Re: [8023-POEP] 2P v 4P and safety [Unscannable attachment] [Unscannable attachment]
Hello Yair and Geoff -
My two cents worth.
Do we really have to account for every possible configuration, or can we just choose to SET the standard and requirements for running PoE plus and govern the use. i.e. You could choose not to have your application worry about 2x2Pair installations!
Geoff is right, many cabling folks make adapters to share sheaths, but I would not call that common in the field. At least in the US, cabling designers and installers have been well educated regarding the need for multiple outlets and multiple cables, and have been following the guidelines of TIA and organizations like BICSI for may years. Many people are moving towards Gig anyway, and making sure their cabling plants support that. I would expect you'll find shared sheath is only done in rare cases using adapters, and people with 2-pair per outlet are probably those with older installations and are either due for an upgrade for 1GBASE-T or to overcome the limit in current capacity of lower grade cable.
Finally, the cabling folks have been working on a DC Power document (shared with PoEP group) which sets the current limit per conductor to 420 mA on Category 5e and 6 (Class D and E) and 315 mA for Category 3. This is well within the safety limit, but is also expected to maintain transmission characteristics. The key for the cabling folks is to ensure minimum temperature rise so reduce aging and insertion loss degradation. The limits could be increased if a higher grade of cable is used.... and the cabling folks would probably LOVE to see you spec Augmented C6 (already needed for 10G) for PoEP :)-
With these limits in mind, you may find that all three options (1x4P-HP PD, 2x2P-HP PDs or 2x-802.3af PDs) will converge as far as concern about capacity.
I can't comment on detection considerations, as I don't have an idea of the complexity involved.
Regards,
SAA
>>> Yair Darshan <YairD@xxxxxxxxxxxxxx> 6/8/05 3:40:36 AM >>>
Hi Geoff
Thanks for the reply.
Yes, I understand that it is no law issue. When I use "legal" I meant
conformance to the relevant standards.
According to your last conclusion, I understand that splitting 4 pair
cable to 2 outlets (each outlet gets 2 pairs) is valid scenario which
PoEp has to address too.
Now we will have the following PD options:
High Power 4P PD
Or two High Power 2P PD's
Or two IEEE802.3af PDs
Do we really want detect and power all options or we wish to detect and
power only 4P or 2P high power or single IEEE802.3af PD and only prevent
damage to two IEEE802.3af PDs connected to the same cable and leave the
decision if to power it or not to the system (implementation specific
etc?).
Yair
________________________________
From: Geoff Thompson [mailto:gthompso@xxxxxxxxxx]
Sent: Tuesday, June 07, 2005 6:56 AM
To: Yair Darshan
Cc: Geoff Thompson; STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: RE: [8023-POEP] 2P v 4P and safety [Unscannable attachment]
[Unscannable attachment]
Yair-
"legal" vs. common...
There is no matter of law involved, only conformance with voluntary
standards.
If you insert a splitter into a single outlet then there is no
conformance issue.
If you split a cable to 2 outlets you are not conformant with TIA-568.
The US has always "required" that you run a single 4-pair cable to each
RJ-45.
The International Standard, ISO/IEC 11801 does not require this. It was
an international fight of long standing. The German national body and a
major international connector manufacturer (US headquartered) bitterly
opposed mandating 4 pair per outlet. Therefore, 2 pair is allowed.
I hope this helps.
Geoff
At 02:46 AM 6/5/2005 , Yair Darshan wrote:
Hi Geoff an all,
Are all the examples you have shown of using the same cable for two
outlets are "legal" within the IEEE802.3?
I guess that if it is not "legal" but people are doing it any way, we
have only to make sure that damage will not be caused but we don t have
to support and detect those cases?
Yair
________________________________
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Geoff Thompson
Sent: Saturday, June 04, 2005 2:24 AM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] 2P v 4P and safety [Unscannable attachment]
Steve-
It is not at all uncommon, particularly in older installations, to have
a single 4-pair cable run to a box with 2 outlets. The idea being that
you could serve 2 applications (e.g. telephone, 10 Mbps, 100 Mbps) with
a single cable. Connector manufacturers also sell a breakout adapter to
do the same thing, i.e. bring out the pairs from a single RJ-45 to each
of two RJ-45s with the correct pinouts for 10/100 Mbps Ethernet.
In the scenario that you describe, someone could could plug a breakout
adapter into an outlet that had one of the following:
1) 2 separate 10/100 Ethernet connections in a single 4-pair
sheath, each connection of which had a conventional 2P 802.3af PoE (or
PoE Plus) provision. This would be used to connect to 2 different 10/100
2P PDs.
2) A single 10/100 Ethernet connection with 4P PoE Plus. This
would be used to connect to a single 10/100 2P PD and an additional 2P
PD that has no data connection.
I hope this helps.
Geoff
At 10:04 AM 6/3/2005 , Steve Robbins wrote:
Wael and all,
In regard to the question, should a 4P PSE = 2 independent 2P PSEs, I
got an interesting question from someone a couple of days ago,
suggesting an application I hadn't thought of.
Basically, the guy was asking the following: Suppose you had a 4P PSE.
Could he make some sort of passive splitter that would allow him to
power two independent 2P PDs? It sounded like the splitter he was
thinking of was just some RJ45 connectors and some wire. He told me
there is already some similar gadget on the market (from Siemon?) that
allows data to be split somehow, and he was wondering if power could
also be split.
Of course I told him I didn't know the answer to that, because a 4P PSE
hasn't been defined yet. But obviously, if you combine an endspan
802.3af PSE with a midspan 802.3af PSE, then you have a system you could
split easily.
I don't know what the specific application is, or if anyone else would
be interested in this. But it might be something to think about in the
2P vs 4P debate.
The main problem with defining 4P=2x(2P) is that it basically forces the
PD to have two separate PD controller circuits, and two separate DC/DC
converters. This increases the cost of the PD, which understandably,
the group is loathe to do.
But the alternative (using diodes in the PD to passively combine the
power from all 4 pairs, into a single DC/DC converter) is messy too. As
Clay Stanford from Linear Tech showed in his presentation at the Austin
meeting, you'd have to actively balance the currents at the PSE end.
Otherwise, small differences in wire resistances and/or diode drops
could lead to big pair-to-pair current imbalances. So it appears this
approach makes the PSE significantly more complex and costly, which the
group is also loathe to do.
The lack of an attractive 4P solution is the main incentive for
considering high-current on 2P. But then of course you get into other
issues.
That's basically where the debate stands right now, with no clear
answers yet, although most people have an opinion.
Steve Robbins
-----Original Message-----
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx]On
<mailto:owner-stds-802-3-poep@xxxxxxxx%5DOn> Behalf Of Wael Diab
(wdiab)
Sent: Friday, June 03, 2005 5:03 AM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] 2P v 4P and safety
Hi Steve, Yair, Chad and all,
Ive been trying to catch up on this thread, which brings up many good
points. One thing that I am not sure we have exhausted yet is Steve's
original point #6 (repeated below for convinience so that you dont have
to scroll through the entire thread).
I dont understand why that would be more complex than just picking one.
Today, in .af, the PD has to accept power on either pair. Increasing
power via a 2P scheme would be similar, with hopefully enhancements to
the classification to differentiate it from leacy 2P power.
Additionally, the PD may have the option to be in yet a higher power
class and draw power over all 4P. In that case we would have to address
the balancing issues that were touched on (but we would have to do that
anyway if we went with 4P regardless).
So we have PSEs that can support a higher power mode over 2P and others
that can go further and support even more power over 4P. In my mind
these could address different needs in the market and I would hate to
have to just pick between one power point and the other unless I really
have to.
As an aside, I think the questions of whether or not 4P = 2x(2P) and how
much power can be drawn over 2P max and 4P are both good ones, but I
would be interested in anwering these first before pre-empting which
scheme we go with and/or whether we allow both options in the standard.
here is the original point so that you dont have to scroll all the way
down
6. Suppose PoE Plus allows both 2P and 4P. How do we define this
without getting into several
different types of PSE and PD, with all the complexity of hooking
them together? I think the
answer is simple: We define a 2P system, and then say that it's
legal to have one or two of these
on the same cable. So, two independent 2P systems make a 4P
system. I think this is really the
only way to go, but of course I'm open to other ideas.
wael
________________________________
From: owner-stds-802-3-poep@xxxxxxxx on behalf of Yair Darshan
Sent: Fri 6/3/2005 1:22 AM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] 2P v 4P and safety
Hi Steve and all,
I absolutely agree with you that the complexity difference between 2P
and 4P are narrow due too the other problems that we have with 2P higher
current.
It is worth to check the advantages of detecting broken wires compared
to other alternatives in order to decide what alternative is good enough
and keep
reasonable cost and reliability for the most popular installations.
Yair
________________________________
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Steve Robbins
Sent: Thursday, June 02, 2005 9:23 PM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] 2P v 4P and safety
I totally agree that we need help from the cable group to determine more
realistic current limits.
When I started this email chain, I was looking for some way to decide
between 2P and 4P on the basis of complexity and cost. Everyone knows
that 4P is more expensive, because it requires more circuitry to switch
and current-limit the extra pairs. But I realized that a 2P system
might need extra circuitry too, if it was pushing the current closer to
safe limits. (That's how the safety issue got included.) So the
complexity/cost gap between 2P and 4P is narrowed by this argument.
That's the main point I was trying to make.
But now I see all this discussion has led to another big point:
Worst-case analysis is greatly simplified if we have circuitry that
detects broken wires, because then we only have to consider one case,
where all wires are carrying nearly equal currents. Without detection,
broken wires just makes a mess of any worst-case analysis. Basically,
where do you stop? How many broken wires are reasonable?
Unfortunately, I don't think "one" is a reasonable answer because if a
cable suffers damage sufficient to cut one wire, there could very well
be two cut. There appears to be a large number of possibilities and I
don't see any good way to trim it down to a manageable few, unless we
rule them all out by making the silicon a little smarter.
-----Original Message-----
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx]On
<mailto:owner-stds-802-3-poep@xxxxxxxx%5DOn> Behalf Of Chad Jones
(cmjones)
Sent: Wednesday, June 01, 2005 8:16 PM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] 2P v 4P and safety
Time to throw into this conversation:
What I read in this email chain is that we need to limit the current on
the PAIR to the limit of one conductor, presently quoted as 420mA. I'm
going to guess that only incrementally raising the current from 350mA to
420mA and raising the min voltage to something like 51V is going to
cause a decent number of present participants to drop out. This is only
21.4W at the PSE for two pair (and 42.8W for 4P). It will certainly
cool my interest in the project. I will also reiterate stated goal of
pushing the physical limits of the cable.
We definitely need help from the cable group with this project. One of
the questions I have is how they derived the 420mA number. Does it
change if we only use 2P in a 4P cable? Also, does it change if only 1
wire in a pair is carrying current (like when one conductor breaks)?
How much margin is built in (in other words do we have use their number
as the max or do we also add margin on top of it)?
Last comment, I would not be opposed to mandating current balance
monitoring within a pair and between the pairs for PDs that need the
high power modes that could cause this 'safety hazard'.
- Chad Jones
________________________________
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Yair Darshan
Sent: Wednesday, June 01, 2005 4:04 AM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] 2P v 4P and safety
Hi Steve and all,
I basically agree with you regarding the situation you have described.
(One wire broken==>Current is doubled ==> temperature rise increased ==>
cable and/or connector parameters may be impaired==>
we need to sense this situation and protect the infrastructure).
Actually we are protecting the infrastructure in the IEEE802.3af by
limiting the current, time and duty cycle. However, we are not sensing a
broken wire in order to protect from doubling the current from the
simple reason that it is not needed IF the time duration peak current
and duty cycle of the event kept low enough so increase in temperature
rise will not happen. The numbers of the IEEE802.3af were chosen in
order to ensure this behavior (there are additional reasons for the
numbers that we have decided in the IEEE802.3af and the end result was
integration of all argument to get reliable operation while keeping the
cost of silicon down)
I guess that we should aim to the same concept in our new project i.e.
set the numbers that in case of failure keep the infrastructure
parameters in their operation range without additional sensing and
features in order to keep the system simple and low cost.
Probably we will need to revisit the protection mechanism by optionally
adding a sense function that will alert of broken wire due higher
current in 2P concept compared to the IEEE802.3; however we need test
results in order to decide the way to go and some of us working on
getting lab data.
Other comments:
1. PD has to have IEEE defined components (PHY or others) otherwise it
is not a compliant PD. PD is a powered DTE. DTE is a device defined by
IEEE802.3. Hence PD must meet IEEE802.3. We don't care about other
implementations that are using PoE technology and are not IEEE802.3
compatible.
2. You are right that a link is not required at the first place;
actually according to the spec PSE operations are independent of data
link status (33.2.4) however the spec allows wide flexibility for
specify implementation specific errors under the variable
"error_condition" which will results by power shut off.
In other words if A PSE vendor/ System designer decides that the PSE
sees something that is not normal (he decides what is normal or not; he
is the vendor and he should know.) and it is not specifically covered by
the spec, (there are many scenarios that meets that definition) it can
be considered "error_condition" as specified by the state diagram.
According to the above, if during operation system reports to PSE that
"something bad has happen" (by getting information that data was good
and now is dead and current is higher than normal for example) PSE is
encouraged to do something about it.
3. I agree with the statement that true worst-case conditions in a 2P
system must assume one wire is broken. In 4P, if we stay around the AF
currents that we don't have to take in consideration that we have
problem if one wire was broken due to the fact that this condition was
tested and we don't have problems with it. If in 4P we will have higher
current than 350mA than we have to test again.
Yair
________________________________
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Steve Robbins
Sent: Wednesday, June 01, 2005 4:12 AM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] 2P v 4P and safety
Yair and all,
Perhaps I could make this a bit clearer. Here is a hypothetical
situation: Suppose we determine that CAT5 cable can carry 800mA per
twisted-pair (400mA per conductor), and under worst-case conditions (max
amb temp, big bundle, etc.) the wire temp stays 10C below the point
where the cheapest insulation starts to soften. So we set I_CUT to
810mA, and it seems safe.
Now suppose that we have actual worst-case conditions somewhere, but
then one wire breaks. The whole 800mA is flowing through one wire, not
two. The wire resistance is doubled, so power dissipation in the
twisted-pair doubles, and the temp rise increases. (I'm not sure that
the temp rise would double, since the power dissipation in the other
pairs hasn't increased. But obviously the wire carrying 800mA will get
hotter, probably significantly hotter.) If it goes up 11C, it exceeds
the softening point.
True, the Ethernet data would be messed up, but who cares? A PD doesn't
even need to have a phy chip. Loss of link is not a reason for the PSE
to shut off power, at least under 802.3af, because link is not required
in the first place. In the above example the load current stays below
I_CUT, so power stays on and the wire temp stays above the softening
point until eventually something bad happens.
The point is, true worst-case conditions in a 2P system must assume one
wire is broken. In a 4P system, I think we'd have to assume one wire
was broken in each of the pairs. (Don't assume broken wires are
statistically independant events. If a wire breaks there is a good
reason, and other adjacent wires could very well be broken too. Once,
when I was at Boeing I had a whole bundle of wires come loose in my hand
when I gave it a small tug. They all broke off right at the connector
pins. Yet every individual connector contact in that bundle had
survived a much stronger pull test right after crimping. A lengthy
failure investigation determined someone had used an improperly
calibrated crimp tool, which made the wire brittle and eventually led to
fatigue. After two years in the field, some were actually broken but
still just barely touching, which led to intermittent equipment
failures.) < br>
Steve
-----Original Message-----
From: Yair Darshan [mailto:YairD@xxxxxxxxxxxxxx]
Sent: Tuesday, May 31, 2005 1:03 AM
To: Steve Robbins; STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: RE: [8023-POEP] 2P v 4P and safety
Hi Steve and all,
Please find attached below few comments on the thoughts presented below:
1. There are basically two roadblocks to higher power:
a. The limited amount of current that can be carried on a single
wire (not a pair of wires).
We must assume that sometimes a wire will break, or a connector
pin will go bad,
so all the current will be on one wire instead of a pair. This
fault condition must define
the limit on how high I_CUT can go, for safety.
b. The affects of heating and current imbalance on the magnetics.
----------------
1a. If a wire or a pin will break the data path will not work due to the
fact that the magnetic will immediately saturate.(Lets assume that at
the worst case all pairs are data pairs such in 1G)
There fore assuming the above condition may not be considered as a worst
case that we have to handle since it is a case that the system is not
working and we don't care what happens as long as we are not causing
damage.
Regarding causing damage:
If one conductor is break, we will have twice the current going through
the other conductor and if we keep the TLIM/TOVLD and duty cycle than
even for 2-4 times the current overload/Ilim numbers we probably will
not have damage issue due to the averaging factor over time.
Connector pin according to its spec need to meet 750mA at 60C. With
cables it is a temperature rise issue which is not an issue for the
overload/ILIM timings and duty cycle.
Regarding safety issue: There is no safety issue here in terms of human
safety. There is an issue of "Infrastructure safety or equipment safety"
in terms of damage however as mentioned above, with TLIM/TOVLD and duty
cycle limitation the equipment is kept un damaged and "safe".
1b. Yes, this is one of the major issues in 2P.
--------------
2. To make sure we get it right in terms of safety, I propose:
a. The objective regarding the goal of higher power should be
amended to include
a few words about safety.
b. Let's look for a worst-case scenario to keep in mind while we
work on the standard.
The best one I've thought of so far is the airline industry.
Assume PoE is used onboard
a jet so that passengers can surf the net or watch DVDs in
flight, without the batteries
in their laptops dying. How hot do you think the FAA will
allow the wires to get? How
much current can a single conductor (in a bundle) carry before
it reaches that temp?
-------
2a - We don't have a mandate to define safety (human safety) in our
standard. We need to address
the reader to current safety standard.
We should address issues that have impact on keeping the
equipment and infrastructure undamaged.
2b - I agree that we should look for the worst case HOWEVER the worst
case scenarios should be real cases and not corner or theoretical cases
in order not to increase the cost of solution or eliminating some
implementation or support some applications.
Regarding the example presented: I guess that it is not the worst case
since in aircraft designs there are a lot of margin in the design
(Aircraft...) so in my opinion it is not representing the worst case.
--------
3. I thought about Clays presentation a lot over the weekend. It
seems to me that reaching
higher power (>30W) will require two things:
a. Current imbalance sensing to detect broken wires, so we can
increase I_CUT beyond
what a single conductor can handle, without compromising
safety.
b. Active current balancing to keep the magnetics happy. Although
this won't solve the
heating problem.
------
3a. The intent is not clear. Do you mean current balance in 4P? If this
is the case than Yes, we need to address this issue and your suggestions
sounds reasonable.
3b- Active and passive current balancing to the magnetic was discussed
in the AF and we didn't want to force this requirements in the standard
due to the following reasons:
- The vendor should keep his system working under his equipment
spec. How he does it is implementation specific.
- Forcing a requirement for active/passive current balancing
will increase the port cost while in most cases it is not required
(above some cabling length there is no issue)
Actually we address this issue in the informative part of the standard
and let the designer to decide how to handle the worst case situation.
In our new project doing active current balancing will dramatically
increase the port cost so the the 2P concept will cost much higher that
the 4P... In addition as you have mentioned, the heat issue still exist.
Bottom line: In my opinion: Current balancing is not cost effective
--------
5. Compare the complexity of a 2P and a 4P system that carry equal
power. The 2P system
would need about twice the current per conductor as the 4P system.
So, while the current on
4P system might just be low enough to avoid the need for ACB/CIS,
the 2P system would almost
certainly need these features. So the complexity difference
between 2P and 4P may not be as
big as previously thought, at least in some medium power range.
For much higher power, 4P
would require ACB/CIS as well, but 2P becomes infeasible because of
safety.
---
5. Agree. From complexity wise, the is no difference between 2P and 4P
high power and in my opinion 4P from system point of view will cost less
than 2P and will be more reliable by far.
(Regarding safety: again there is no safety issue here (the system
may not work... but no safety issue))
----
6. Suppose PoE Plus allows both 2P and 4P. How do we define this
without getting into several
different types of PSE and PD, with all the complexity of hooking
them together? I think the
answer is simple: We define a 2P system, and then say that it's
legal to have one or two of these
on the same cable. So, two independent 2P systems make a 4P
system. I think this is really the
only way to go, but of course I'm open to other ideas.
-----
6. Basically I agree with this concept which is what the 4P concept
(part of it) represents: Let's take two improved AF power channels,
combined them together to have twice or more the power by having the
4P..
The problem that I have with part of your suggestion is that it will
allow at the PSE side, PSE that supports only 2P high power and PSE's
that support only 4P which leads to the undesired environments were we
have many different PSE's and PD's that will cause us complex system.
I suggest looking at the 4P as the only HP concept that will address all
PD types.
By using this approach, the 4P design will be efficient, and simple.
And when it will be integrated, the difference in chip level between 4P
and 2P will be small enough to stay cost effective.
From system point of view (cost, reliability, flexibility etc) 4P will
represents better cost numbers than 2P.
Yair
________________________________
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Steve Robbins
Sent: Monday, May 23, 2005 7:50 PM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: [8023-POEP] 2P v 4P and safety
PoE Plus Team,
Here are some thought I had over the weekend regarding 2P vs. 4P, and
safety. I checked with Mike, and he wrote that it's okay to post this
on the reflector and get a converstation going.
1. There are basically two roadblocks to higher power:
a. The limited amount of current that can be carried on a single
wire (not a pair of wires).
We must assume that sometimes a wire will break, or a connector
pin will go bad,
so all the current will be on one wire instead of a pair. This
fault condition must define
the limit on how high I_CUT can go, for safety.
b. The affects of heating and current imbalance on the magnetics.
2. To make sure we get it right in terms of safety, I propose:
a. The objective regarding the goal of higher power should be
amended to include
a few words about safety.
b. Let's look for a worst-case scenario to keep in mind while we
work on the standard.
The best one I've thought of so far is the airline industry.
Assume PoE is used onboard
a jet so that passengers can surf the net or watch DVDs in
flight, without the batteries
in their laptops dying. How hot do you think the FAA will
allow the wires to get? How
much current can a single conductor (in a bundle) carry before
it reaches that temp?
3. I thought about Clays presentation a lot over the weekend. It seems
to me that reaching
higher power (>30W) will require two things:
a. Current imbalance sensing to detect broken wires, so we can
increase I_CUT beyond
what a single conductor can handle, without compromising
safety.
b. Active current balancing to keep the magnetics happy. Although
this won't solve the
heating problem.
4. Maybe it's time for an acronym here. How about Active Current
Balance (ACB) and Current Imbalance
Sensing (CIS). They seem to go together, so "ACB/CIS"?
5. Compare the complexity of a 2P and a 4P system that carry equal
power. The 2P system
would need about twice the current per conductor as the 4P system.
So, while the current on
4P system might just be low enough to avoid the need for ACB/CIS,
the 2P system would almost
certainly need these features. So the complexity difference
between 2P and 4P may not be as
big as previously thought, at least in some medium power range.
For much higher power, 4P
would require ACB/CIS as well, but 2P becomes infeasable because of
safety.
6. Suppose PoE Plus allows both 2P and 4P. How do we define this
without getting into several
different types of PSE and PD, with all the complexity of hooking
them together? I think the
answer is simple: We define a 2P system, and then say that it's
legal to have one or two of these
on the same cable. So, two independant 2P systems make a 4P
system. I think this is really the
only way to go, but of course I'm open to other ideas.
Steve Robbins