you very much for brainstorming, and finding out pros and
cons of this subject.
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.
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.
anyway, what we are facing today is a new power standard,
for application which will need more power than ever.
this new application there will be new PSEs and probably
need for better cables.
.at we already specified CAT5E as minimum requirement, so
CAT3 cables might not work for 25W applications.
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?
I would like to address also other points from Jeff and
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.
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
good point Jeff regarding the back-feeding that can mess
is a very good point Jean.
would add we should also consider the back feeding from a 2
pair connected .bt PD to the unpowered pair (e.g. midspan
Design Center Manager
East Carrillo Street, Suite D
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think there is one important case that is overlooked in this
are a LOT of PD equipments using a DC adapter input, which
means it can be powered from PoE or the adapter.
need blocking diode(s) to avoid the adapter being
short-circuited by the PSE power supply through the port
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.
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).
in Advanced PHYsical Communications Technology
NOTE NEW EMAIL ADDRESS. THE OTHER WILL STILL WORK, BUT
PLEASE USE THIS FOR CME BUSINESS)
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.
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.
a brand new installation, for high power PoE application, we
are specifying cabling performance anyway (cable category,
bundle size etc..).
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.
not trying to get rid of active bridge in application where
plug&play and backward compatibility are requested and
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)
so, there will be no need for designing PSEs with voltage
auto-selection as Goran is proposing.
think this would add extra-cost and complexity to the PSE,
and also could impact system efficiency since extra-switches
is data-only feature. The transceiver will cross the data
path internally but the physical connection of the power
delivery path remain the same.
would assume that it is a data feature only.
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.
have taken a look at what could be done at the PSE side.
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".
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.
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.
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.
consider a PSE switch where those transceivers with Auto-MDI
feature are present.
it automatically adapt the port voltage polarity as well?
Or is it a data-only feature?
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.
Original Message --------
Subject: Re: [802.3_4PPOE] Are diode bridges really
From: Christian BEIA <christian.beia@xxxxxx>
Date: Wed, February 05, 2014 10:51 am
crossover” is a cable which inverts voltage polarity
on both 1,2 / 3,6 pairs and 4,5 / 7,8 pairs.
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.
don’t know if such a cable complies with any
standard, but it seems that exists on the market.
I understand correctly?
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.
the best - Val
[802.3_4PPOE] Are diode bridges really needed
was not aware about this case.
there anyone who can tell if those “4-pair
crossover” cables are standardized or not?
Hauer, Goran (Goran) [mailto:ghauer@xxxxxxxxx]
Sent: Friday, January 31, 2014 4:41 PM
Re: [802.3_4PPOE] Are diode bridges really needed
haven't participated in any meetings live, but I
have been subscribing to this list for quite some
time. Great work you guys!
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.
Chad Jones (cmjones) [mailto:cmjones@xxxxxxxxx]
Skickat: den 31 januari 2014 16:14
[802.3_4PPOE] FW: Are diode bridges really needed
the reflector should be able to handle attachments
up to 2MB in size.
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.
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.
HW ENG, Cisco Systems
IEEE P802.3bt 4PPoE Task Force
Date: Friday, January 31, 2014 at 6:02 AM
To: Yair Darshan <YDarshan@xxxxxxxxxxxxx>,
Fred Schindler <fred@xxxxxxxxxxxxxx>,
Dave Dwelley <ddwelley@xxxxxxxxxx>,
"Picard, Jean" <jean_picard@xxxxxx>,
Chad Jones <cmjones@xxxxxxxxx>
Cc: "Yseboodt, Lennart (lennart.yseboodt@xxxxxxxxxxx)"
"Abramson, David" <david.abramson@xxxxxx>,
"Kousalya Balasubramanian (kousalya.balasubramanian@xxxxxxxxx)"
Subject: Are diode bridges really needed
trying to find out if having a defined voltage
polarity at the PD PI is an achievable goal.
look at the short presentation in attachment. It
summarizes the spec we have today.
this brief research, my understanding is the
Alt-A: reverse polarity at the PD happens only if a
MDI-X cable is used and there is no auto-detect mode
Alt-B: reverse polarity at the PD should never
those assumption correct?
they are correct, I would like to understand if
auto-detect mode is a common feature or it is
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?
you for any comment
not using the reflector since I’m not sure I can
attach presentations there.
it is ok I will forward this email to the reflector.
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