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


The standard already mandates a polarity on Alt-B pins at the PSE and allows either polarity on Alt-A pairs depending upon the MDI/MDI-X data configuration.  With auto-negotiation capability, a PSE can be either MDI or MDI-X, and likewise, PSE's exist in each of the allowed Alt-A polarities. 

Table 33-2 (PSE Pin-out alternatives), does have an issue where post-negotiation, a connection may exist that doesn't align with the table because the data can swap MDI/MDI-X configuration and the PoE polarity doesn't.  Specifying the polarity of the pins rather than making it dependent on the data connection would resolve this and would provide the known polarities being discussed.  For example, it could be 1,2,7,8 Negative and 3,4,5,6 Positive for very high power PSEs.  This would be fully backward compatible and would allow the use of PDs without bridges, even if they are outside the scope of the standard.  More importantly, it would allow swap-out of equipment in such installations in the future.

Best Regards,

On 2/7/2014 9:07 AM, Christian BEIA wrote:


Thank you very much for brainstorming, and finding out pros and cons of this subject.


I would like to state again my position: My only goal is to have a defined polarity at the PSE output, which may allow well-defined systems to work without a full input bridge.

I don’t want to get rid of diodes at all, I like the plug&play feature, and as a customer I don’t like to bother about the right cable to use.


But, anyway, what we are facing today is a new power standard, for application which will need more power than ever.


For this new application there will be new PSEs and probably need for better cables.


In .at we already specified CAT5E as minimum requirement, so CAT3 cables might not work for 25W applications.

What is the difference between this requirement, and the new one for .bt, which might be: use CAT6 , straight, 37-max bundle-sized cables if you want to reach 99.9W?


Then I would like to address also other points from Jeff and Jean:


In general all the reverse-polarity issues can be solved with a single diode (FET) at PD input. To drive a low-side FET (on the lower voltage leg) is simpler than a FET bridge, and the losses are halved.


About backfeed voltage and auxiliary sources I see some issues with FET bridge also (it is currently an issue). It won’t stop the reverse current automatically. So the requirement in the standard must stay there. It will be a system implementation issue.


Thank you


Best regards




From: Darshan, Yair [mailto:YDarshan@xxxxxxxxxxxxx]
Sent: Friday, February 07, 2014 12:52 AM
To: STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx


Very good point Jeff regarding the back-feeding that can mess detection.



From: Jeff Heath [mailto:jheath@xxxxxxxxxx]
Sent: Friday, February 07, 2014 1:21 AM
Subject: Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx


This is a very good point Jean. 


I would add we should also consider the back feeding from a 2 pair connected .bt PD to the unpowered pair (e.g. midspan case).




Jeff Heath
Design Center Manager

Description: Linear Technology Corporation




402 East Carrillo Street, Suite D


Santa Barbara, California 93101







Thank you.


From: Picard, Jean [mailto:jean_picard@xxxxxx]
Sent: Thursday, February 06, 2014 3:10 PM
Subject: Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx


Hi everyone,


I think there is one important case that is overlooked in this discussion.


There are a LOT of PD equipments  using a DC adapter input, which means it can be powered from PoE or the adapter.


You need blocking diode(s) to avoid the adapter being short-circuited by the PSE power supply through the port connection.







From: George Zimmerman [mailto:george@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx]
Sent: Thursday, February 06, 2014 2:38 PM
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






From: Christian BEIA [mailto:christian.beia@xxxxxx]
Sent: Thursday, February 06, 2014 9:13 AM
Subject: Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx


Thanks Goran, Kenghua,


So, if we agree that Auto MDI-X is a data-only feature, and the PoE voltage will not be affected by this feature, I would propose not to mention Auto-MDI-X when talking about PoE since it creates confusion.  


The only reason to mention auto MDI-X is to say that this feature makes crossover cables not needed anymore. And I know it is very widely-adopted feature.


For a brand new installation, for high power PoE application, we are specifying cabling performance anyway (cable category, bundle size etc..).

So, it seems to me that specifying to use straight cable would be fair as well. It would add no cost, and it would make lose no feature.


I’m not trying to get rid of active bridge in application where plug&play and backward compatibility are requested and appreciated feature!


I would like to specify a single voltage polarity at new 4PPOE PSE output, so that the new 4PPOE PD would not need diode bridges for high power application (e.g. > 51W)


Doing so, there will be no need for designing PSEs with voltage auto-selection as Goran is proposing.

I think this would add extra-cost  and complexity to the PSE, and also could impact system efficiency since extra-switches are needed.


Best regards






From: Chuang, Keng Hua (HP Networking) [mailto:kenghua.chuang@xxxxxx]
Sent: Thursday, February 06, 2014 4:00 PM
Subject: Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx




It is data-only feature. The transceiver will cross the data path internally but the physical connection of the power delivery path remain the same.




From: Hauer, Goran (Goran) [mailto:ghauer@xxxxxxxxx]
Sent: Thursday, February 06, 2014 5:44 PM
Subject: [802.3_4PPOE] SV: [802.3_4PPOE] Are diode bridges really needed (2).pptx


Hi Christian, all,


I would assume that it is a data feature only.


However, it would be really nice to remove those rectifying bridges from the PD, to both increase efficiency and reduce cost and complexity in the PD.


I have taken a look at what could be done at the PSE side.

If you could consider that a 4-pair powered device (PD) would accept power a bit differently from the old PoE standard, by instead powering the pairs 3,6 and 4,5 with the negative voltage and the pairs  1,2 and 7,8 with the positive voltage, the polarity at the PD side wouldn't change regardless of straight cable, "Val's Crossover Cord" or "Goran's Crossover cord".

The PSE would then need to be able to swap polarity between two of the pairs (for instance between pairs 4,5 and 7,8) to maintain compatibility with the old PoE standards.


When typing this down, I came to think about that it could maybe be solved even more elegantly by supplying the pair 4,5 with the negative voltage and the pairs  1,2 and 7,8 with the positive voltage regardless, and after identification of the PD type, the PSE would either power the pair 3,6 with positive voltage for an old (2-wire) PoE PD or with negative voltage for a 4-wire PoE PD. The 4-wire PD would then need to be able to connect the pairs 3,6 and 4,5 together in the powered state. It would also need to be able to rectify the voltage on the pairs 3,6 and 4,5 during detection and negotiation (classification). This reduces the total number of needed transistors at the PSE and PD but adds a rectifier (that only need to support detection and negotiation currents) and a switch (transistor) to the PD. This PSE solution would also only support 2-wire powered PoE PD that accepts power on the data pairs.


There should be other possible solutions to be able to get rid of those rectifying bridges, I would be happy to see some more brainstorming about this.


Best regards,



Från: Christian BEIA [mailto:christian.beia@xxxxxx]
Skickat: den 6 februari 2014 09:53
Ämne: Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx


Thanks Peter, all,


Let’s consider a PSE switch where those transceivers with Auto-MDI feature are present.  

Would it automatically  adapt the port voltage polarity as well? Or is it a data-only feature?


best regards,





From: Peter Johnson [mailto:peter_johnson@xxxxxxxxx]
Sent: Wednesday, February 05, 2014 7:14 PM
Subject: Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx




It might be telling that there are 10/100/1000 PHY transceivers that automatically adapt to crossover (Auto-MDI) with BOTH the "data" pairs (1,2/3,6) and the "spare" pairs (4,5/7,8) independently.



Pete Johnson




-------- Original Message --------
Subject: Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx
From: Christian BEIA <
Date: Wed, February 05, 2014 10:51 am

Hi Val,

a “4-pair crossover” is a cable which inverts voltage polarity on both 1,2 / 3,6 pairs and  4,5 / 7,8 pairs.

In other words 1,2 pair at one end is connected to 3,4 at the other end, and 4,5 is connected to 7,8.


I don’t know if such a cable complies with any standard, but it seems that exists on the market.



did I understand correctly?


Bet regards




From: Valerie Maguire [mailto:Valerie_Maguire@xxxxxxxxxx]
Sent: Wednesday, February 05, 2014 4:21 PM
Subject: Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx


Hi Christian:

What is a “4-pair crossover” cable?  Is a modular cord that is wired T568A on one end and T568B on the other?  If so and this cord is used as designed, it would ensure that the same pair combination is "split" (e.g. terminated to pins 4,5 and 3,6) on both ends of the channel.

All the best - Val

From:        Christian BEIA <christian.beia@xxxxxx>
To:        STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx,
Date:        02/05/2014 01:55 AM
Subject:        Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx

Thank you Goran,
I was not aware about this case.
Is there anyone who can tell if those “4-pair crossover” cables are standardized or not?
Thank you
Best regards
From: Hauer, Goran (Goran) [mailto:ghauer@xxxxxxxxx]
Friday, January 31, 2014 4:41 PM
Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx
Hi guys
I haven't participated in any meetings live, but I have been subscribing to this list for quite some time. Great work you guys!
When I saw this presentation however I must say that I have seen (and used) crossover cables intended for 1000BASE-T that crosses all four pairs, whether they are standardized or not that I can't tell, but they are out there.
Best regards,
Från: Chad Jones (cmjones) [mailto:cmjones@xxxxxxxxx]
den 31 januari 2014 16:14
[802.3_4PPOE] FW: Are diode bridges really needed (2).pptx
Christian, the reflector should be able to handle attachments up to 2MB in size.
All, I am forwarding this on behalf of Christian. He is looking for some help to decide if we can pursue removal of auto-polarity in the PD.
Christian, I am going to forward this to Pat Thaler (I’m not sure that she’s subscribed to our reflector). I will copy you. It was her comment in AF that eventually led to us mandating autoMDIX support.
Chad Jones
MGR, HW ENG, Cisco Systems
Chair, IEEE P802.3bt 4PPoE Task Force
From: Christian BEIA <christian.beia@xxxxxx>
Friday, January 31, 2014 at 6:02 AM
Yair Darshan <
YDarshan@xxxxxxxxxxxxx>, Fred Schindler <fred@xxxxxxxxxxxxxx>, Dave Dwelley <ddwelley@xxxxxxxxxx>, "Picard, Jean" <jean_picard@xxxxxx>, Chad Jones <cmjones@xxxxxxxxx>
"Yseboodt, Lennart (
lennart.yseboodt@xxxxxxxxxxx)" <lennart.yseboodt@xxxxxxxxxxx>, "Abramson, David" <david.abramson@xxxxxx>, "Kousalya Balasubramanian (kousalya.balasubramanian@xxxxxxxxx)" <kousalya.balasubramanian@xxxxxxxxx>
Are diode bridges really needed (2).pptx

Hi Guys,
I’m trying to find out if having a defined voltage polarity at the PD PI is an achievable goal.
Please look at the short presentation in attachment. It summarizes the spec we have today.
After this brief research, my understanding is the following:
About Alt-A: reverse polarity at the PD happens only if a MDI-X cable is used and there is no auto-detect mode
About Alt-B: reverse polarity at the PD should never happen.
Are those assumption correct?
If they are correct, I would like to understand if auto-detect mode is a common feature or it is something “advanced”.
I can imagine that a Midspan probably won’t have an auto-detect feature. But what is the reason for using MDI-X cables between the midspan and the PD?
Thank you for any comment
Best regards
I’m not using the reflector since I’m not sure I can attach presentations there.
If it is ok I will forward this email to the reflector.

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