pins are utilized in the cable for bi-directional communication whereas in 10/100 only 4 pins are utilized.

If I buy an el cheapo PoE injector that says it's 802.3af compliant will the data connection still remain at gigabit speeds, or will it get knocked down to 10/100 speeds? My guess is that if it get's knocked down to 10/100 it's because the power pins on the POE side are not connected to the data side. Can I just jumper the pins on the connectors with some diodes/resistors/or something to make the injector work at gigabit speeds?

Of course, I could just go buy a gigabit PoE injector, but I'm cheap and DIY is usually what fits the budget.

brwainer · January 11, 2016

At work we sometimes use power injectors that look like laptop power bricks from the early 2000's, they are no name but reliable and seem to work at gigabit. But normally we use a POE switch, especially gully managed ones that let you disable power on a port to reboot the AP - something we do fairly often despite using proper enterprise APs from Ruckus and sometimes Meraki.

pbelf · January 11, 2016

The way that the power is actually delivered down the cable with 802.3af is not just a case of using some of the pins for power and some for data. The power and data are actually delivered down the same individual wires. There is actually quite a sophisticated electrical handshaking that happens at connection to work out if the device at the end of the cable supports 802.3af as sending power to device that isn't expecting it isn't a great idea.

If you injector supports gigabit and I'd be suprised if there any that you buy today that don't then it won't degrade to 100Mbit.