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[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.