Power Over Ethernet (PoE) implementation

I'm reading about PoE and I wonder why there are 2 alternatives (A and B) to implement a PSE. Which are the benefits and drawbacks of each of them?

I am designing an end-span Ethernet switch with PoE (PSE) and I don't know which alternative is better. The IEEE 802.3af allows a PSE to use either of the two alternatives, but not working simultaneously. So, products the like in the market, which alternative do they implement?

It is cheaper to implement arternative B because you don't need a special transformer. So, what's the point in using alternative A?

Regards, Chesi

Reply to
chesi
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(snip regarding power over ethernet)

This is commonly done for TV antenna amplifiers, including large and small satellite dish amplifiers. Though in many cases it is done with

60Hz AC instead of DC to avoid electrochemical problems in a damp environment. I am not sure where people use PoE, but it might also be true there.

At TV frequencies it isn't hard to do with LC or RC filters. It is harder to do with ethernet and keep that signals balanced. I think my choice would be center tapped transformers, though I believe it has been suggested that it isn't so easy.

This reminds be of a discussion before about whether 10baseT will work with a 120VAC common mode voltage on the pair. I never got around to actually (and carefully) testing it, though.

-- glen

Reply to
glen herrmannsfeldt

Can you point us to descriptions of the two options?

Reply to
William P. N. Smith

Ah, found it at

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/* The spec does not allow both sets of wires to be used ? a choice must be made. The Power Sourcing Equipment (PSE) applies power to either set of wires. The Powered Device (PD) must be able to accept power from both options.

*/

If you are designing a gigabit switch I don't think you have any choice, as you don't have any spare pairs. I suppose you could read this as allowing transformer-coupled power over the 4/5 and 7/8 pairs, but if you are going with transformer-coupled PoE, why not do it over the 1/2 and 3/6 pairs and avoid the confusion?

If it's only a 10/100 switch you could probably get away with the 'spare pairs' thing, but I'd have to believe that phantom-power Ethernet magnetics are going to be as easy to come by as the regular type in the near future, and the phantom-power over 1/2 and 3/6 'feels' more compatable to me.

Dunno why they allowed the spare-pairs thing, must be some backwards-compatability thing to some pre-standard kludge.

Doesn't someone use phantom power on all pairs to get 26W at the terminal device? I suppose that's out of spec as well...

Reply to
William P. N. Smith

Also found

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which tries to make it sound like switches use phantom power and midspan injectors use spare pairs (and "PowerDsine High Power Midspan" does both simultaneously).

Reply to
William P. N. Smith

same is true for 10/100 that use Telco connectors (12 interfaces on a 50 pin connector) - no spare pairs.

there are implementations that support more than one type of PoE - a lot of the cisco stuff supports their prestandard system as well as 802.3af (they more or less have to since a fair number of cisco end points like IP phones havent been updated to 802.3af).

FWIW i cant see why you could put both flavours of 802.3af into a switch and let end point discovery switch on the right version on a per port basis?

after all plenty of networks are built as multi vendor and the last thing a switch manufacturer is going to want to do is to reduce the size of their potential market.

i seem to remember that the spare pairs system was meant to be easier to build for in a simple device (at the client end i think) - but it cant be used everywhere so we end up with 2 flavours.

Reply to
stephen

I have also seen an installation where a single 4 pair cable is used to feed two 10/100 connectors. Again, no spare pairs.

Reply to
James Knott

There may be situations where the spare pairs aren't available.

Reply to
James Knott

According to the spec, linked to in another note, the powered device must be able to work with either method.

Reply to
James Knott

I'm not sure I'm parsing the above properly, but there's no need to do both, as the spec (apparently) says the end-point has to support both, so the switch only has to provide one.

[Plus or minus support for the pre-standard stuff, and the post-standard high-power stuff... That's the wonderful thing about standards, there are so many to choose from!]
Reply to
William P. N. Smith

The magazine EDN had a very clear article on the subject not so long ago.

formatting link
and look for PoE :-)

Reply to
Gerard Bok

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