Documentation/networking/pse-pd/pse-pi.rst

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PSE Power Interface (PSE PI) Documentation
==========================================

The Power Sourcing Equipment Power Interface (PSE PI) plays a pivotal role in
the architecture of Power over Ethernet (PoE) systems. It is essentially a
blueprint that outlines how one or multiple power sources are connected to the
eight-pin modular jack, commonly known as the Ethernet RJ45 port. This
connection scheme is crucial for enabling the delivery of power alongside data
over Ethernet cables.

Documentation and Standards
---------------------------

The IEEE 802.3 standard provides detailed documentation on the PSE PI.
Specifically:

- Section "33.2.3 PI pin assignments" covers the pin assignments for PoE
  systems that utilize two pairs for power delivery.
- Section "145.2.4 PSE PI" addresses the configuration for PoE systems that
  deliver power over all four pairs of an Ethernet cable.

PSE PI and Single Pair Ethernet
-------------------------------

Single Pair Ethernet (SPE) represents a different approach to Ethernet
connectivity, utilizing just one pair of conductors for both data and power
transmission. Unlike the configurations detailed in the PSE PI for standard
Ethernet, which can involve multiple power sourcing arrangements across four or
two pairs of wires, SPE operates on a simpler model due to its single-pair
design. As a result, the complexities of choosing between alternative pin
assignments for power delivery, as described in the PSE PI for multi-pair
Ethernet, are not applicable to SPE.

Understanding PSE PI
--------------------

The Power Sourcing Equipment Power Interface (PSE PI) is a framework defining
how Power Sourcing Equipment (PSE) delivers power to Powered Devices (PDs) over
Ethernet cables. It details two main configurations for power delivery, known
as Alternative A and Alternative B, which are distinguished not only by their
method of power transmission but also by the implications for polarity and data
transmission direction.

Alternative A and B Overview
----------------------------

- **Alternative A:** Utilizes RJ45 conductors 1, 2, 3 and 6. In either case of
  networks 10/100BaseT or 1G/2G/5G/10GBaseT, the pairs used are carrying data.
  The power delivery's polarity in this alternative can vary based on the MDI
  (Medium Dependent Interface) or MDI-X (Medium Dependent Interface Crossover)
  configuration.

- **Alternative B:** Utilizes RJ45 conductors 4, 5, 7 and 8. In case of
  10/100BaseT network the pairs used are spare pairs without data and are less
  influenced by data transmission direction. This is not the case for
  1G/2G/5G/10GBaseT network. Alternative B includes two configurations with
  different polarities, known as variant X and variant S, to accommodate
  different network requirements and device specifications.

Table 145-3 PSE Pinout Alternatives
-----------------------------------

The following table outlines the pin configurations for both Alternative A and
Alternative B.

+------------+-------------------+-----------------+-----------------+-----------------+
| Conductor  | Alternative A     | Alternative A   | Alternative B   | Alternative B   |
|            |    (MDI-X)        |      (MDI)      |        (X)      |        (S)      |

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