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Doubt about AP power consumption

Hi guys,

 

According to the AP-315 datasheet, the worst-case power consumption is 14.4W (PoE+) and 13.6W (PoE). This is the output I got:

 

power1.pngpower2.pngIf the worst-case is 14.4W, why LLDP has negotiated the unneeded 20.8W? And can I assume my AP is consuming around 5.5 W in the last minute?

 

Regards,

Julián

Aruba

Re: Doubt about AP power consumption

The AP will attempt to reserve the maximum amount of power needed, assuming worst-case conditions.

By default, that includes a total of 6.4W to allow for the USB port to source as much as 5W to an attached device.

That explains why the AP is asking for 20.8W.

You could use the ap-poe-power-optimization provisioning parameter to disable USB and only ask for 14.4W.

You are correct that in most cases the actual power consumption in normal use will be significantly lower than the worst-case numbers we document in the datasheet.

/Onno

Re: Doubt about AP power consumption

Hi Onno,

 

What a slip! I didn't realize the datasheet specifies it excludes the power consumed by external USB device as you say:

 

·Maximum (worst-case) power consumption: 14.4W
(802.3at PoE), 13.6W (802.3af PoE) or 12.7W (DC)
--Excludes power consumed by external USB device (and
internal overhead); this could add up to 6.3W (PoE) or
5.9W (DC) for a 5W/1A USB device

 

Taking into account this extra 6.4W, the negotiated 20.8W fits exactly. One more question about this. I planned to take a 2530 24G switch to power 24 AP-315, if I disable the USB as you say and take the worst-case, they would need 24x14.4=345.6W, but the 2530 24G switch has 195W of PoE power. What would happen with the APs if the switch reaches its 195W of PoE power? I suppose they will take power saving measures if IPM is enabled, thought the datasheet says this only applies when powered by an 802.3af PoE source, and in this case the switch is 802.3at and not 802.3af.:

 

- For the 310 Series Access Points, the IPM power-save
feature applies when the unit is powered by an 802.3af
PoE source.

 

Regards,

Julián

Aruba

Re: Doubt about AP power consumption

A couple of things:

- The 14.4W (or 20.8W) is the worst-case consumption of the AP itself. Depending on the type and length of Ethernet cabling you use, the actual consumption per AP could be 1-2W higher. The switch would typically adjust for that when receiving the POE budget request (LLDP)

- The AP has no knowledge of the total powere budget of the switch, so it will not know if the total consumption exceeds the 195W you mention.

- I'm not sure what the behavior of the switch will be when the APs draw more than 195W. The power to one or more switch ports may get disabled?

- 195W gives you a budget of 8.125W per AP. Even with cabling losses, you'll have at least 7.5W average per AP. That may be plenty for many use-cases, but there's a risk.

- When enabled, IPM will simply kick in when the AP exceeds the available POE budget. If the POE source is an 802.3af one (15.4W at the source), it will kick in when the AP consumes over 13.5W. IPM wil be idle when consumption remains below the budget. If the source accepts the LLDP budget, IPM will never kick in.

/Onno

Re: Doubt about AP power consumption

Hi Onno,

 

Thank you for the clarification. Two more doubts:

 

  1. Then, I assume IPM will kick in when the AP exceeds the available PoE budget, when powered by an 802.3af or 802.3at source, although IPM will kick in at different PoE budget thresholds. The datasheet and user guide only mention IPM will operate when the AP is powered by an 802.3af source.
  2. I guess the worst-case power consumption is when the AP has all radio chains enabled, max transmit power, dual-band, many clients, many features enabled, etc., isn't it?

Regards,

Julián

Highlighted
Aruba

Re: Doubt about AP power consumption

1. When using an 802.3at POE source, the power budget is 25.5W. IPM is meaningless, since this AP will never reach the limit of this budget.

2. That is correct, but it also accounte for component and calibration variations, thermal conditions, etc.

/Onno

Re: Doubt about AP power consumption

OK, thank you very much for your interest!

 

Regards,

Julián

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