[8023-POEP] ALLOWING EACH 2-PAIR SET TO OPERATE AUTONOMOUSLY
802.3AT
4-PAIR SYSTEMS
ALLOWING EACH 2-PAIR SET TO OPERATE AUTONOMOUSLY
CLAY STANFORD
LINEAR TECHNOLOGY
5-2-06
RECOMMENDATION
Assuming 2-pair 802.3at is able to address the power
needs for the majority of PDs (i.e. 2P > ~ 25 watts)
Implement 4-pair system as two fully autonomous 2-pair
systems with these limitations:
4-pair PDs must maintain isolation between pairs
4-pair power classification is handled as two,
autonomous 2-pair ports
4-pair PDs may only operate at power levels above
Pmax2pair (~25W)
We do not need to define pair-to-pair power turn on
timing.
We maintain requirement that if port is to be
powered, it is to be powered within 1 second of
detection
We do not allow combining two 802.3af ports onto one
cable
DETAILS OF RECOMMENDATION
It would be possible to allow 4-pair systems to
operate as 2 totally autonomously 2-pair systems.
However there are some problems associated with this
architecture detailed as follows:
Expensive:
Point #1) Even if PD is in plastic housing, it must
contain two independent and isolated front ends. This
includes signature, classification circuit, load
isolation (hotswap) switch, and DC/DC converter. If
the front ends are not isolated, problems with (top
rail) ground leg current balancing can occur if driven
from a single PSE. Additionally, ground loop problems
can occur if driven from two separate PSEs.
Point #2) If we allow 4-pair PDs to operate within
the 2-pair power range we should mandate that the PD
must also be designed such that it can be powered by a
2P PSE. Otherwise an interoperability problem will
exist. For example, assume Pmax2pair = 30W, and we
allow a 24 watt PD to be operated from a 4P PSE (two
12-watt ports). If we dont mandate that the PD must
also be able to operate from a 2P PSE, the user may
have a 30W 2P PSE that can not power a 24W PD. In
addition, there are problems with the classification.
If the 4P PD advertises 12 watts on each port, then a
2P PSE would not be able to power it. This can be
fixed by encoding the total power in the
classification, but this adds additional complications
and the two ports are no longer autonomous. TO AVOID
THESE PROBLEMS, I WOULD RECOMMEND THAT 4-PAIR PDs BE
RESERVED FOR 4-PAIR POWER LEVELS.
Power turn-on problems may occur. Because of the
time needed to perform detection and classification,
there may be a significant difference in turn-on time
for two ports used to power one 4-pair PSE.
Additionally, the first port may turn off before the
second port turns on due to DC disconnect timing
(300mS). Note: It would be possible to specify
port-to-port turn-on timing requirements. In this
case, the ports would no longer operate autonomously.
The PD must contain advanced Power Good circuitry to
determine when both ports are fully powered. This
power good circuit must know what type of PSE is
connected to each port. If two .at PSEs are connected
and ready, it can give the green light. However, when
many other combinations are connected, it must give
the user an indication of a problem. (For example, 1
.af PSE, 2 .af PSEs. 1 .at PSE and 1 .af PSE, 1 .at
PSE and 1 broken wire, etc.). If 24 watt 4-pair PSEs
are allowed (as in the example above), the power good
circuitry must be able to determine if it is powered
by a 2P .at PSE or a 4P .at PSE. In this case, it may
be difficult to know if it is getting 12 watts or 24W
from the 2P PSE. I WOULD SUGGEST WE DISALLOW THE
EXAMPLE 24 WATT 4P PSE.
Will we allow 4-pair .af ports on one cable? This
sounds like an easy way for users to up the power
using existing hardware. But if we allow the use of a
Y cable to combine two .af ports onto a single cable,
the differences in wiring within the installed base
will make for an interoperability nightmare. I
RECOMMEND AGAINST THIS.
System administration will not have good visibility
as to what is plugged into system.
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306655284-4pair.doc