Solved: 802.3 af standard

sarahr202 · ‎12-21-2008

hi every body!

I have few questions about 802.3af standard.

A link at www.flukenetworks.com about PoE describes:

" A 802.3af compliant powered device is able to accept power either using the alternative A or B"

then in the same paragraph, I find:

" this is not a mandatory option on either the part of the PD or the PSE,however,both must support this feature in order to properly utilize it"

1)What can we be sure about 802.3af compliant PD? that it can accept power using alternative A or B or not?

2)my cisco press book defines classes for PD as follows:

classes of device

class 0 15.4 watt

class 3 15.4 watt

So if a device use 15.4 watt, what class does it belong to ,class 0 or class 3?

3) How class of device is determined?

My book just does not elaborate on the process of determining the class of PD.

I did not find any good link describing the working of that process.

I want to know the detail of the process

If anybody knows a good link about it, please forward to me.

Thanks a lot!

viyuan700 · ‎12-21-2008

"What can we be sure about 802.3af compliant PD? that it can accept power using alternative A or B or not?"

802.3af complaint PD (powered device) or PSE (power sourcing equipment) can accept power using alternative A or B.

Vendor can have speicifc POE technologies but that may not work with 802.3af complaint.

Maybe something like ISL/802.1Q trunking, ISL is specific to cisco so it will work with any cisco device but will not work with other vendor. But different vendor eqquipment can talk using 802.1Q

"How class of device is determined?"

Line below it says default class is used if either the PSE or PS, doesn't support or attempt the optional power class discovery.

That column list maximum power offered by PSE but PD has max power level 0.44 to 12.95 for default class and 6.49-12.95 for class 3.

as per my understanding if PSE or PD doesnot support class 3 then default class is used.

View solution in original post

viyuan700 · ‎12-21-2008

to measure whether a PD needs power or not PSE sends a volatge signal of 2-10V then it measure the impedance.

This process tells the PSE whether it has to send power or not.

Once it decides it has to send power it has to decide how much it needs so again it sends a volatge of 15-20V , PD draws current (volt-amp slope). PSE then adjust it pwer accordingly.

Below link expalins the process.

www.akrossilicon.com/products/docs/poe_overview.pdf