Building my own WISP II: Teach me about POE injectors.

July 28, 2014 3:43 PM

As part of my continuing attempt to build a wireless bridge to bring the internet to my house, I have obtained two used Tranzeo TR902-11f 900Mhz wireless access points sans any power supply. Please help me find (cheaply) POE injectors for these WAPs.

According to the data-sheet[pdf] they are supplied by POE at 18v 1.1a (output) with 7 watts max. I have never set up a POE device before, but I assume that I need two POE injectors at 18v 1.1a (output) but, do they also have to be specifically at 7 watts or is the datasheet telling me that 7 watts is the maximum output of the WAP?

Also, in case anyone is wondering, trees with leaves do indeed form an intermittent but effective barrier to 2.4Ghz wireless signal transmission even at short distances (< 500ft) with no other barriers. Also, cheap 2.4Ghz panel antennas are not particularly directional. In addition, it's possible to find much cheaper non-Ubiquiti branded 900Mhz WAPs on your favorite internet auction website.

posted by ennui.bz to Computers & Internet (4 answers total)

I think the confusion here is that it's referring to two separate things. Had you bought these new (or had they come with the power adapters), you'd have a PoE injector that would require a power brick of its own that outputs 18VDC @ 1.1A. This would then inject normal PoE-levels of electricity over the ethernet cable to the device, and it'd pull 7 watts off of that. You can only draw so much (Wikipedia says either 12.95W or 25.5W, depending on which specific PoE spec) so you'd need to know that when planning out the network (you could not run two of these on the same line using an earlier PoE injector, for example). The device will draw whatever wattage it needs from what's available - as long as the voltages match up you're good to go, and PoE is a standard so you don't even really need to worry about any of that. You'd just buy an injector or a hub that supports that or something and plug it in. For example, this Netgear switch at NewEgg provides PoE on four ports. (This is not a recommendation - just the first useful thing that popped up when I searched.)
posted by mrg at 4:22 PM on July 28, 2014

Response by poster: So, say, I had a midspan power injector with a power brick rated at 18v 1.1a output then, say, I could power a tr902 and connect it to a router?

Basically, the PoE injector just sends along the ethernet cable whatever juice the power brick is putting out keeping in mind the total watt draw from the connected device(s)?
posted by ennui.bz at 4:39 PM on July 28, 2014

The output from the injector's power brick is irrelevant. The one that would have come with that unit just happens to pump out 18V at 1.1A.

The PoE spec provides at least 44VDC at 15-25 watts (though, as with anything else, having more wattage available is not a bad thing). PoE also has some logic in it to sense devices that need it and all that so it's not just straight dumping voltage on the line. Here's probably more than you wanted to know about it (also, maybe more readable, here). But to answer your actual question, yes, you essentially just grab something that'll provide PoE (injector, switch, whatever) and hook that dealie in there and it'll just work. Doesn't matter what you use - the PoE spec defines what's actually available over the ethernet cable.
posted by mrg at 6:00 PM on July 28, 2014

There are two varieties of "PoE".

There's the standardized IEEE 802.11af which provides 44-57v at 350mA. That's what you'll get out of an ethernet switch with PoE or any other standards-conforming implementation. In this case the injector/switch and the device negotiate the required amount of power before the injector throws voltage on the line. This is the "right way" to do PoE.

Then there's ghetto vendor-proprietary PoE where they just shove 12 or 24v or ?? on the cable and hope for the best. If the device at the end of the wire is expecting the same, then it all works out. If the device at the end if from a different vendor, or expects real 802.11af PoE, then maybe not.

I think your Tranzeo access points are using ghetto PoE, not standard 802.11af. The giveaway is (1) 18v at 1.1A power adapter, and (2) no mention of 802.11af in the spec sheet. So I'd stick with the matching vendor power injectors because I doubt the access points will work with a run-of-othe-mill standards-compliant PoE switch.

The wikipedia article discusses standard vs. non-standard implementations. Check it out for more details on voltage and current levels.
posted by ryanrs at 10:45 PM on July 28, 2014 [1 favorite]