Tuesday, 27 May 2014
Unifi UAP-AC with 802.3af and 802.3at PoE switches
These are really good APs, and we recommend and sell them. However, I was caught out by a slight problem this last week.
We are running the latest beta test code for these. This is, of course, a risk we are taking. We fully understand that risk. But what happened was not actually an issue with the beta code.
We upgraded to 3.1.12 from 3.1.10 and encountered a problem. The APs kept crashing. We were lucky if they ran for an hour, and often way less. This made the WiFi almost unusable.
It had been fine on 3.1.10, or so it seemed, so we assumed it was an issue with the new beta s/w. Shit happens, we are running beta s/w, so not a surprise. Ubiquiti tried hard to find the issue. We sent logs and config and so on. It was clear from the beta forum that others had the same issue. The PRO models were fine, only the UAP-AC units were crashing.
We found the problem, to our surprise.
These come with there own in-line power over ethernet (PoE) power supplies. Some of the lower models are a passive PoE, but the PRO and AC units claim to be 802.3af and 802.3at compliant. To be honest we had not done a lot of PoE, and we got some nice PoE switches. What we had not spotted is that they are 802.3af only, and that is only 15W of power. The UAP-AC units need 22W so the 802.3at (25W) switches are needed.
Now, my understanding is that 802.3af and 802.3at should negotiate the power requirements. This should mean that a device needing more than 15W should not negotiate with an 803.2af switch and so should not work at all, perhaps indicating a power fault. Sadly, either the switch or the AP does not do this and it tries to work anyway. Somehow the UAP-ACs were working but the s/w update clearly increased the power usage and caused crashes.
Having worked this out, changing to use the provided in-line supplies fixes the issue. This is messy when we have PoE switches anyway, so we are going to try and find some 802.3at PoE switches.
It is a shame. If the negotiation logic had worked we would have realised our error when installing these, rather than much later on a s/w upgrade. As I say, I don't know if the switch (netgear) or the AP is to blame (though latest comments on the forum suggest it is the AP), but it is a cautionary tale none the less.