Hello,

I am trying to use the hEX PoE platform in place of old fiber media converters so that I can add an Aruba AP-635 nearby. The AP-635 cannot boot up functionally (it can power up enough to stage a basic config but it can't power it's radios on) using anything less than 802.3at POE.

The spec sheet for the hEX PoE...
https://mikrotik.com/product/RB960PGS#f ... ifications

Lists that 802.3at PoE is one of its capabilities.

PoE-out
Details
PoE-out ports Ether2-Ether5
PoE out 802.3af/at
Max out per port output (input 18-30 V) 1 A
Max out per port output (input 30-57 V) 450 mA
Max total out (A) 2 A

I have the Mikrotik powered using a 54.6V 2A power supply. Each hEX will only be powering a single AP (30W max).

When the AP boots up it shows that it is only receiving 802.3af power. Is there an obvious setting that I have overlooked which is required to allow the hEX PoE to supply 802.3at power? I have tried the "PoE LLDP Enable" and that didn't make a difference. I am running the 7.12 router OS. Is there a bug in that code version?


APBoot 2.6.2.3 (build 80087)
Built: 2021-04-29 at 10:04:03

Model: AP-635
DRAM: 2 GiB
Flash: Detected MX25U6435F: total 8 MiB
MMC: 0 (eMMC)
PCIE: link up
Power: 802.3af POE, port0
Radio: qcn9072#0, ipq6010#1,ipq6010#2
Reset: cold
FIPS: passed

I cannot say if it applies to your case, but 30W@54.6V mean 550 mA.

From what I understand 802.3at means "up to 600 mA" (while 802.3af means up to 350 mA), and Mikrotik provides max 450 mA as there is anyway the limit in the standard to 25.5 W per device and 25.5/0.45=56,67 V, which is in practice the max voltage of 57V, so it is 802.3at compliant see:
viewtopic.php?t=182546

I wouldn't be surprised that your AP expects to get 600 mA to consider the power source "fully" 802.3at (while in reality one would need 802.3bt to exceed 25.5 W)-

In any case if the AP really-really is 30W, it needs 550 mA@54.6V and the hex Poe per specs won't deliver that amount of current, and you have to consider anyway some "slack" or loss of voltage.

I was afraid of that. Mikrotik was playing too fast and loose with the "802.3at" label. Standards are standards for a reason and if they can't support the standard they definitely shouldn't list it on the spec sheet.

Well, I am not particularly a "defender" of Mikrotik, but - as often happens - in this case it's the "standard" that is - if not downright ambiguous - prone to different implementations but the basic Laws of physics and electricity cannot be changed.
The 450 mA at the maximum allowed voltage of 57 V are almost exactly the maximum 25,5 W allowed.
A 30W device is NOT 802.3at compatibile, period.
If you had (say) a 24 W device and you wanted to power it with 45 V (at the PD, likely with around 48 V at the PSE with some cable length between the devices) you would need some 535 mA which would be (IMHO) within the standard specs BUT that a Mikrotik device POE won't allow that would be something to debate.

In short: I agree that MT is frequently specifying their PoE devices as 802.3at compliant unjustified, they are 802.3af with maximum power exceeding standard (but that's it). And it would only be fair to fix the product pages and brochures.

I'm usually referring to wikipedia articles when it comes to various technical things. And there's a pretty extensive article on 802.3 PoE, which includes a table with some standard features. From the table it's clear that 802.3at (indirectly) requires power source of PSE (switch) which provides at least 50V ... and MT doesn't supply such power adapter by default with any of devices. And for hEX PoE in particular MT doesn't list a 50+V power adapter as optional item ... so in order to make device 802.3at compliant, one has to use 3rd party power adapters. And that's not OK IMO.

But to praise MT at least a bit: 802.3af specifies up to 15W power outputs (up to 350mA, which means output voltage of 42.9V) while hEX PoE can output 21.6W (if using optional 48V power adapter). But the question remains: is hEX PoE supporting actual power class negotiation as per standard (see table Power levels available in the wikipedia article)? And if it does, does it support only power classes 0-3 (which makes it 802.3af device) or also power class 4 (which would make it 802.3at but device clearly can't deliver up to 600mA required by power class 4).

Also - maybe I am re-stating the obvious - there are losses in the cable depending on cable length and type of cable (not the case of the OP if the AP is "nearby")

Voltage/power needs to be anyway at least pre-calculated, there are dedicated calculators online, here is one:
http://poe-world.com/Calculator/

To use an example a hypothetical 25.5 W device (thus within the 802.3at but at its upper limit) with 48 V at the PSE at 20 meters on el-cheapo Cat5E would be actually powered at 44 V and consume 580 mA, or, even at 56 V and with a "premium" Cat6 cable it would come out at 460 mA, i.e. still above what the Mikrotik is stated to be able to provide.

I have not any specific experience with PoE on Mikrotik devices, but (personally) I wouldn't trust *any* vendor when the end device power requirement is near the edge of the specs or the length of the cable is more than a few meters and I would rather choose a separate PoE injector (which IMHO is also easier to replace when/if needed and - in my perverted mind - it is expendable in case of spikes/issues/whatever, better to have it fried than the switch/router port).

Only as an example, possibly useful for the OP requirements an Ubiquiti U-Poe-at-eu, Poe injector and power supply, see here specs:
https://dl.ubnt.com/datasheets/poe/PoE_Adapters_DS.pdf
is rated for 48VDC, 0.65A and can be found for 15-20 Euro.

The Aruba APs I'm using typically only consume about 8-12W of power... the hEX PoE, even if it can only do 450mA, should be more than capable of providing the necessary power for the device. I think the issue here is the negotiation of the standard.

From this article...
https://fiberroad.com/resources/article ... r-ethernet

...it seems like this hEX PoE is not correctly doing this type of negotiation with the AP.

2-event classification – for PDs of 802.3at Class 4
When the PD is identified as a Class 4 device, the PSE will use a second event to verify that the PD really needs a higher level of power. This second event can be either of the two following methods:

Hardware-based 2-event classification
PSE first performs 1-event classification as described above. If it reads the Class 4 current value from the PD, it only supplies Class 3 power and repeats the voltage impulse for the second time. If after this 2nd event it is confirmed that the PD is Class 4, the PSE provides Class 4 power to the PD.

Software-based LLDP classification
PSE first performs 1-event classification as described above. If it reads the Class 4 current value from the PD, it only supplies Class 3 power and requests confirmation from the PD via Layer 2 LLDP protocol on whether the PD is indeed Class 4. If after this 2nd event it is confirmed that the PD is Class 4, the PSE provides Class 4 power to the PD.

2-event classification support
The IEEE 802.3at standard defines that Class 4 PDs must support both hardware-based 2-event and software-based LLDP classification, while PSE must only support one but may support both. PoE+ injectors typically only support hardware-based 2-event sort. Many PoE+ switches support both methods.

I will submit a case with support to ask if it is working as intended. The 802.3at standard is a STANDARD... and if you list it on the specs it should work.