Power over Ethernet or PoE technology describes a system to pass electrical power safely, along with data, on Ethernet cabling. The IEEE standard for PoE requires Category 5 cable or higher for high power levels, but can operate with category 3 cable for low power levels. Power is supplied in common mode over two or more of the differential pairs of wires found in the Ethernet cables and comes from a power supply within a PoE-enabled networking device such as an Ethernet switch or can be injected into a cable run with a midspan power supply.
Powered Devices (PD) are discovered using Link Layer Discovery Protocol (LLDP) which is a vendor-neutral Link Layer protocol in the Internet Protocol Suite used by network devices for advertising their identity, capabilities, and neighbors on a IEEE 802 local area network, principally wired Ethernet.
In the case of PoE devices, power class is also reported.
Power over Ethernet (PoE) (IEEE 802.3af)
The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA) to each device. Only 12.95 W is assured to be available at the powered device as some power is dissipated in the cable
In 2003, the IEEE 802.3af standard for Power over Ethernet introduced a new aspect to Ethernet data networking, it allow for the delivery of DC power in tandem with the 10/100/1000 Mbps data.
When power over Ethernet (POE) IEEE 802.3af was ratified in 2003, its nominal power delivery at 12.95 Watts of DC power was more than adequate for the early adopter applications such as your standard VoIP phones, security cameras, and wireless access points (WAPs).
The specification for PoE calls for two devices: the power source equipment (PSE) (POE data switch) and the powered device (PD) (IP phone, WAP, IP camera, IP speaker).
The (PD) powered device can be one of many different devices, including the IP phone or wireless access point.
The 802.3af standard also provides for five power classes to which a device might belong. The PSE vendor does not have to implement all these classes and can choose to support the maximum of 15.4 Watts. It should be noted that even though a powered device might support IEEE 802.3af-2003 power classification, the PSE might not, and 15.4 Watts max delivery is the common denominator.
Class 4 can only be used by IEEE 802.3at (type 2) devices, requiring valid Class 2 and Mark 2 currents for the power up stages.
An 802.3af device presenting a class 4 current is considered non-compliant and, instead, will be treated as a Class 0 device
Power over Ethernet+ (PoE+) (IEEE 802.3at)
The updated IEEE 802.3at-2009 PoE standard also known as PoE+ or PoE plus, provides up to 25.5 Watts of power.
The 2009 standard prohibits a powered device from using all four pairs for power. Some vendors have announced products that claim to be compatible with the 802.3at standard and offer up to 51 Watts of power over a single cable by utilizing all four pairs in the Category 5 cable.
With the introduction of network devices that require more than 15.4W has compelled the IEEE to develop a new PoE standard that can deliver even more power than defined in the IEEE 802.3af standard. This new standard, IEEE 802.3at, is designed to deliver at least 30W per port of inline power.
Transmitting more than 15.4W of power per port poses some significant challenges. One such challenge lies in the physical characteristics of copper cabling, which can overheat or get damaged when transmitting power above certain thresholds. The IEEE is exploring different means of transmitting higher levels of power subject to these limitations.
Another challenge is backward compatibility with the IEEE 802.3af standard. This interoperability could be crucial to the successful adoption of 802.3at. Therefore, the IEEE is working to make sure that 802.3at-compliant PSEs are able to interoperate with 802.3af powered devices and the reverse.
PoE+ brings more power and better classification techniques to an already established industry of PoE networks. To be compliant, a PSE must source 30 W to the data or spare pairs, and a PD controller must draw no more that 25.5 W at the input of the RJ45 connector. A PoE+ compliant PSE must be able to perform single-event hardware classification, while the new 2-event classification and LLDP data layer classification are optional high-power classification mechanisms.
On the powered-device side, the PD must be able to respond to 2-event classification (by the PD controller) and LLDP (by the PD microprocessor). PoE+ compatible designs are already being deployed.
Note: BB-24 will add the IP Phone class to its value.
Example: IP-560g with BB-24 will be 6.2 W idle, 9.2 W active, and 13 W reported via LLDP.