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Re: [8023-POEP] [Bulk] [8023-POEP] AW: [8023-POEP] August interim


On 6/5/09 2:40 AM, Unterdorfer, Pascal wrote:
Hello Everybody,

I have a question about the PSE Interface, Detection and Safety!

In the actual IEEE 802.3af standard, 33.2.9 Power supply allocation, the behaviour of connecting a PSE PI with another PSE PI is not explained.
That would be because it is a requirement of 802.3af that you do not turn power on until the PSE positively identifies (Discovery and Classification) that the device at the other end of the link segment has the probe signature response that is unique to an 802.3 PD.

If you are worried about a cabling topology where a UTP cable is daisy-chained to multiple (i.e. more than two) interfaces then you are out of scope for 802.3 as well as most other applications for Category 5/6/7 cabling. If that is the case, then you are on your own and we can't help you.  Note that it talks about a "link."  A "link" is a single point-to-pont connection.

33.2.9 Power supply allocation
A PSE shall not initiate power provision to a link if the PSE is unable to provide the maximum power level requested by the PD based on the PD's class. Where a PSE does not provide the optional classification function specified in 33.2.7, all PDs are treated as Class 0.

The PSE may manage the allocation of power based on additional information beyond the classification of the attached PD. Allocating power based on additional information about the attached PD, and the mechanism for obtaining that additional information, is beyond the scope of this standard with the exception that the allocation of power shall not be based solely on the historical data of the power consumption of the attached PD.

If the system implements a power allocation algorithm, no additional behavioral requirement is placed on the system as it approaches or reaches its maximum power subscription. Specifically, the interaction between one PSE PI and another PSE PI in the same system is beyond the scope of this standard.

In the industrial field, line and ring topologies are very common, so my question is:
Does the new IEEE 802.3at standard explain this issue?
No. In the text above a "system" refers only to what is within the scope of 802.3, specifically the 802.3 (and not the higher layer aspects of the same devices) portions that are hooked to a single point-to-point link. For that reason that any connection "in a line" or "in a ring" of Ethernet interfaced devices has to resolve all issues of how to hook multiple Ethernet point-to-point link segments together within whatever higher layer device is used to hook together the multiple Ethernet interfaces.  An 802.1D Bridge is the usual device for doing this and allows devices to be hooked in a line quite nicely. If you try to hook 802.1D devices in a ring then it will undo one of the segments in the ring as the topology violates the Spanning Tree protocol.

What happens if I connect two PSE PI's together?
Neither PSE will try to supply power

Can anybody help me with this question?
Hopefully, this answers your question.

Best rregards,

Geoff Thompson


Manny thanks!


With best regards, mit freundlichen Grüssen

Pascal Unterdorfer
Product Manager Industrial Networking


Hirschmann Automation and Control GmbH
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