[8023-POEP] Some thoughts on 2 pair power systems
I had some thoughts on 2 pair power systems - here are some definitions,
rules and situations....
1. For both the PSE and the PD only 2 pair power delivery is specified.
2. PSEs may be deliver power over the primary pair (AB - the pair used
for autonegotiation) or the secondary pair (CD). I think that we should
refer to these as AB-PSEs and CD-PSEs (or similar nomenclature). All
midpsans must be CD-PSEs. Endpoints may be either PSE type but if they
are implemented as CD-PSEs then they are functionally equivalent to
midspan PSEs.
3. In all cases, a PD must not draw more power than it has requested.
For CD-PSEs this is limited to the power class presented; for AB-PSEs
this may be modified by the L2 power management.
4. PSE system configurations may be AB-only, CD-only or multiple PSEs
co-located.
5. PD configurations:
A system with a single PD must be an either/or device (like .3af).
It presents a discovery resistor (and classification if supported) on
2 or more sets of pairs. After it
is powered it disappears from all of the pairs. It can use simple
diode bridges to accept power on
any of its supported pair sets. A .3af PD would be classed as an
either/or PD presenting detection on
2 pair sets although nothing would prevent a .3af system that could
accept power on multiple ports
connected in an either/or configuration.
A system with more than one PD may be configured as a both/all device.
It presents discovery resistors (and classification) on multiple pair
sets. At its simplest this would be
a 4-pair PD, needing power on both AB and CD pairs. Each PD must
follow the rules of discovery
and classification; it must follow rule #3. The system may function
with less than all of its PDs receiving
power while continuing to present discovery resistors on its (as yet)
unpowered PDs. According to the group
objective it will be mandatory to display some form of indication if
at least one PD is powered but more
power is still required.
All PD systems must be either/or or both/all configurations.
6. A PD system receiving power over the AB pair must support L2
management for the AB pair, a PD receiving power over the CD pair does
not support L2 management for that pair.
Some situations
a) Any simple PSE connected to an either/or PD - works same as .3af (but
may be >15W if both .3at).
b) Both AB-PSE and CD-PSE connected to and either/or PD: First to detect
& power supplies the system, the other doesn't detect. If both detect
simultaneously and both supply power - so what! Power drawn will be less
than classification level limit. NB - if PD intends to use L2 to request
more power than L1 class limit, then it must be sure that it is getting
power from AB-PSE.
c) Combo PSE system (co-located AB-PSE & CD-PSE) connected to either/or
PD system. May use discovery of either/or PD device to supply via
multiple pairs & gain efficiency. No need to specify transmission
efficiency mechanism as transmission efficiency has always been beyond
the scope of the standard.
d) Single PSE connected to both/all PD system: PD must not draw more
power than classification level limit; indicators may be recommended or
mandated to show that more power is needed.
e) Separate PSEs connected to a both/all PD system: each PSE-PD pair
runs according to the rules, AB pair may use L2 management.
f) Combo PSE system connected to both/all PD system: may allow special
treatment but need to beware of split cases. I suggest that the
potential efficiency gains should be outside the scope of the standard.
Still a concern...
Need to be sure that classification mechanism lets a >13W PD know that
it has been detected by a .3at PSE. The PD must understand whether it
can draw the extra power.
Mixed old & new PSEs connected to an either/or PD that requires more
than 13W. The PD may need a means of determining whether either PSE is
capable of >13W and ensuring that only the new PSE successfully detects
and powers. This might have to be mandatory - needs more thought.
That's all for now,
Hugh.