IEEE 802.3af: Power over Ethernet

The IEEE 802.3af specification defines a standards-based mechanism for providing Power over Ethernet (PoE) to devices. Power is provided inline, using two of the four available pairs (4 wires) of the 8-pin modular jack used for twisted-pair Ethernet connections. The standard introduces two types of devices:

• Power Sourcing Equipment (PSE): Includes devices such as Ethernet switches and power injectors that provide inline power to powered devices
• Powered Devices: Includes devices such as IP Phones, access points, and IP cameras that receive power from PSE

The specification defines a nominal voltage of 48 volts direct current (min 44 Vdc to max 57 Vdc), with a maximum power output of 15.4 watts per PSE (switch) port. To support more intelligent power management, the PSE might optionally classify powered devices. Table 1 shows the currently defined power classifications.

Table 1: IEEE 802.3af Power Classifications 

Class	Max Output Power (Watts)
0		15.4
1		4.0
2		7.0
3		15.4
4	Treat device as Class 0

If the PSE cannot determine the power classification of a powered device, it should default to a Class 0 device with a maximum of 15.4 Watts. Class 4 is reserved for future use. The IEEE 802.3af task force is currently working on an extension to PoE, commonly referred to as PoE+, which might extend the amount of power supplied by a PSE to 24 watts per port.

In Cisco TelePresence deployments, IEEE 802.3af-based PoE is supplied by the primary codec to the attached IP 7975G Phone (Class 3 device) associated with the CTS endpoint. Power can also be provided locally to the IP 7975G Phone if desired. PoE is also supplied to the primary and auxiliary cameras connected to the primary codec. If the CTS endpoint supports multiple codecs, such as with the CTS-3200 or CTS-3000, the secondary codecs supply PoE to their associated primary camera Ethernet connections as well.