Re: [8023-POEP] What's the max voltage drop thru a midspan?
Hi Martin,
I believe that the way you look at it is reasonable since it reflects
the current practice.
I wonder why some Endspan PSE vendors choose to use ALT B since it
doesn't make sense from supporting 2P cable point of view. Today 2P
cable may be less relevant however from operational point of view having
data and power on the same pair has many advantages.
I guess that the number of ports supported by those vendor (using ALT B
in Endspan) compared to the total ports in the market is not high to
cause us interoperability issues (especially that there is an IT manager
solution in terms of adapters or other equivalent interface to convert
from alt B to ALT A etc.)so high even it does allowed by the current
standard.
Yair
-----Original Message-----
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Patoka, Martin
Sent: Monday, June 12, 2006 6:22 PM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] What's the max voltage drop thru a midspan?
Another way to look at this is that the presence of the detection
backoff algorithm implies that an endpoint/midspan combination will
occur per 33.2.3.1 (although not specifically allowed/encouraged by fig
33-4). In the event that the midspan did not actually power the PD, it
would then have to carry the endspan currents.
I think that this addresses your concerns when combined with the
paragraph that David quoted.
Regards,
maritn
Martin Patoka
Systems Engineer
Texas Instruments
214-567-5487
mpatoka@xxxxxx
-----Original Message-----
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Steve Robbins
Sent: Monday, June 12, 2006 10:00 AM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] What's the max voltage drop thru a midspan?
David,
Unfortunately I don't think this text solves the problem. My
interpretation is that it does the following:
1. The midspan must pass data thru it without degrading data integrity.
2. As the midspan injects power on the spare pares, it must block that
power from going back to the endspan.
So it doesn't say anything about passing power thru on the data pairs.
Steve
-----Original Message-----
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx]
On Behalf Of David Law
Sent: Monday, June 12, 2006 3:46 AM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] What's the max voltage drop thru a midspan?
Hi Steve,
Do you think the following text contained in the third from last
paragraph of subclause 33.4.8 'Midspan PSE device additional
requirements' of IEEE Std 802.3-2005 covers what you are looking for:
---oo000oo---
Configurations with the Midspan PSE in the cabling channel shall not
alter the transmission requirements of the "permanent link." A Midspan
PSE inserted into a channel shall provide continuity for the signal
pairs. A Midspan PSE shall not provide DC continuity between the two
sides of the segment for the pairs that inject power.
---oo000oo---
Regardless, I agree that the IEEE 802.3at specification will need to be
written carefully to ensure that, for example, it doesn't
retrospectively place a higher current carrying requirements on existing
IEEE Std
802.3-2005 compliant Mid-Span PSEs.
Regards,
David
owner-stds-802-3-poep@xxxxxxxx wrote on 10/06/2006 05:34:39:
> Guys,
>
> As you all know, we're planning to run current from the endspan thru
> the midspan in a 4P system.
>
> Although I think its unlikely that we'll see Af-midspans with fused
traces
> or delaminated boards, technically we can't prove that it won't happen
> because 802.3af doesn't specify a min current rating for this path.
There
> seems to be no max limit for dc resistance either. (If these specs
> are
in
> 802.3af and I've missed them, then someone please point me to the
applicable
> table or paragraph.)
>
> We should make sure 802.3at specifies both these parameters. (But
> only
for
> midspans that output >15.4W so we're not retroactively putting new
> requirements on Af-midspans.)
>
> Any comments?
>
> Steve
>
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