(Summary from Deploying Brocade FastIron-based PoE and PoE+ Solutions in the Campus Network)
The IEEE 802.3af and 802.3at specifications define the standards for delivering power over existing network cabling infrastructure and providing multicast-enabled full streaming audio and video applications for converged services, such as Voice over IP (VoIP), WLAN access points, IP surveillance cameras, and other IP technology devices.
PoE technology eliminates the need for an electrical outlet and a dedicated Uninterrupted Power Supply (UPS) near IP-powered devices.
PoE capability is needed in the campus network access layer only. Distribution and core switches do not need PoE capability.
The PoE standard defines power classes for up to maximum of 15.4 watts 
of power. The device gets less power after power dissipation (loss) over the copper wire.
IEEE 802.3af PoE standard classes
Standard PoE Class Usage Min Power at PSE Max Power at PD
802.3af 0 Default 15.4 0.44 – 2.95
1 Optional 4 0.44 – 3.84
2 Optional 7 3.84 – 6.49
3 Optional 15.4 6.49– 12.95
4 Unused
The IEEE 802.3at PoE+ (also called High Power) standard doubles the power at the PSE ports to a maximum of 30W.
The new PoE+ standard is a superset of the legacy PoE standard, so switches supporting PoE+ by default support legacy PoE capability as well. This is accomplished by bundling legacy PoE classes 0 through 3 (Type 1) and using the unused class 4 for high power (Type 2).
Standard PoE+ Class Usage Min Power at PSE Max Power at PD
802.3af (Type1) 0 Default 15.4 0.44 – 2.95
1 Optional 4 0.44 – 3.84
2 Optional 7 3.84 – 6.49
3 Optional 15.4 6.49 – 12.95
802.3at (Type2) 4 Optional 30 12.95 – 25.5
The 802.3at standard currently supports POE+ on 10/100/1000 Mbps Ethernet ports operating over standard Category 5 Unshielded Twisted Pair (UTP) cable or better.
There are two ways for the powered devices to dynamically request the PoE power from the PSE—proprietary protocols such as the Cisco Discovery Protocol (CDP) used by Cisco IP phones or the standard-based Link Layer Discovery Protocol (LLDP) used by variety of IP phones and access points
LLDP is a Layer 2 network discovery protocol described in the IEEE 802.1AB standard. This protocol enables a device to advertise its capabilities to and to discover other LLDP-enabled devices in the same 802 LAN segments.
LLDP Media Endpoint Devices (LLDP-MED) is the Layer 2 network discovery protocol extension to LLDP described in the ANSI/TIA-1057 standard. This protocol enables a PoE switch to configure and manage connected powered devices that need to send media streams across the network (for example, IP phones and security cameras). LLDP enables network discovery between network connectivity devices (such as switches), whereas LLDP-MED enables network discovery at the edge of the network, between network connectivity devices and media endpoint devices (such as IP phones).
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