Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx

Hi George,

I believe Christian's proposal is not really aimed at 'getting rid of (all) the diodes'.
The proposal is to have a fixed polarity at the PSE side in all cases. Whereas a 'reversed' polarity on the data pairs would be allowed in 802.3at, it would no longer be allowed in bt.
That's it as far as the standard goes.

Because the polarity is now fixed, the PD vendor has the _option_ to choose not to implement a rectifier.
For most 'typical' PoE products today that would not make sense: the cost of a rectifier is very low, and supporting all cable configurations has real value.
I would expect that most devices would still implement rectifiers.

For high density PoE applications or applications at the high end of the power scale the situation is different: the rectifier is substantially more expensive and/or lossy and/or bulky.
Lighting is an obvious candidate: installation is done once, in bulk, so no chance of getting cross cables and the efficiency gain is about 1.4% on a typical full power PoE installation.
On large installed power, that is a very significant number.

Lennart Yseboodt
Research Scientist - Solid State Lighting
Philips Research

From: George Zimmerman [george@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx]
Sent: Thursday, February 06, 2014 20:38
To: STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx

Christina -

I would confirm what Keng-hua reports, that I know of many PHYs with the crossover correction, on both sets of pairs independently.  I agree that this feature in the PHYs works data-only, and will not effect powering.  However, because of this feature, we therefore do not see any trouble calls from mis-connected crossover cabling.  Therefore we do not seem to have a good way to see whether the two sets of pairs are crossed over independently in the field.


One reason that the feature is popular is that the equipment vendor no longer has to worry about the configuration of the wiring (with regards to crossovers).  Specifying a specific configuration for 4-pair POE applications seems to be a step backwards, and would lead us back to the days when the end-user would have to have multiple sets of patch cords handy when connecting a new link so that he could make sure he had the right crossover combination at the end point.  Remember that there are several connections in the link topology where a crossover may or may not occur (either by mistake or by design).




George Zimmerman

Principal, CME Consulting

Experts in Advanced PHYsical Communications Technology