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IEEE 802.3at-2009 Table 33-7 shows minimum power supply (PSE) levels that seem to be less than the peak power that the powered device (PD) could draw for Class 1 and 2. Can anyone clarify how these are compatible or where I am misinterpretting the standard?

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Red Dragon
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1 Answers1

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IEEE 802.3 Power over Ethernet Power Requirement Nuance

Background

  • Power Supply Equipment (PSE) supplies power to a Powered Device (PD) at a Power Interface (PI) across a link section (cable).
  • Pclass defines the average power the PSE needs to supply. (IEEE 802.3at-2009 Table 33-7)
  • Pclass_PD defines the average power the PD is allowed to consume. (IEEE 802.3at-2009 Table 33-18)
  • Ppeak_PD defines the peak power the PD is allowed to consume. (IEEE 802.3at-2009 Table 33-18)

Nuance

  • It isn't always obvious in the standard that Pclass is an average value. It can be misleading that for Class 0 and 3 Pclass > Ppeak_PD which might lead you to believe Pclass was the peak power the PSE would need to supply. Pclass < Ppeak_PD for Class 1 and 2 is a sign that this is not correct.
  • The internet is flooded with references that get this wrong or hide this nuance.
  • https://ethernetalliance.org/wp-content/uploads/2018/04/WP_EA_Overview8023bt_FINAL.pdf Table 13 helps provide clarity. Attached below.
  • PSE peak power (Ppeak_PSE?) is never defined. You must derive it yourself as done in above paper. Note margin has not been added and thus slight Pclass differences vs IEEE 802.3at-2009.

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Red Dragon
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