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Re: [8023-POEP] IEEE 802.3at Vport ad hoc, Oct. 11 meeting materials



Hi Fred,

I'm not sure that I'll be able to attend todays call in person so I wanted 
to comment on the proposals I saw in the slide set 'Vport PSE current 
requirements v3.pdf'.

Thanks and regards,
  David


Comment 184 - subclause 33.3.5.4
--------------------------------

I have a couple of comments in relation to this subclause and the proposed 
change.

[1]  The current text is the draft states 'At any operating condition the 
peak current shall not exceed PPort max/VPort for more than 50ms max and 
5% duty cycle max. Peak current shall not exceed IPort max.'. Now the 
first shall statement yields a peak current of Pport_max/Vport = 12.95/36 
= 360mA with the additional constrains of time and duty cycle. The second 
shall statement simply states that the peak current is 400mA. This has 
always seemed contradictory, particularly since the second shall statement 
has a higher value, yet has no time or duty cycle constrains. I have 
wondered if the intent was that the first shall statement only applies 
after any startup transient - hence the text 'operating condition' - and 
that the second shall statement applied always including during startup. 
But then 'any operating condition' could also be read to include startup 
transients. Regardless these conflicting shall statements should be 
clarified and I don't see that in the proposed change.

[2] Using the formula (Pport_max/Vport) x (400/350) where Pport_max is 
12.95W (Table 33-12, item 2) and Vport minimum is 36V (Table 33-12, item 
1) seems to allow a maximum current of (12.95/36) x (400/350) = 411mA. Now 
when I look at ILIM (Table 33-5, Item 10, PSE output current - at short 
circuit condition) it has a minimum value of 400mA and a maximum value of 
450mA. This minimum value places a requirement that a PSE has to supply at 
a minimum 400mA - but the maximum value appears to place a requirement on 
the PSE to current limit at less than 450mA. Hence a PSE is required to 
provide a current limit somewhere in the range 400mA to 450mA. I therefore 
don't think we should be changing the PD specification to allow it to draw 
in excess of the allowable minimum current limit value of a PSE. I suspect 
the issue here however is the use of the Vport minimum of 36V as specified 
rather than the 'average' Vport minimum of 37V which isn't.

On the assumption that it is the intent that the formula be changed in 
such a way that it yields a maximum of 400mA, having this followed by 
another shall statement that states the maximum shall be less than IPort 
max (which is also 400mA) seems redundant. If the intent is that the peak 
is 400mA and that this should occur for a maximum of 50ms with a 5% duty 
cycle then the text should simply be changed to read 'At any operating 
condition the peak current shall not exceed IPort max for more than 50ms 
max and 5% duty cycle max.'.


Comment 183 - subclause 33.2.8.4
--------------------------------

I'm not too sure why we should use VPort = 57V in the calculation on slide 
7 since the title of subclause 33.2.8.4 includes the text '.. at min 
output voltage'. For VPort > 44V IPeak = 17.6/VPort yields a maximum IPeak 
of 17.6/44 = 400mA. Item a) and b) therefore have the same IPeak value of 
400mA which supports increasing the peak operating current a PD can draw 
to 400mA since that appears to be what IEEE Std 802.3af already requires a 
PSE to be able to source. I'm therefore certainly in support of simply 
deleting item b) (or replacing it) as it seems to currently be redundant.

In respect to the proposed new formula on slide 10 I don't think we should 
have Rch (the channel resistance) as a variable. We should select the 
worse case maximum channel resistance base on the cable category that IEEE 
P802.3at is going to support and place that figure in the formula. If we 
don't we can't ensure the correct value will be used and this could impact 
the ability for a PSE to support the cable plant that the standard is 
supposed to support.




"Fred Schindler (frs)" <frs@cisco.com> wrote on 09/10/2007 18:58:59:

> Hello,

> The materials for the Oct. 11, IEEE Vport ad hoc meeting are attached.
> Anoop will run this meeting because I am out of the office.

> 
> We have completed the review if the SOA comment bucket.  Please confirm
> that the information collected is acceptable to the group.  If accepted,
> it will be presented to the Task Force during the Richfield meeting.
> The second presentation will help us determine if the legacy PD is a
> constant power load.  Understanding this will enable us to create a PSE
> current requirement.

> 
> Please ensure that you review the IEEE-SA PatCom slides [
> http://standards.ieee.org/board/pat/pat-slideset.pdf ] prior to the
> start of the conference call. At the start of call I will ask that
> everybody confirms that they have read this material - if anybody hasn't
> we will pause the call until they confirm that they have done so.

> 
> Thanks,

> Fred

> 
> ---------------------

> Hello,

> 
> The IEEE 802.3at Vport ad hoc will be meeting to cover the following:

> 
> 1) PSE current requirements to fit on Figure 33-9a, of D0.9.

> Please see
> http://grouper.ieee.org/groups/802/3/at/public/sep07/vetteth_2_0907.pdf
> for our starting point.

> 
> 2) Comment review and recommendations.

> a) Powering off the PSE PI when the voltage is out of specification.
> Comments 178, 137

> 
> b) Short-circuit removal of power. Comments 111, 11

> 
> c) SOA and response time. Comments 186, 185, 96, 78, 122

> 
> d) PSE current limits and type-1 considerations. Comment 28, 23

> 
> Our goal is to create a presentation for the Richfield IEEE Interim
> (Oct. 18 - 19) that address the above points.

> Thanks,

> 
> Fred Schindler

> Technical Leader

> CISCO Systems

> 170 West Tasman Drive

> M/S SJ-19-3

> San Jose, CA 95134-1706

> Tel. (408) 525-9859

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> 
> [attachment "C.htm" deleted by David Law/GB/3Com] 
> [attachment "Vport SOA comment bucket.pdf" deleted by David Law/GB/3Com] 

> [attachment "Vport PSE current requirements v3.pdf" deleted by David
> Law/GB/3Com]