For this trip into the world of the IEEE802.3bt Task Force, I want to dive into the material that I presented at the November Plenary Session. My presentation, 4PPoE: Maximizing Interoperability with 802.3-2012 Devices (and other things), addressed a method for meeting the backwards compatibility objectives that 4-Pair Power over Ethernet (4PPoE) study group adopted. I hope that you share any feedback that you have by either commenting on this blog or emailing me at david.abramson@ti.com.
The core of the presentation is an idea, called the Connection Check, in which the 4-pair PSE (4PPSE) interrogates the device on the other end of the cable to determine if it is connected to a single PD interface or two PD interfaces. This method would allow a 4PPSE to understand if it should consider the power requested by the PD to be:
- The total power for both pair sets (for the single PD interface).
- The power for each pair set respectively (for the dual PD interface).
This idea would also take advantage of the fact that all current PD interfaces, 802.3af and 802.3at, are required to accept power over either pair set (in practice this is accomplished by using a diode or FET bridge at the input of the PD). The connection check would use this fact to power current PDs over all four pairs.
Let me give an example to make this idea a little clearer. If a 4PPSE was connected to a class 3 PD, the 4PPSE would use the Connection Check to determine that it was probing the same class 3 PD interface over each pair set. It would then provide class 3 power (15W) total for both pair sets combined. However, if the Connection Check indicated that it was probing two different class 3 PD interfaces over each pair set respectively, it would provide 15W individually to each PD. This is illustrated in the figure below.
You can read the rest of the details of the presentation, such as how we might implement the Connection Check, for yourself. However, I want to point out one more thing that is not in the presentation itself: If the Connection Check is implemented in 4PPSEs, PD manufacturers could immediately implement 51W PD designs by using two class 4 802.3at PD interfaces. This would allow them to move to higher power quickly with almost no design effort.
What do you think? Do you believe the Connection Check is a good idea, that it should be implemented in 4PPSEs? Any other ideas on how to address the issues in the presentation?
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