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Hello Classmates,

How are you ?

I would like to ask why is PoE even in the objectives of CWNA ? CWNA is wireless and we are talking about wires again ? I have the practice tests and flashcard questions and i see like nearly 60 questions of PoE. I ask myself why is it so relevant ?

Since it is in the objectives and i need to master it, could someone please inform me the following :

How does the process take place, i know that if a PSE finds a 25oms resistence it detects a poe compliant PD, however what is with the signature detection, how does that work, i found no clue ?

Othe question, i saw in two white papers the following remarks for power class:

Class Usage Maximum Power Levels
at Input of Powered Device
(watts)

0 Default 0.44 to 12.95
1 Optional 0.44 to 3.84
2 Optional 3.84 to 6.49
3 Optional 6.49 to 12.49
4 Reserved

However when i fail a question in the practice exam it gives a completely different power class measurements stating for instance 15.4watts for 0 and 3.

What's the difference ?

thanks
rafael

The power levels as you stated:

Class Usage Maximum Power Levels
at Input of Powered Device
(watts)

0 Default 0.44 to 12.95
1 Optional 0.44 to 3.84
2 Optional 3.84 to 6.49
3 Optional 6.49 to 12.49
4 Reserved

are for the amount of power measured at the device (AP) end. The higher level of 15.4 is the power measured at the PSE.

The reason POE is on the exam is that POE is used to deliver power to many deployments of APs and Sensors within the enterprise. The use of POE is part of planning and deploying WLANs. Knowledge of POE is useful in the field.

Hi Bryan,

Thanks for the effort however we are not there yet. I knew about the 15.4w PSE that becomes 12.95w on PD.

How does that loss happen ? You copied the same thing i did while in the practice test it is totaly different. Why 's it different for all class power, what;s right or wrong ?

Also, how does that signature works, any hint ?

thanks again

This is not the answer but a link to the IEEE site to search the standard:
http://grouper.ieee.org/groups/802/3/purchase/index.html
There are a lont of little quirks and what if's inside 802.3-2005 clause 33. Your best bet is to read the standard.

Hello,

Reading about the POE, i faced one question. What happens if the PSE doesn't detect the signature from PD?

I would say it doesn't classify the power class, does not provide the power meaning it is not powered, and the PD is seen as a nonPOE compliant device.

I have read the it gives 15.4W and is classified as Class 0, if it is true, how? wouldn't it burn the nonPOE device? Is this wrong or right ?

thanks

I think you are confusing the detection signature with the classification signature.

For a Powered Device to be 802.3-2005 Clause 33 compliant, it MUST reply with a 25 Ohm detection signature.

However, the classification signature is optional, and if it is not detected, the PSE will classify it as 0 (Default) with a 15.4 Watt reserve.

Hi Collins,

I think i understood it now, however you know the reason it would classify a signature that was not detected or does not exist ? Sounds like weird..., because if a PD is not 802.af compliant then it would be still ranked 0 and receive 15.4w of power ?

thanks
rafael