Power Over Ethernet (PoE)

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Power over Ethernet (PoE) is a means of supplying electrical power from a switch port over ordinary data cabling to a powered device (PD), such as a voice over IP (VoIP) handset, camera, or wireless access point.

• PoE is defined in several IEEE standards:
  • 802.3af allows powered devices to draw up to about 13 W
    • Power is supplied as 350mA@48V and limited to 15.4 W
      • but the voltage drop over the maximum 100m (328 feet) of cable results in usable power of around 13 W
  • 802.3at (PoE+) allows powered devices to draw up to about 25 W, with a maximum current of 600 mA
  • 802.3bt (PoE++ or 4PPoE) supplies up to about 51 W (Type 3) or 73 W (Type 4) usable power
• PoE-enabled switch is referred to as endspan power sourcing equipment (PSE)
  • When a device is connected to a port, the switch goes through a detection phase to determine whether the device is PoE enabled
    • If so, it determines the device’s power consumption and sets an appropriate supply voltage level
    • If not, it does not supply power over the port and, therefore, does not damage non-PoE devices
  • Powering these devices through a switch is more efficient than using a wall-socket AC adapter for each appliance
  • also allows network management software to control the devices and apply energy saving schemes
    • such as making unused devices go into sleep states and power capping
• If the switch does not support PoE, a device called a power injector can be used
  • aka midspan
  • One port on the injector is connected to the switch port
  • The other port is connected to the device
  • overall cable length cannot exceed 100m