Re: [8023-POEP] 2P v 4P and safety
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
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
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.)
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