Implementation
There are two system components in PoE -- the Power Sourcing Equipment (PSE) initiates the connection to the second component, the Powered Device (PD). The current is transmitted over two of the four twisted pairs of wires in a Category 5 cable. The standard defines two choices for which pairs of wires are used to transmit the power. In one method, the power goes over the spare pairs that are not used by 10BASE-T and 100BASE-T. In the other method, the data pairs are used (without negatively affecting data transfer performance). The Power Sourcing Equipment (PSE) can take either approach. All Powered Devices must support both.

When used in conjunction with a UPS system and integrated with network management tools, critical network-based facilities can maintain high availability, even though the electrical power may be down.

The Internet Engineering Task Force (IETF) has been working in parallel with the IEEE to extend its Simple Network Management Protocol (SNMP) to apply to PoE ports as well. It has developed an Internet Draft that extends the Ethernet-like Interfaces MIB (RFC 2665) with a set of objects for managing Power Source Equipment and Powered Devices. IEEE 802.3af defines the hardware registers that would be used by a management interface. The IETF draft defines management data objects based on the information read from and written to these registers.

The most important feature of the standard is that Power over Ethernet is completely compatible with existing Ethernet switches and networked devices. Because the Power Sourcing Equipment (PSE) tests whether a networked device is PoE-capable, power is never transmitted unless a Powered Device is at the other end of the cable. It also continues to monitor the channel. If the Powered Device does not draw a minimum current, perhaps because it has been unplugged or physically turned off, the PSE shuts down the power to that port. Optionally, the standard permits Powered Devices to signal to a PSE how much power they require.

There are two main implementations of Power over Ethernet dependent on the manner in which a DC voltage is “Injected” into the Category 5 cable; “end-span” and “mid-span”.

In an “end-span” device the Power over Ethernet features are integrated into the Ethernet switch itself, and no other components are necessary.

The “mid-span” device acts like a patch-panel, residing between a conventional Ethernet switch and the terminals. This is also referred to as a “Power Hub”. Power is added to the spare wires on the cable, with data uninterrupted.

Network devices which can accept the injected DC power directly from the Category 5 cable through their network connection are considered to be “PoE Compatible” or “Active Ethernet Compatible”.

Devices which are not PoE Compatible can be converted to Power-over-Ethernet by way of an “Active Ethernet Splitter (also known as a DC “Picker” or “Tap”). These devices take the DC voltage that has been injected into the Category 5 cable, by the injector, and make it available to the connected device via a conventional DC power socket.

Two basic types of “Active Ethernet Splitters” are available; passive and regulated.

A passive Active Ethernet Splitter simply takes the voltage from the Category 5 cable and directs it to the device for direct connection.

A regulated Active Ethernet Splitter takes the voltage from the Category 5 cable and converts it to a different voltage. Several standard regulated voltages are available; 5VDC, 12VDC, 18VDC and 48VDC. This allows a wide range of non-PoE devices to be powered through a Category 5 cable.

Standards
The US based IEEE (Institute of Electrical and Electronic Engineers) approved the standard for Power over Ethernet products in June 2003. The 803.af-2003 standard ensures that:

The amount of power available is limited. There are four power classes in the 802.3af specification, from 4 watts to a maximum of 15.4 watts to be delivered per port. The maximum power available at 100m is approximately 13W.

All PowerDsine products starting with “4”, for example PD-AS-401/12, are products that pre-date the 802.3af standard [June 2003].

All PowerDsine products starting with a “6”, for example PD-6012, are fully compliant with the 802.3af standard.

The PowerDsine 8000 Series, High Power over Ethernet, products infringes the 802.3af standard but will conform to a new standard yet to be approved, 802.3at. The power available from High Power over Ethernet mid-spans at 100m is 26W to 30W but is expected to be 39W when the standard is adopted.

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