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knat53
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Only 450mA PoE output for 802.3at?

Mon Jan 24, 2022 3:41 am

I'm having trouble powering up some PTZ cameras using Mikrotik PoE switches and routers (PowerBox Pro, CRS112-8P-4S-IN, hEX PoE). I thought the 802.3at standard required up to 600mA of current to be available from the power sourcing equipment (PSE) but I notice that many Mikrotik products that claim to support 802.3at only provide up to 450mA per port (PowerBox Pro, CRS112-8P-4S-IN, hEX PoE), as per the official Mikrotik product specifications.

The PTZ cameras draw up to 530mA when the infrared LEDs are on and when panning, tilting, or zooming the camera, but these Mikrotik devices are only supplying up to 450mA at 48V (using the 48POW adapter).

Do these Mikrotik devices not properly support the 802.3at standard as they claim? Or am I wrong in thinking that the 802.3at standard requires PSE to supply up to 600mA per ethernet port?

Thanks!
 
mkx
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Re: Only 450mA PoE output for 802.3at?

Mon Jan 24, 2022 8:53 pm

If one can consider wikipedia article relevant, then: 802.3 af specifies max current at 350mA (and minimum voltage at PD to 37V). 802.3 at extends that towards 600mA (minimum voltage at PD 42.5V).

So yes, mentioned devices don't implement at revision, but they implement af with some margin.
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Znevna
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Re: Only 450mA PoE output for 802.3at?

Mon Jan 24, 2022 9:12 pm

In MikroTik world you always have to read the datasheet / specs provided in order to see to what parts of some standard were implemented.
As per the topic title, yes, only 450mA for 802.3at as specified in the specs for said devices (I only checked PowerBox Pro).
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knat53
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Re: Only 450mA PoE output for 802.3at?

Mon Jan 24, 2022 9:17 pm

Yes, according to Table 33-1 of the IEEE standard (https://ieeexplore-ieee-org.uml.idm.ocl ... nt/5306743), it appears to me that the 802.3at implementation must support up to 600mA. Mikrotik advertises that PowerBox Pro, CRS112-8P-4S-IN, hEX PoE all support 802.3at but this does not seem to be the case if they only provide up to 450mA of current. I purchased these PSE with the understanding that they could power up devices requiring 802.3at implementations and they are failing to do that.

Unless I am overlooking something? Perhaps somebody else could she some light on this.
 
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Znevna
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Re: Only 450mA PoE output for 802.3at?

Mon Jan 24, 2022 9:21 pm

Max out per port output (input 30-57 V) 450 mA
I don't see how you could expect 600mA when they specify the max supported.
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knat53
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Re: Only 450mA PoE output for 802.3at?

Mon Jan 24, 2022 10:58 pm

What I expected is that the switch would be able to power all 802.3at devices since the specs for CRS112-8P-4S-IN explicitly state "autosensing 802.3af/at PoE/PoE+ and Passive PoE."

Does this mean that the CRS112-8P-4S-IN is not fully compliant with 802.3at?
 
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Re: Only 450mA PoE output for 802.3at?

Mon Jan 24, 2022 11:14 pm

On the same page, from where you took that autosensing line, at https://mikrotik.com/product/crs112_8p_4s_in
You find:
Max current is 1 A per port if input voltage is 18-28 V, 450 mA if 48-57 V. Total limit is 2.8A@24V and 1.4A@48-57V.
Are your findings different than the values mentioned?
Last edited by Znevna on Tue Jan 25, 2022 9:09 am, edited 1 time in total.
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knat53
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Re: Only 450mA PoE output for 802.3at?

Mon Jan 24, 2022 11:31 pm

On the same page, from where you took that autosensing line, at https://mikrotik.com/product/crs112_8p_4s_in
You find: Max current is 1 A per port if input voltage is 18-28 V, 450 mA if 48-57 V. Total limit is 2.8A@24V and 1.4A@48-57V.
Are your findings different than the values mentioned?
Most PDs don't specify current draw, but instead just state max power consumption and that they require 802.3at. According the CRS112-8P-4S-IN documented specs, the switch can provide sufficient power for the PD and it's stated that it implements 802.3at but often there is no way to know if the current output is insufficient until it is tried.
 
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Re: Only 450mA PoE output for 802.3at?

Mon Jan 24, 2022 11:42 pm

Reading the full specs of both devices (PSE and PD) usualy takes care of such problems.
Also:
However, 802.3at allowed Type-1 PSE’s to evolve in ways that gained many of the IEEE 802.3at feature enhancements described above even if they continue to limit minimum output power to the 15.4 watt range.
I see nothing wrong here.
For easy math you can use this: https://www.rapidtables.com/calc/electr ... lator.html
Per the specs provided by MikroTik:
Screenshot_20220124_234912.jpg
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mkx
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Re: Only 450mA PoE output for 802.3at?  [SOLVED]

Tue Jan 25, 2022 12:01 am

The problem with spec sheets specifying power consumption is that it's not easy to map that to standard. Standard specifies max current (350 mA for af, 600 mA for at) and also specifies voltage ranges both on output of PSE and input of PD. And indirectly also maximum voltage drop on cable between them. So you need to calculate back and forth between Amperes, Volts and Watts while taking real numbers into account (actual supply voltage, actual voltage drop on cable which depends on actual current draw). Or you can go with worse case from standard: 12.95W max for af or 25.5W max for at but at the same time observing also max allowed voltage drop.

But: if PD requires say 14.5W, does it require af or at? It depends: if cable is short, then af will suffice. With longer cable it requires at.

BTW, at standard allows max 57V. If one manages to get hold of 57V power adapter for MT and considering 450mA limit, this translates to 25.65W ... which is only so slightly above minimum required power at PD for at (25.5W). So with a short cable and when using 57V power adapter, Mikrotiks can qualify as supporting at. OTOH at standard requires PSE output voltage between 50V and 57V which is not possible achieve when using power adapters sold by Mikrotik (nothing above 48V).

All in all, support for IEEE PoE on Mikrot8k devices is far from ideal ...
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knat53
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Re: Only 450mA PoE output for 802.3at?

Tue Jan 25, 2022 12:12 am

Yes, the PD is drawing around max 530 mA at 48V, which translates to a power consumption of 25.4 watts. If I used a 57V power supply, then at 450mA I could provide 25.7 watts of power, which most of the time would be sufficient. However, the PD specs state a potential maximum consumption of 33 watts. I haven't observed this much power consumption but it's possible according to the PD manufacturer. Unfortunately, the CRS112-8P-4S-IN is inadequate even with a 57V power supply as that would only provide 25.7 watts max.

The 48V power supply is hard enough to find. Where can I find a 57V adapter?
 
knat53
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Re: Only 450mA PoE output for 802.3at?

Tue Jan 25, 2022 12:17 am

The problem with spec sheets specifying power consumption is that it's not easy to map that to standard. Standard specifies max current (350 mA for af, 600 mA for at) and also specifies voltage ranges both on output of PSE and input of PD. And indirectly also maximum voltage drop on cable between them. So you need to calculate back and forth between Amperes, Volts and Watts while taking real numbers into account (actual supply voltage, actual voltage drop on cable which depends on actual current draw). Or you can go with worse case from standard: 12.95W max for af or 25.5W max for at but at the same time observing also max allowed voltage drop.

But: if PD requires say 14.5W, does it require af or at? It depends: if cable is short, then af will suffice. With longer cable it requires at.

BTW, at standard allows max 57V. If one manages to get hold of 57V power adapter for MT and considering 450mA limit, this translates to 25.65W ... which is only so slightly above minimum required power at PD for at (25.5W). So with a short cable and when using 57V power adapter, Mikrotiks can qualify as supporting at. OTOH at standard requires PSE output voltage between 50V and 57V which is not possible achieve when using power adapters sold by Mikrotik (nothing above 48V).

All in all, support for IEEE PoE on Mikrot8k devices is far from ideal ...
For reference, where did you find the "minimum required power at PD for at (25.5W)" requirement in the IEEE at standard? Thanks!
 
mkx
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Re: Only 450mA PoE output for 802.3at?

Tue Jan 25, 2022 9:07 am

For reference, where did you find the "minimum required power at PD for at (25.5W)" requirement in the IEEE at standard? Thanks!

As I stated in my post #2 above: the numbers are from wikipedia article. I've no reason not to trust it, you're welcome to verify them by reading actual standard.

And again from wikipedia article: at specifies max power at PSE as 30W (at lowest required output voltage which is 50V) or 25.5W (at lowest required input voltage at PD which is 42.5V). If your camera requires as much as 33W, then this is clearly out of at specs ... so camera should be requiring at least 802.3bt type 3. Which is easier to write than to implement as bt standard requires to use all 4 pairs for PoE (with 600mA max current per pair for type 3).
I agree, finding appropriate power adapter is hard ... for proper at (and beyond) power supply voltage should be at least 51V (allowing slight voltage drop on device input, Mikrotiks with multiple power input options tend to have a diode on power input to block voltage leaks and diodes drop voltage by around 0.4V) ...

I would say that camera is as 802.3at compliant as Mikrotik ... only at the other end of spectrum, sadly (for you as user) they don't seem to overlap much.
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