PoE — Power over Ethernet
Definition
Power over Ethernet (PoE) is a set of IEEE 802.3 standards that deliver DC power over twisted-pair Ethernet cable together with the data. The power available at the switch port (PSE) is 15.4 W for 802.3af (Type 1), 30 W for 802.3at (Type 2, PoE+), 60 W for 802.3bt Type 3 and 90 W for 802.3bt Type 4. Cable losses reduce the power guaranteed at the powered device (PD) to 12.95 W, 25.5 W, 51 W and 71.3 W respectively. The name PoE++ is a commercial term for 802.3bt. A switch also has a total PoE budget shared by all its ports, which is often lower than the per-port maximum multiplied by the number of ports.
In the field
PoE planning compares the total budget of the switch with the sum of the actual draw of the connected devices, which manufacturers publish as a PoE class or a maximum power. A 24-port PoE+ switch with a 370 W budget, for example, cannot supply 30 W on every port at once, because that would require 720 W. When the budget is exceeded, the switch refuses or removes power on some ports according to its port priority settings.
Sources
- Ethernet Alliance, Overview of 802.3bt, Type 3 : 60 W au PSE, 51 W au PD ; Type 4 : 90 W, 71,3 W · ethernetalliance.org
- Sifos, IEEE 802.3 PoE Technology Overview, Type 2 (802.3at) : 30 W au PSE, 25,5 W au PD · sifos.com
- NETGEAR, Overview of Official PoE Standards, 802.3af : 15,4 W au PSE, environ 12,95 W utilisables au PD ; 802.3at 30 W ; 802.3bt (PoE++) Type 3 60 W, Type 4 90 W · netgear.com
- Microchip, Next-Generation PoE: IEEE 802.3bt White Paper : classes de puissance et rétrocompatibilité 802.3af/at · ww1.microchip.com